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Was Thomas Edison Right about DC Power?
Declan McCullagh writes "Everyone knows the alternating vs. direct current wars ended with Thomas Edison and Nikola Tesla. But now DC power is being seriously considered for data centers. DC advocates say that plugging servers into AC power is inefficient, and switching to DC cuts down on waste heat and component failure. The University of Florida has even bought 200 DC servers."
Was Thomas Edison Right about DC Power?
:-)
Oh, well, nothing sensationalist about that headline. (*rolls eyes*)
DC advocates say that plugging servers into AC power is inefficient, and switching to DC cuts down on waste heat and component failure.
In this case they're right. With that much hardware that close together, it's easier to treat the entire room as a single device. As the article suggests, this cuts down on waste heat produced by inefficiencies in AC->DC conversion. In fact, it significantly cuts down on the amount of equipment needed in the entire room. The concept can be taken as far as to cutting down to a single power supply per rack.
The amusing part about this is that the resulting racks might look a lot like Big Iron servers with pluggable motherboards.
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I thought it was Westinghouse for AC.
(No, I'm not proud of having graduated from FSU, but I am proud of having not graduated from UF)
I heard of this new power system. Seems like a mix of AC and DC, to create the ultimate power form. AC *lightningbolt* DC was the name, and with a lightning bolt in the name, it has to strike you like thunder.
No, he was having a pissing match with Tesla.
"We returned the General to El Salvador, or maybe Guatemala, it's difficult to tell from 10,000 feet"
Although Alternating current has provided easier long range transfer of power, it does seem to be causing problems with machines. A recent study conducted by Maxtor (sorry, I lost the link, probably on google), found that AC power generates weak antistropic fields. Although this is normally not a problem, as the power supply converts the flow to DC, a poorly shielded power supply can leak the antistropic magnetic field, sometimes corrupting data on the harddrive. Although not usually a problem, this can be dangerous around production machines, so either switch to dc, or check your power supplies carefully. Usually, it is not a problem of a poorly built power supply, but simply one who's shielding fails over time.
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In Washington State, Verizon (Was once GTE) runs almost all DC powered servers and Telco equipment in their Data Centers. Many of the IBM server my company buys support DC power.
TFA says
The current travels through massive copper bus bars that are bolted together, but joints must be inspected regularly. Loose joints are a big problem.
I am no electrical techie... but why 'bars' of copper? Cannot DC be safely and efficiently transmitted with thicker 'wires' as opposed to 'bolting' bars together? Hmm.. time to catch up on Electrical Engg stuff...
Here, take a cookie. I promise, by the time you're done reading it...oh, well!
Tesla and Edison were both right...and wrong. Like many Slashdotters do when debating which operating system is best for any given job, Tesla and Edison wanted to apply one power system to every job. Its like having a toolbox with only a screwdriver in it. Ever try to drive a nail with a screwdriver?
For moving power over long distances, AC is king. But for short distances with most modern electronics, DC would win. The first thing a desktop system or server does with AC is converts it to DC. So if you have a number of machines all in the same room, why not do the conversion in one spot, and eliminate the redundancy in every machine.
Would it benefit the average user with one or two machines? Not at all. But for a major center with many machines in the same room, I can see quite a bit of benefit with going with DC.
I want a new quote. One that won't spill. One that don't cost too much. Or come in a pill.
How come there is no real difference? Because both modern AC and modern DC supplies start out by converting the power to high frequency AC (on the order of several kHz), and operate on that. That's what you actually want as input, if anything.
The article states:
In other words, the DC supplies they use are more efficient than standard AC supplies, which are the cheap crap and notoriously inefficient.
Yes this is good in certain applications. I could imagine places like homes and server rooms having this. The conversion from 120V AC to DC at every device is pretty ineffecient. Having one large AC->DC converter would probably be much better. For one, you could locate the device outside. A majority of the heat generated by servers is from the power supplies. However, you would still need DC->DC converters which waste heat too, although I don't think it's that much.
For home use, just imagine getting rid of all those ugly AC power adapters for everything! You could have much smaller DC plugs and fit 20 ports on one outlet.
For industrial settings, you still need AC. It's just the best way to distribute power to things like motors and high power AC systems (120, 208, 240, 480V, etc...)
It's simply not possible to make a DC power transformer because only alternating current provides the changing magnetic field that makes them work. Power transformers are required for stepping up the voltage before transmitting the power over long distances in order to reduce the power losses.
I'm pretty sure I'm just joking about that idea...
Seastead this.
Computers already use DC power?
Is this not the entire point of the PS? To convert AC to DC?
So basically all these new DC computers would be is a computer that relies on yet another source to convert AC to DC then still requires some sort of internal component to convert THAT DC to the correct voltage for the various devices within the computer itself?
Bzzt. Tesla held the patent on AC and licensed it to Westinghouse (and eventually abandoned the patent to help AC by reducing costs). Tesla worked for Westinghouse. Edison promoted DC and founded his own power company, Edison Electric (now part of ConEd).
This is simply not true.
No.
English is easier said than done.
You've got it completely backwards. Edison promoted DC, while Tesla was backed by Westinghouse.
What a stupid headline. Sure he was right about DC power, but he was wrong about AC power and that still has nothing to do with the article.
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If DC was so great it would've succeeded in the 19th century. The argument ended around that time, too.
CASE CLOSED. AC WINS.
Brilliant!
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Uhh.. is a fucking arrogant subject line and even worse when you're fucking wrong.
Err, reverse that. Edison thought AC power was much too dangerous. Tesla didn't care.
Sure, Edison pushed for AC...in electric chairs. He lobbied for the criminal system to use the AC current, knowing that people wouldn't want the same type of power running through their homes. In fact, Edison tried to get people to say that criminals got "Westinghoused," not "electrocuted."
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Nope, sorry. Please play again. http://en.wikipedia.org/wiki/War_of_Currents Tesla and Westinghouse patented all of the AC equipment. Edison wanted to sell his stuff. He even went as far as designing electric chairs with AC to prove it was "more deadly."
All the elephants in the University of Florida server room are probably breathing a sigh of relief over this one.
Telco cabinets are DC and have been for some time.
The HP Blade chasis has an AC to DC PDU outside the chasis.
One thing to remember about DC VS AC cabling. DC requires thicker gaugue cable to push the same wattage. If you think the back of your server cabinet looks cluttered now, wait.
STFU & GBTW
for crap's sake, dc powered servers are nothing new, many have config option of "-48VDC standard telco" supply.
Is it worth pointing out that the phone company, whose switches and local distribution network all required DC to drive the (first) electromechanical components and (then) electronic ones, never made the switch? Commercial power was (and still is, AFAIK, although I've been out of that business for years) used to charge massive banks of 48V batteries that actually power the central office equipment. Once they made the decision to have UPS on that scale, AC/DC/AC conversions were expensive and hence minimized. Modern conversions are much more efficient than they were in the old days; but unless they're cheaper than the electricity, at some point it makes sense to convert once then distribute DC.
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Stodgy old telcos figured this out years ago - the bulk of the switching center equipment runs on 48 VDC. This also makes power backup simple - lead-acid cells in big batteries...
This works especially well when combined with solar panels, since they output DC anyway; why waste energy converting to AC and then back again? Especially since the power is generated on-site so it doesn't have to deal with the resitive loss of travel down miles of power lines, although I realize there can be a lot of wiring inside the building too. Seems like if you're running a large facility with big machines that are contantly running, it makes sense to have some DC infrastructure. The supermarket down the street has about a megawatt of PV on the roof, and uses all DC refrigerators and air conditioning.
DC power is not very good for distributing power over anything other than short distances, in particular given how trivial AC-to-DC conversion is using modern solid state power supplies. Once you reach the end user, then DC starts making more sense.
I've seen houses wired with 12V DC from mini hydro and solar - but in those cases it was a long way to the nearest transmission wire and would cost a fortune to get mains power onto the site.
Telco switches normally ran on 48V DC back in the electromechanical days, and standard telco offices have rooms full of big honkin' batteries to act as a UPS for the building. And yes, power gets distributed on fat copper busses that you don't want to drop wrenches on. As electronic switching systems replaced the old mechanical ones, the capacity increased rapidly while the floor space for electronics decreased, but there's been enough opportunity to fill it back up again.
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Perhaps we'll see the AC group hitting back with demonstrations of how dangerous these DC powersupplies are to the hamsters and other wildlife native to big server rooms.
Incidentally, that's how the electric chair came about:
[Edison]AC is dangerous! Just watch what happens to these various animals when I close this circuit!
Edison electrocutes some horses
[US_Gov]Ooooo... I'll bet that works on people too!
US_Gov introduces new grisly method of executions, while disregarding the main point of Edison's demonstrations.
The story has a good post script too... some reporters came to Edison to get his take the new, modern form of executions. When asked what name he would give to the method, Edison, in an attempt to forever link his competitor's name with electricity's most grusome application, offered "to Westinghouse someone."
Procrastination Man strikes again!
The long distance power distribution network relies on AC power, for reasons that I assume many/most slashdotters are aware of (high voltage to minimize voltage drop across lines, with transformers at the ends). That said, I've long thought it would be really nice to have a big DC power supply with a DC power distribution system in buildings. Just think about getting rid of all the wall warts and power supplies that we currently have to deal with and instead just having regular straight cables to plug DC-powered electronics in to the wall. I believe it would also greatly reduce wasted electricity that people complain about now. I know that many wall warts and other power supplies just waste electricity all day when they're plugged in but not being used. With a well designed large central DC power supply for the whole building, efficiency could be greatly improved. Of course, there are still definite applications where AC is useful. Big appliances with motors (washers, driers, vacuums, etc) are much better suited to running on AC.
Two wrong posts don't make a right. Edison only said AC was dangerous because he wanted to spread FUD with the public that rivil AC was inherently more dangerous than DC. He even tried to turn Westinghouse into a word meaning "to be electrocuted" and pushed for the first use of an alternating current electric chair.
One line blog. I hear that they're called Twitters now.
He also raised turkeys. That was on his turkey farm.
Not that there's a reason the telco world has been running it's stuff on -48v power for, what, forever? Try to make that work well across town, though.
That said, if space and cooling are an issue it might well make engineering sense to get the transformers, capacitors, and rectifiers out of the computer boxes. Big 5v/12v power busses wouldn't even need to be insulated. So while the reporter badly mangled the story, the engineering sounds reasonable to me.
Where I work (Small regional wireless co) 80% of our equipment is DC. Granted that most of that power is for the telco switches a great deal are sun and other servers. The advantage for dc is only having to convert power once. We have a power plant (inverters,rectifiers and a huge battery bank) that takes up an entire room. To keep the battery bank charged requires converting from AC->DC. If the power goes out the batteries take over, end of story. If we were all AC in the switch room we would have to do another conversion from the DC in the batteries back to AC meaning more equipment, losses from the conversion and so less efficiency and more points of failure.
Therefore, if you live farther away than, what was it? 20 miles? TURN OFF ALL YOUR POWER! DC current can't go that far.
However, if you enjoy distributed power, no, no he certainly was not.
You can't take the sky from me...
Actually, that's the norm across the phone industry. Everything, and I mean everything, runs on -48V DC. Okay, not the fluorescent lights....
This goes back to the telephone talk battery, which is -48 V DC. That powered the phones via old cord switchboards, and was the voltage of electromechanical (stepper, and later crossbar) switches, which basically used relays. Electronic gear was then designed to run on the same power plant. A telephone building has a big bank of batteries, powered by multiple "rectifiers" (DC supplies) which, btw, are normally engineered to not run over 40% of load. (That way they can still run the systems and recharge the batteries when one of them is kaput.)
If you then put anything else into one of their buildings, the Network Equipment Building Standards (NEBS), which are Telcordia documents that practically carry the force of law, dictate that equipment be DC powered. Among other things -- NEBS gear has to meet the brick schytthaus test. (Sun Netras and many Cisco routers meet NEBS. Your basic rack server doesn't. And aluminum racks are STRICTLY forbidden; it has to be steel.)
So because of the talk voltage on analog phones, lots of computing equipment is engineered for -48 V DC power. Sort of like the legend (I know, that one is not really true) about the railroad track gauge being based on Roman chariots. But in this case it's surprisingly effective.
The reason you want to use DC is that a computer's power supply converts AC into DC. The power supply of most computers isn't that efficient at it. This basically converts some of your electricity into heat. (Heat in a 1U server in a big rack of 1 U is really bad.) In theory the data center's big AC to DC converter is more efficient and better cooled. Thus you save money in power bills, air conditioning, and rack space (less heat, and power draw means more servers per rack). Plus in theory your servers should last longer as the power supply is one of the more likely points of failure.
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Wrap the casket in copper, replace the headstone with a magnet, and expose corpse to this article. As Tesla turns in grave, free power.
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"Swinging both ways" effectively gives you twice the power by increasing by a factor of 2 your opportunities for coupling. :-o
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Rackable declined to comment for this story because it is in a stock exchange quiet period, after announcing a plan to sell 3.3 million shares in a secondary offering to raise new financing.
So then... Slashdot exposure during the quiet period is all OK then right? Shankland watches this particular market segment pretty closely. I wonder if he has any options? *cough* *just a theory* *cough*
Have you Meta Moderated t
IBM Has had DC Power in their mainframes for years. The latest ones convert redundant 3-phase power to 480v DC for distribution between the frames. All the details you want can be found here
That's true, but DC doesn't travel well, but I'm not sure on the figures. Either way, it loses more than AC, hence why wall power is AC, but I can't see there being much loss from one end of a data center to the other, the losses come from miles of cabling. So.. bring on the DC!
DC is way better than AC for very long transmission lines. For lines longer than 1000 miles, AC results in very unfavorable reactances and even radiation resistance. So, if you want to build a power line across the top of Siberia, you should use DC.
As for distribution within an equipment rack, you're going to use on-board regulation anyway. Thus there's no particular advantage for AC. Switching regulators are now easy and cheap so you can get all the different DC voltages from a single source.
Anyone how has any understand of electricity knows it is much easier and more efficient to transfer large amounts of electricity over large wires with huge potential volt differences on AC than to use DC current. Much less line loss and greater distances. Whether DC is better in a specific application is an entirely different consideration than if Tesla or Edison was more right about our power grids. If Edison won we would have generators every two blocks...
Tesla promoted DC. Edison (backed by Westinghouse) pushed for AC.
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The future, as I (and a lot of others) envision it, will be essentially DC-based. Some day they will standardize battery form factor, and I'm talking about things like our curent ion/Lith batteries here, and you will basically only have that for each of your appliances. Shapes will range from watch battery to big "cellpacks". This will be the so needed end of wires. We will have only one outlet to recharge them all in the whole house, or even dispose of them if they last a very long time.
Of course, this will require a revolution in batteries technology, but I'm sure deep down in the googleplex some mad PhDist with an ugly assistant is already working on it.
So, yes Edison was right, but he may have been slightly optimistic about the pace of change.
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Of course, this just vindicates Tesla. Shipping low-voltage DC around a facility, whether ancient telephone switching center or modern server farm, is a lossy affair without big, fat wires to keep the voltage drops down (losses rise as the SQUARE of current; P = I^2 R). Centralizing the AC-DC conversion a few yards away where heat can be handled better is sensible (just making power supplies which can be cooled with ambient air would save a lot on A/C), but moving it even a fraction of a mile makes no sense without superconductors.
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A 200 watt DC server uses the same power as an AC. The argument is how many conversions must one go through before the power is used.
In an DC plant, you will have a AC to DC power supply deliverying power to a bunch of batteries. The batteries will deliver (with the supply) typically -48 VDC to the servers. The servers then need to convert this 48 volts to DC that it wants (ie. +/- 12 and +5v). There are several conversions steps here. The AC to DC of the rectifiers; possibly the electo-chemical conversion from DC to battery and then battery to DC; and the DC to DC conversion in the server. Non of these steps are 100 % efficient.
In an AC plant you may or may not have a UPS. I will include the UPS in this flow...
AC to the UPS that gets converted to DC (battery electro-chemical conversion may be involved) and then conversion from DC to AC to be delivered to the server. The server will convert AC to the various DC voltages needed.
Again, many conversions and all not 100 percent efficient.
At our company (UnitedLayer) we have found that is is about break even.
Tim
48Volt DC is the standard in the telco space. I once was going to colocate a bunch of stuff in a CO and when I told the folks there it was AC they said "well, we might have an inverter somewhere around here" It was all DC.
Blizzard's World of Warcraft is all run on HP blade server using a DC power source. They have thousands of blands all connected into these really cool looking big DC distribution hubs.
What Rackable is really pushing is a system where AC to 48VDC conversion takes place in a unit at the top of the rack, and 48VDC is local to the rack. That, at least, simplifies the cable management.
One big advantage of 120/240VAC power distribution using US standards is that the connectors are standardized and reasonably idiot-proof. That is, if you can plug it in, you won't overload the power cord or the connector, and if you overload the branch circuit, a breaker will trip. Outlet strips have circuit breakers, so you can't overload the cord to the outlet strip without a breaker trip. There are NEMA standard power plugs for 15A, 20A, and 30A circuits, 120/240VAC, and single and three phase configurations. All this is standardized nationally and enforced by the National Electrical Code.
In contrast, there are no simple standards for 48VDC. Most 48VDC gear has big screw terminals. There are no standard plugs and sockets. Somebody, preferably a licensed electrician, has to check all the data plates, add up the current loads, calculate voltage drops, size the wire and breakers, and torque the big screw terminals to the correct torque, using the correct lockwashers. Every time you add or change a load, somebody has to recheck the math. Errors can cause a fire. None of this is all that hard if you have basic power technician skills, but you can't just go casually plugging stuff in.
Although, since the development of the low-cost clamp-around DC ammeter, things have become easier in the DC world.
I'm surprised no one has mentioned it yet, but the big advantage comes from removing *two* convertion steps when you use a UPS. Normally a power supply takes AC current and converts it to DC. A UPS does the same thing: takes AC power and converts it to DC to charge the battery. But then it takes the DC current out of the battery and converts it back to AC so that you can plug the power supply into it! So effectively we get AC -> DC -> AC -> DC. Obviously this is stupid since you waste some energy with each convertion. When you have whole racks of computers and network equipment all plugged into a UPS, it would be much more efficient if they could accept DC current coming out of UPS. Then they wouldn't even need power supplies since they wouldn't need to convert current. *That* is why DC equipment is more efficient. Of course the article didn't explain that.
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In the socially superior world of cisco, most routers have a plug in the back for a direct DC connection for people with dc power running in their racks. It seems the significance of this is that people will be getting DC power going straight into the building rather than AC. People here seem to be thinking that they are just getting rack mounted AC to DC power supplies, and running DC power to each device.
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The reason AC won is transmission. Transmitting DC over long distances requires different feats of engineering. That's not to say someone can't figure out a clever way to overcome the limitations today, but it was just an expensive fire hazard back then.
...is more likely if you get electrocuted from a DC source than an AC source. Hyperbole aside, DC really is more dangerous than AC.
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http://www.nanotech-now.com/news.cgi?story_id=1373 8
Contrast this with a properly designed DC system a la old-school telco: The same front-end of the UPS is used, with a 10% loss converting AC to battery voltage. Then you run that into DC supplies that, with modern electronics, are going to be doing a lot better than an AC supply, so let's say 5% loss. That puts you at better than 85% efficiency.
The critics cited in the article are actually probably not far off in calling the Rackable solution over-hyped, if you only take into account the isolated-rack design. Rackable puts 2-3U of beefy redundant supplies at the top of the rack and does DC to the servers. Efficiency-wise this is only fractionally better than a bazillion AC supplies, and quite possibly dead even because of the DC->DC losses in each server on top of the AC->DC->AC->DC setup implicit with AC-based UPS systems. However, AFAICT from a glance at their site, Rackable's systems are designed to drop right into existing DC datacenters, which eliminates the AC supplies at the top and the DC->AC->DC stages.
The issue is what kind of infrastructure is needed to feed the selected DC voltage (which is going to be -48VDC) into the racks with the lowest bus losses, but this is someone I would expect is either a) already solved by the decades-old telco industry, or b) going to be solved in at the appropriate 384-cores-and-100TB-per-7ft-rack scale RSN, by "the market".
I know that if I were in the position of designing a big datacenter right now, I would be looking very hard at DC systems.
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Here's a page including schematics of boost and buck converters, and one with a schematic for a glow-plug converter.
Time is Nature's way of keeping everything from happening at once... the bitch.
Rectifiers take up that much space and cause that much heat?
AC power has a single advantage over DC power: you can easily step up or step down an AC supply to a higher or lower voltage. When you have massive lengths of cable to get the power from point A (eg: Latrobe Valley) to point B (eg: Melbourne -- a distance of some 130 km), the difference in loss from resistance becomes very significant.
This means we're not going to see a move from AC transmission to DC transmission (in the general case). However, to have the conversion from AC to DC in a single place could well be a significant advantage. To do it right, though, you need a standard, with pinouts for +5V, -5V, +3.3V, -3.3V, +12V, -12V, +48V, -48V, etc. You'd have one set of rails delivering one voltage; another set of rails delivering another; etc., etc. A server would plug into whichever sets of rails it needed for its voltages.
Then you have the problem of redundancy, so you need two sets for each voltage. After all, if the 12V line blows out, your servers are going to be dead in the water, even if they still have 48V or 5V. This gets messy, very quickly. DC-DC converters? If you're going down that path, you might as well have a good quality AC power supply in the server.
And what about the size of the cables required to deliver all that power? The copper bars for delivering good quality DC at high currents, as would be necessary for a data center, get pretty damn thick, pretty damn fast.
Then there's the issue of switching. DC power is notorious for being difficult to switch. With DC, as you draw the connections apart, you'll get an arc, especially with high power connections. The higher the power in the DC line, the longer the arc can be, and hence the greater the distance the connections need to travel to stop the flow. With AC, because the current momentarily stops twice in every cycle, you don't have that problem; the arc disappears of its own accord. (I'm dumbing all of this down considerably, by the way.) Not an easy task to solve. It could be an issue for individual servers, if they need enough power (maybe a top-end E15k?); it would definitely be an issue for the point just after transformation (to be fair, the obvious answer is to pull the AC power before the transformation, but there could be good reasons why that may not be a good thing to do.)
The next question that needs to be asked: who is suggesting going down this path? Oh, look, it's a vendor of DC power supplies. Gee, might there be just a little bit of vested interest here?
Nice idea. Not really workable in practice, IMO. And for the record: I did two years of electrical engineering before dropping out to finish a CS degree instead.
Gone are the days I guess where on every block was a buzz-cut 10-year-old who, while soldering his Heathkit, could easily master such new space-age concepts as Ohm's law.
Now for those of us who might relate better to power in terms of Makeup-mirror-equivalent (MME) units, who don't want to be bothered with such chores as multiplying by powers of 10:
While old tom did all he could to derail AC power he could not overcome the main short coming of DC power, line loss. If Edison had his way we would have to have power station every few blocks. When Tesla invented the polyphase generator it was possible to step up the generated power, transmit it long distances and step it down for home use.
What ended the DC-vs-AC "war" was not some house wiring but long-range transmission. Simply put, there are no DC transformers (now there are, but they are less efficient and much more complex). That 48V DC power comes into the house through some high-voltage lines. If it was 48V DC all the way from power plant, it would all dissipate in the transmission lines.
Wiring house or data center to DC is really a local issue which has nothing to do with what Edison was fighting Tesla for. My low-voltage garden lights are DC, my model railroad is DC, the data center is the same thing on somewhat larger scale. Think "low-voltage lights in a large park". The scale is about the same, the power consumption is similar, but fewer buzz words so less shock factor.
The origin of the 48 volt number is that it was convenient, and now it just sneaks under the 50-volt "low voltage" cutoff in the NEC, which I think was written with telcos in mind. The glorious thing about this is that you don't need licensed electricians to do power wiring in a central office.
And the reason it's negative with respect to ground goes all the way back to the telegraph system: Western Union initially ran bipolar lines and noticed that the positive ones corroded much faster. Sodium ions (from dissolved salt) are negative, and thus repelled from lines that're also negative. The whole phone system was built with positive ground because of this, and it's saved incalculable maintenance costs. It does tend to mess with people's heads the first time, if they're used to negative ground systems, but you get over it quickly. (A number of traditions use blue for "hot" and black for ground/return, to help escape your "red equals positive" association.)
DC power as used by telcos is also always redundant. There's an A-side and a B-side for everything, and the cables are sized so that the entire load can run from just one side. This leads to some very fat copper, which is cheap compared to downtime. You don't achieve five-nines reliability with a system that contains single points of failure!
Now, about rack-mounting: This was also invented by the telcos, originally in a very wide (40-inch?) format, for the panelboards and Strowger switches. Some of the old crossbar equipment is still in those huge racks, but the 23-inch width is infinitely more common now. All telco equipment is mid-mounted, with the ears approximately in the center of gravity on the shelf, so the force on the screws is shear. There's no torsion on the mounting flange unless you step on the front or back of the shelf. Cooling is always convective bottom-to-top, or occasionally front-to-back with fans. This leads to a "cool" front aisle and a "warm" back aisle between alternating rows of equipment.
Now, the pro audio industry borrowed the rackmount idea fairly early on, but they were mostly mounting control panels and mixers, which are very shallow, so flush-mounting made sense. They also changed the every-inch Western Electric mounting holes to an alternating-spaces "EIA" standard, and narrowed the rack from 23 to 19 inches.
Somewhere along the line, an absolute idiot decided that computers should be rackmounted, but they should be 19 inches wide, flush-mounted, and use EIA hole patterns. I'm sure this has something to do with mainframe legacy getting perverted by peecee people. The current mishmosh of mounting standards (19" vs 23", two-post versus four-post, flush versus mid, inch versus RU, front-cable versus rear-cable) is what every datacenter tech deals with on a daily basis. Throw overhead racks versus raised-floor cabling into the mix, and you've got a recipe for frustration!
If you're familiar with the concept of "blade servers", where common components are separate from processor resources in the shelf, congratulations. Telco hardware has been built like this since the invention of the circuit board. Actually, the concept of replacable plug-in units goes back before that, but it got vastly easier with printed wiring boards and card-edge connectors in the sixties. Most of the "good ideas" in serious computing circles are actually century-old ideas in the telco industry. Spend a week shadowing a central office tech before you design a datacenter, please!
Also consider: If your datacenter is already built for DC, throw some solar photovoltaic panels on the roof. Inverters are a large part of most PV systems' expense, and you can skip that part. Why not start offsetting your grid demand now?
Also also: Edison was flat-out wrong about DC. The modern switching power supplies that make DC transmission lines practical didn't exist in his day. Besides, long-distance power transmission is an entirely other discussion.
What the hell? Why even post?
japan is on DC power, or at least their power is transfered over lines in DC.
If we can power more things from DC supplies, then it will be easier to add photovoltaics to our energy supplies. If I could run my house off solar DC it would be more efficient and cheaper than having things go through an inverter just to be transformed back down into some other DC voltage.
To make downconverting easier (DC-DC voltage upconverting equipment is expensive) the common supply will have to be as high voltage as any draw from it. The article talks about 48v distribution, but I was hoping for 24v or 12v since those are more common to get from solar panel systems.
Start Running Better Polls
The article conflates several things.
First off: Digital electronics generally requires several voltages. And they're all low, requiring high currents, massive conductors, and local filtering and regulation. So even if you're providing DC power from outside the room, you'll have a switching power supply (or several) in each piece of equipment to convert whatever the rough DC power is to whatever you need, smooth it, and regulate it.
But while some electronic devices use a common switcher to generate all the voltages with one conversion step, others use a "roughing" supply and a bunch of local supplies. Part of that is to get better regulation - part is because the roughing supply must run from 60 (or 50 or whatever) Hz and thus requires big caps to tide you over the low part of the cycles - caps you don't want taking up space near the components.
If you're going to do it in two stages anyhow, you can put your roughing supply OUTSIDE the room and only have the final supplies inside. The roughing supply has a lot of heat dissipation so you save a bunch on your cooling.
Second: There are two standards for power distribution in electronics rooms:
- Your local power line stuff. (120/240/480/208-3-phase in the US)
- The telco standard: x2-redunant 48V DC.
A lot of equipment - especially networking equipment - is manufactured for sale to tellcos and other operations that use the standard. They might have initially used it because some of their equipment was co-located in tellco sites, where only 2x48VDC is available - and they got a quantity discount for buying a bunch of the same stuff and went to 48V for their own sites. Or they might use it because it's MUCH simpler to do backup power with floating batteries and century-old technology than with a building-sized UPS. (Note that a UPS CAUSES at least one outage when first installed and on the averate at least one more within the first year of operation from some malfunction. And a UPS dissipates more power than a roughing power supply or a battery charger.)
But the standard for 48VDC is REDUNDANT 48VDC supplies, with the equipment only requiring one (and typically doing "cutover" with diodes B-) ). With the equipment already set up for redundant supplies it's not a lot of cost or work to wire both sides and put in two 48V feeds to the equipment room. (Four diodes are a LOT cheaper than a pair of 120V roughing power supplies at each box, too.) So of course the users of such equipment normally give it dual supplies. (Even if it's a single rack and so they just put two roughing supplies in the rack fed from two different 120V feeds.)
The result is that all the equipment has redundant power supply, and keeps operating glitch-free through a number of kinds of partial outages - AND power supply repair and replacement. This is what's responsible for much of the claimed increase in reliability.
The whole Edison/Tesla DC/AC war had to do with the economics of CROSS-COUNTRY power transmission. AC beat DC there because a century or more ago it was virtually impossible to jack DC voltages up to levels suitable for long-distance transmission and back down to levels safe for distribution within houses, while AC could do that easily and efficiently. So Westinghouse/Tesla could ship cheap power from Niagra Falls to New York City while Edison had to build fuel-burning power plants IN the city. It has essentially nothing to do with shipping the power around within a single building.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
http://it.slashdot.org/article.pl?sid=05/11/11/141 0221
Tesla originally worked for Edison, but they had a bit of a falling out, which is possibly why the AC/DC competition was so heated. Edison embarked on a pretty ruthless and gruesome campaign to discredit AC power, at least by modern standards. He electrocuted stray dogs and cats with AC current in public demonstrations intended to show how dangerous AC power was.
In one instance, he even electrocuted an elephant...
During the construction of Luna Park on Coney island, an elephant used as a beast of burden went out of control and killed a couple of people. Topsy, as she was called, was condemned to death. However, there was a wee bit of a problem. Elephants aren't the easiest critters to kill. What happens if you walk up and fire a shotgun at it's head, only just to piss if off? They do have rather thick hides, and we are talking about a homicidal elephant the size of a couple SUV's here. There weren't any cliffs handy to stampede poor Topsy off of, and I doubt dynamite was ever seriously considered. Edison, being the generous person he was, gladly volunteered to execute the elephant with AC current and filmed the whole thing. He showed the resulting film, "Electrocuting an Elephant" (1903) publically on many occasions. It is quite probable that many a cat and dog escaped a crispy fate thanks to this film. If you decide to track down a copy of "Electrocuting an Elephant" today, please be warned that it's a rather gruesome little piece of history, and is not for the faint of heart, or SPCA members.
in my server room and have wondered if there wasn't a simple way to hack together a few big ones and let them work together to power all the machines. Then I could focus on cooling them in one place and deal with the other hardware on its own.
It never looked economical to do.
The problem with quotes on the internet, is that nobody bothers to check their veracity. -- Abraham Lincoln
I work for a telecom network analysis company, and all are products can run on standard 120V AC or 48V DC (you have inputs for +48 and -48) depending on the power supply. I haven't tried it yet, but I'm pretty sure you can run an AC one and a DC one at the same time (redundant power supplies). From what I understand, a lot of the telecom companies (especially US ones) require DC in there labs (and have for awhile now). Also, talking to a couple of guys who have setup equipment, DC is much more painful to get shocked by than AC.
I am d3matt
Edison and Tesla were both right. Remember, the DC vs. AC wars were fought back when the load was mostly made up of lights, motors, very utilitarian things. AC is fantastic for transmission over long distances (and for running three phase motors, but that's another story). DC happens to be better at running precision equipment like computers -- heck, they all run on DC already. All we're really talking about here is taking advantage of an economy of scale by doing one big power supply (or a few, for redundancy) instead of one for each machine.
Ever seen a telco rack? Everything runs on -48VDC. Everything. A telco rack always includes a couple of DC power supplies, and all the equipment just ties in to a common DC bus. The best part of all: the UPS simply consists of four "car batteries" (not exactly, but you get the idea) wired in series and tied directly into the bus! No pesky inverters to deal with.
The telecom industry has been doing it this way for decades. It's about time the computer industry got on board.
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Is this a new Slashdot cliche in the works? Will it be added to soviet russia, elderly koreans and the like?
In Soviet Korea, elderly dead people spin YOU!
It seems to me that the only disadvantage with DC has to do with interconnecting it with the existing AC grid. From this wikipedia entry and from reading the book Infrastucture by Brian Haynes, which states "some of the longest, highest capacity power tranmissions lines carry direct current", I get that it is very efficient. It even makes our AC power more stable by linking electricity producing AC grids that aren't in sync. Other advantages are: resistive losses are lower for a given conductor size, and only two wires are need instead of three, thus reducing the materials needed (such as less wire, fewer insulators, and smaller towers) on long runs.
So why do you say that it isn't any good for long distance power distribution?
I'll take a swag at it, because the code laws change radically at 50 VDC?
You left out the part that mentions WHY only Cisco, Sun Netras and very little else is NEBS compliant: it costs $$$$$$ to get certified, there are few people that actually buy/use it and those that do generally already use Sun and Cisco gear (or other Telecom based stuff like Adtran, etc), and designing stuff to meet NEBS requires alot of extra homework. This all adds up to a very large price difference between your generic AC powered server and a DC powered one, cause if its DC powered, its probably NEBS (at least I havent seen any non-NEBS DC systems). If they were to come out with a DC system that cut out the NEBS tax, the whole DC movement would catch on alot faster.
Tm
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Other than the standard stuff we alredy have in AC, what if you accidentally put your fingers in the hot wire? AFAIK with AC current you are able to pull out, but with DC you can't move your muscles!
Telecom rooms are FULL of DC-only equipment. Telephone switches, MUX's, etc., have been running on 48V DC power for decades. It's about time this idea has been brought to the IT space, since more and more of this equipment sits right next to the DC equipment in telecom companies' switch rooms.
Just make motherboards, chips, etc work with A/C, problem solved.
I dont care what voltage type they use in the enterprise. We seriously need 5V DC in the house. I have way more electronics that need 12V,5V and 3.3V than ones that need 110V. So everything needs a walwart that dies every year.
"Give orange me give eat orange me eat orange give me eat orange give me you." -Nim Chimpsky
But, Edison **was** wrong because the context was how to do reticulation using the technology of the time, not powering server rooms or moving power across continents or using switchmode power supplies
Engineering is the art of compromise.
In the data centers I've seen, The only things you could install were Sun servers with DC power supplies. I was shocekd, because I'd never seen one...
That was about 5 years ago. So I think this has been a thing for while now.
dave
I work for a small IOC telco. Our 10 or so servers and our Ethernet switches are all AC powered. Two of our three routers are DC though.
Seems like every time I've looked at DC powered servers and switches they are almost twice the cost of the AC versions. You'd think they'd be cheaper or almost the same.
Anybody know why this is the case?
I hate the worship of Edison. He simply hired hordes of scientists and engineers, had them do the work, then took all the credit. I don't know if the story you tell is true, but I certainly can belive it.
No, I don't trust in god. He'll have to pay up front, like everybody else.
Back when I worked for Sun Microsystems in 1999 we got a tour around one of the data centers in Palo Alto were shown a transformator side-room which provided DC power to plugs around the floor of the huge data-center.
Is all the extension cords I'd have to run up I-95 and the Turnpike.
-- Of course I'm paranoid. I'm a sysadmin.
Looking behind my desktop computer, I see: two power strips, with wall warts for router, network switch, USB hub, printer server, printer, external drive, speakers, and phone. Couldn't all that be in one box?
NEVER!
If you decide to track down a copy of "Electrocuting an Elephant" today...
karma whoring... (link to video in External Links section)
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That was pretty good for 1970, but not today! A 300W PC power supply at 63% efficiency has about 475W input, which means 175W loss. 175W would turn it into a crispy critter--more heat than a high end CPU, with less effective heat removal.
Assuming 70-80% efficiency for today's off-the-shelf power supplies and perhaps 15% potwntial power savings would be realistic, but that streches out the financial pay-back time. Not that it would make a big difference: most equipment buyers don't care how much power is wasted, as long as the servers keep running. Power conservation matters only when the circuit breakers trip or the system shuts down from overheating. The utility bills get paid from another department's budget.
The true benefit is moving some heat generation out to the DC power source, away from the servers. That improves the server room cooling budget, so you can squeeze in a few more racks before paying for a major facility upgrade. But this is a one-time shot. The next stage must be reductions of load power consumption. There isn't enough loss in a 90% efficient power supply to cover the next round of upgrades.
The Tesla vs. Edison dispute was over transmission of power. AC won because the transformer and 3 phase power made it more economical over large distances. It still is.
However, over small distances, DC has lots of advantages. Telephone Central Offices have historically used -48 volt DC power and modern telephone switching systems have the same kinds of power requirements as servers. Why not power servers the same way as a Central Office Switch? Battery chargers constantly float charge large banks of station batteries with the 48 volt DC power distributed to the individual racs via bus bars. Highly efficient DC-DC converters at the point of use step down the voltage. If the system is designed cleverly, isolation could be eliminated, simplifying the down converters and boosting their efficiency. The whole system is a great big UPS with lots of hold-up time.
Wansu, th' chinese sailor
For now, I would be very happy with a super power supply that I could use to replace the huge number of wall warts I have. One cord to AC, and a bunch of connectors and cables to replace the original warts.
For example, for my network connection, I have a DSL modem, and switch to break out the ip's, a couple of routers, all plugged in to a UPS. I don't need any AC here, everything connected to the UPS is a wart that converts it back to DC. Everything would cooler, faster, and longer on backup if the DC was distributed directly.
Is that quite so? Wouldn't there be taps on the transformer for approximately 12V and approximately 5V, and then the potentials finely adjusted using DC-DC regulators? Wouldn't that have less loss?
Taking this a bit further, why not have an entire rack power supply that can deliver a rail of 3.3V, 5V, and 12V to each server, thus eliminating the need for a high-current DC-DC converter on the target board? I am excluding things like the exotic voltages for CPU and RAM, but still it is the 12V and 5V rails that would have to be able to source significantly more current.
"Everyone knows the alternating vs. direct current wars ended with Thomas Edison and Nikola Tesla."
Yeah, hell everyone knows that...
Erm, I didn't... and I studied Electrical Engineering in University...
People who use the term 'Everyone knows...' really annoy me no end, as there are very, very few cases where it can be true... if any.
RIIA is investigating how they could profit from having AC/DC playing in all the data centers across the nation
It seems that we should be able to have a static charge collector, to collect atmosphere/cloud energy, even on a clear day.
Is there some reason why this technology stopped with sending a key up a kite string, 200 years ago?
Telecoms used to use DC equipment for cellular equipment. They want clean power to make sure all the RF stuff doesn't have issue with power supply noise. You just end up with a very real electrocution risk instead. At the datacenter/switch office I worked at there are still some old equipment that needs DC power (hint hint: that new stuff all runs on AC). Where as your typical AC power cord is relatively thin the DC power supplies has huge metal bars. Low voltage but high power demands means that was the only way. The result is everything has to be shielded inside a plastic case. Anyhow it cost much less to just run everything off AC. You might think that using DC means you can make the UPS system more efficient and simple. However, serious UPS systems uses batteries only for the gap between power failing and generators coming online and the generators outputs AC.
I, for one, prefer Marvel Powered Super Heros..
ok, ok, bad joke.. -1 off topic...
"People who use the term 'Everyone knows...' really annoy me no end"
Im sorry I didnt know that.
PSU init in PC should work on DC as it is.
Mine works on 300V DC (Europe, mains voltage 230V DC) without a problem.
Even more, if one has a bit newer unit with cos phi correction, one can take correction subunit out, since it's not needed for DC and get better efficiency. Only snag is that at least my PSU unit wants 400V DC after power correction unit amputation, since this is what it had been output at that point by old circuitry...
I say why not? Make an efficient 12V regulating system, and have it built for your necessary processor and 5V for other utilities and such, this would run just fine, I'd think, since it's all really stepped down. Hell, I say go one step further and build your own power source out of good deep-cycle 6V series-paired parallel-connected Trojan T-105 batteries (to make a high amp-hour 12V system) and charge from solar. Given the newer processors that use far less power, I'd tend to think this is a reasonable way to go. You cut out the power company altogethr on that part by maintaining your own power system for a separate operation (plus you can custom-build your own power redundancy to your requirements, and it's really not THAT hard to design) and save money in the long run!
Still waiting on Serviscope_minor to wake up to fucking reality and realize that Jessica Price isn't going to fuck him.
On se Internetz nobody noes your German.
So, instead of using 3 phase 208, or 120 wiring, you want to lower the voltage, which necessitates increasing the current, and thus the required cross-section of wiring. The article mentions the 12 volt internal power supply on PCs... if we use that for a baseline, at best you're looking at a 10:1 ratio (120V/12V) for heavier power bus. If a rack takes 25kw, that's about 2100 AMPS, which is going to require some major buss bar to keep I2R losses low.
It's AC out of my wall, 100V.
....
50 Hz in the north half of the country, 60 Hz down here in the south half. Used to have to check your appliances to make sure they took the right power. A fan my wife bought back in her college days has a timer with two scales printed on it because of that.
These days I think pretty much everything converts to DC, so frequency at the tap doesn't matter. At least our fridge and washer both now say 50/60 Hz.
But AC. Definitely AC out of the wall.
Now, as to whether the high tension transmission might be DC, I'd have to ask my wife's little brother. I suspect he'd know, since he's a licensed electrician. (Used to tell me I should switch from computers, and I have more than once seriously considered it.)
I'll have to pay more attention next time. I know I've seen some huge open-air capaciters at local power conversion stations.
I'll say one thing, I'd welcome something to get rid of all the octopus taps with clots of transformers and wire knots around our house. I've often thought, with all the 9V and 24V converters in the mix, it'd be nice to have 9V and 24V coming out of the wall for the phone, the iBook, the Mac Mini, the various hubs (which, starting late this year could start being replaced with Motorola's UWB wireless, if iNTEL would just give up their NIH piggishness on pseudo-UWB that's less than half the efficiency, less than half the potential speed, and noisy to boot), the stereo,
Telcos use DC power. At Sun, we make DC versions of some of our servers just for them.
Im suprised you did not hear about the Edison/Tesla battles (both scientific and economic) during an Electric Eng. degree.. I think most geeks who love HV know abit about Tesla :-)
Yep, the power is in DC. Washington, DC, lobbying capital of the world. Next question, please.
Money for nothing, pix for free
It wasn't Declan who wrote the story: it's far too technical for him. Of course, getting the facts straight on something even as simple-minded as politics is a stretch for the guy.
The same technology is used in many other places now - for a number of reasons not necessarily to do with current carrying (for instance - ability of a DC line to sync with otherwise out of phase transmissions that go via different distances of wires to the same place).
I've worked inside telco facilities where the bus bars were 2" x 4" and even larger - all at 48 volts. (back in the late '60s - fun swinging from the bus bars with DRY hands ;)
The reason that AC won out in the past is that the technology to "fix" voltage drop due to transmission line losses was easy - a transformer - loops of wire around a soft-iron core in various types and flavors of windings (step-up, step-down, saturated-core constant voltage, etc.) They didn't require the types of technology we now have, high efficiency diodes, transistors, thyristors, etc. so we could do high efficiency (relativly) DC-DC voltage changes (actually they are DC-AC-AC-DC as in DC-> high-freq -> transformer -> rectifier -> filter -> DC)
By the same reasoning (lack of other technology) the telcos used 48 volts because it was easier to float the whole load across a bank of batteries and continuously charge them than it was to switch in the batteries if/when the main power failed. Today's UPS circuitry that can detect a drop within a single cycle of the 60Hz main and switch in the inverter from the otherwise minimally charging battery simply didn't exist.
The bottom line is that if the "extra" 33% current carrying capacity of a given wire size at a given voltage gives a cost/benefit, then DC makes sense - or, if the distribution system has some other necessary characteristic (float the whole thing across a bank of batteries so we don't have to switch them in if the power goes down for example) then DC distribution might win.
On the other hand, if you're in a facility where you don't control everyone and their addition of equipment to the pool, just one screw-up with power of the wrong type being applied to the wrong equipment at the wrong time will make the use of DC a giant pain in the ass.
The telecom industry has grown up with 48volts - their techs use it and understand it and their engineers know how to spec the wiring for it. If you've never wired a bank of Strouger switches using 000 wire (and a hammer to bend it into the right form) then I suggest you stick with "traditional" (this century) AC wiring.
Power in minus power lost in translation (from the input to the required voltages) is what you have to work with - but total power in including the losses in conversion are what you have to get rid of in the form of heat. The efficiency of the conversions to the necessary voltages determine what actual "work" you get from the whole setup. If doing a single major conversion to what is the major voltage required (12 volts? Maybe it should be 5?) and then dealing with the (in)efficiencies of the DC-DC conversion to the other voltages necessary might in some cases increase the total efficiency enough to justify the extra cost for the heavy current carrying wires at installation - and maybe even the cost of replacing the idiot's mis-plugged machine every couple of years
Or maybe not. It depends.
Been there, done that, paid for the T-shirt
and didn't get it
The equipment in telecommunications facilities and labs has run on 48 VDC for a long time, now. This is nothing new; data centers just now appear to have reached the critical mass where they finally realize the inherent inefficiency of having hundreds of tiny AC-DC power supplies for each box. In telecom, the DC supply paradigm also facilitates the centralization of highly efficient chemical battery backup arrays. The telecom lab where I work has a lot of very heavy, very thick copper bus plates not only in the battery room, but also at the distribution/breaker boxes located throughout the building to feed the equipment.
If you think about it, this shouldn't be surprising. The standards of uptime for telecommunication facilities have always been the highest, even higher than any data center. DC distribution and supply is the only way to go.
Welcome to the big leagues, boys.
With both AC and DC distribution, there are losses due to the resistance of the wire (I-squared-R losses). The way to minimize these losses is to increase the voltage (V) and decrease the current (I) while transmitting the same power, but there is a limit to how high the voltage can be increased. Air breaks down at about 3x10^6 V/m. To avoid this dialectic break-down, you continue to raise the height of the power line as you increase the voltage.
With AC distribution lines, there are also losses related to the capacitance between the power line and the ground. Increasing the height of the power also minimizes the capacitive losses.
With both AC and DC there are reflections between the source and load which cause further trips from one end to the other. Each reflection is smaller than the previous one, but remember how many people are using electricity and the fact that everyone is constantly adding and removing load from the system. So, even in a DC system, the line voltage will be constantly changing.
Then, we have the conversions. Conversions from one AC voltage to another AC voltage is accomplished with a step-up or step-down transformer. This converstion isn't free, and it doesn't work for DC. It is very efficient and economical however, to convert from a higher DC voltage down to a lower one -- even for moderately high currents. it is very painful however to step a lower DC voltage up to a higher one. There are circuits to do it, but typically (or at least through 1990), it has been easier to convert to AC, go through a step-up transformer, and and then convert to DC. Also, the circuits for up-converting DC to DC are usually fixed at multiples of 2x, 3x, 4x, etc. using diodes.
So, let's put it all together. I can believe there are long-distance DC transmission lines where the savings in capacitive losses are worth the significant capital investment required at both ends of the line for the conversions, conditioning, and to match the source to the line and the line to the load, but in general, in a DC distribution scheme, the DC voltage drops continuously along the line and must be periodically stepped-up by some hard-to-determine amount because it depends on the age of the wire, the distance from the last step-up, and the demands of the load at that moment in time, but the circuits for doing it are inflexible (can only do multiples).
With AC, you get the flexibility that each sub-station is monitoring its own load and it can control the variable-step-down transformers to achieve the desired neighborhood voltages. Ready to increase the height of the lines? Step-up. Ready to drop the height of the lines? Step-down.
In a data center, as several people have said, everything is in one place, so the problem is different. You want to pick a high enough DC voltage so that it is always higher (even at maximum load across the entire room) than any voltage you might need. Then, you use the cheap and economical DC-to-DC conversions _at the point of use_ to take that down to the +5V and +12V that your equipment needs. You may pay marginally extra for larger cabling to handle higher currents, but you save money by not needing step-down transformers in each power supply. Let weight, more compact, and more efficient.
I wondeer if they know what happens when you suddenly switch all things off ! Inductance of that copper wire isn't so small and consumption very high too on such servers. large capacitors on the end of that DC line and something like zener diodes should probably do the trick when switching all off. Very simple solution is creating circles of racks around same power supply. :=) but that would look just too futuristic :))) I think this thing could work for servers very close to each other but if you have longer distance DC/DC converters are not so eficient. That idea with 48 volts would be litle bit more eficient only when standard transformer and rectifier would be used and then DC/DC converters for a circle of racks.
smash.
I run: Windows, OS X, Linux, FreeBSD. Just because you have a hammer, doesn't mean everything is a nail.
A wacking great DC supply to power a server room is a good idea as one big converter has got to be more effecient than many smaller ones, also as previously mentioned it will reduce server room clutter and remove the need for inverters in the UPS as 6/12V goes very nicely into 48V.
Some people seem to be having an issue with the low voltage high loss in teh cabloes but this could be overcome with bigger cables (buzz bar?) and the fact that the power only has to travel maybe 200m tops.
In the not too distant future, next Sunday A.D.
I think a lot of people are missing the most basic of facts.
1) AC is the most common form that electrictity is generated in. Yes, there is such a thing as a DC generator, but it is less efficient than an AC one.
2) AC is a much better choice for transmitting over long distances. Yes, you could do it with DC, but it would require a lot more stations along the way to keep the voltage at a constant level.
3) AC, just like the name suggests, alternates. It requires a much smaller size of wire to deliver the same amount of power.
4) The majority of the electricity delivered is consumed by electric motors. AC motors are more efficient... especially three-phase motors.
for failing to mention hot grits or petrification.
+++ATH0
"Everyone knows the alternating vs. direct current wars ended with Thomas Edison and Nikola Tesla..."
Only in America.
In the rest of the world (where most of the research went on), AC was seen as the obvious way to carry power long distances. The first domestic use of inventions like electric lighting were not in the US.
The Tesla/Edison battle was a typical American legal battle of greed, patents and bribery between entrepeneurs, NOT scientists or engineers.
Most of the heat is generated when the capability of power supply meets the needs of your hardware. In other words when you have 350W PSU and your hw needs 350W - that a lot of heat in PSU.
It's easy to go beyond this with PSU's that are far beyond your needs (like 1500W), but who cares...
Other than this you already have DC in your PC (server, etc) and you won't get much with DC powerline.
www.worldbank.org/html/fpd/em/transmission/technol ogy_abb.pdf
Above is a link to a pretty good summary of HVDC around the world.
I guess you guys should go update Wikipedia and set them straight on the "fact" that DC is not suited to long distance transmission. Here's what Wikipedia has to say in the article entitled: Electric Power Transmission
High voltage DC (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is required to be transmitted over very long distances, it can be more economical to transmit using direct current instead of alternating current. For a long transmission line, the value of the smaller losses, and reduced construction cost of a DC line, can offset the additional cost of converter stations at each end of the line. Also, at high AC voltages significant amounts of energy are lost due to corona discharge, the capacitance between phases or, in the case of buried cables, between phases and the soil or water in which the cable is buried. Since the power flow through an HVDC link is directly controllable, HVDC links are sometimes used within a grid to stabilize the grid against control problems with the AC energy flow. One prominent example of such a transmission line is the Pacific Intertie located in the Western United States.
Go show 'em up guys.
I don't know that much about electronics, so please excuse me if this is a ridiculous idea, but I think it'd be cool if the mains in houses had both AC sockets and DC sockets. The AC-to-DC would happen at one spot and the outlets would be augmented with a new little socket for the DC. It'd be even cooler if that socket could be a USB socket with the power wired up, ready to use with all of those little USB-powered gadgets you can get to use with your laptop. I'm sure someone's going to tell me that it'd be hard to supply USB-compatible power over an entire house with the conversion in one spot, but it'd sure be nice if it were possible! :)
Can the submitter of that headline elaborate on precisely what Edison said about DC power?
Ever since I got my first 386 in the early 90s, all my computers have had switched-mode power supplies.
Are you sure you're not thinking of the IBM PC, instead of the more modern PC compatible?
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http://www.powerstream.com/Wire_Size.htm
Telco exchanges around the World are largely running on DC (last time I looked). From memory 50V DC provided by huge banks of 2V lead acid batteries, which are kept charged from an electricity supply from the street. This 50V DC being carried around the building through huge copper "bus bars" from memory which were about 1cm thick by maybe 15cm tall. We called them "buzz bars" because of the incredible power they could carry. All those batteries were capable of vaporizing a misplaced shifting spanner, no problem. I remember seeing warning photos of a guys hand, after he was careless with a shifting spanner. His hand looked like lightly cooked pork.
At any rate, with the amount of electrical gear in telco exchange buildings, they choose DC probably because it is efficient for the short power runs which they use within a building (no pun intended, short is bold because the DC voltage drop on these short runs does not pose a problem). However, since PC's use different voltages for different parts, +12, -12, +5, -5, +3.3, etc, DC to DC conversion is still required. Common computers as they currently exist (those used in server rooms, desktops and peoples homes) are not as suited to a single DC supply as telco equipment is and could only provide a very small efficiency gain. I don't see these sorts of computers being designed for this sort of supply, because they would be going back to specialist designs for those systems, which would be moving away from the economies of scale benefits which current server gear gains from using mainstream computer part designs.
War crimes, torture, lies, illegal spying... Would someone give Bush a blowjob, already, so he can be impeached?
The AC-versus-DC debate ended when the switched mode power supply was invented.
Switched-mode power supplies use an oscillator to convert DC to AC at a much higher frequency than the mains, allowing it to pass through a much smaller transformer {the longer the current spends flowing in any one direction, the heavier the steel core needs to be; at 50Hz one cycle takes 20ms giving 10ms in each direction, but at 50 kilohertz the current is spending only 10 microseconds flowing in each direction}. The AC coming out of the transformer is then rectified to convert it to DC, and negative feedback is applied to the oscillator side of the circuit to adjust the amount of power going through. Switched-mode supplies have to use very high quality rectifier diodes on the secondary side because they are rectifying high frequency currents.
Now, I said that a switched mode supply starts with DC. This DC is had by rectifying the mains with {hopefully} a bridge rectifier {a single diode half-wave rectifier would appear to work, till everybody on the same housing estate was using one at the same time and blew up the substation}. You can run a computer from DC. Notice how, to an AVO, the input terminals of the power supply {left and right} look like a non-polarised capacitor. This is not surprising, since downstream of the first fuse are four diodes in a bridge configuration and a huge capacitor wired so that it will always charge regardless of which pin is positive and which negative. Note that the capacitor will charge up to the peak voltage, which in the case of a sinewave is the RMS voltage * sqrt(2). That's about 325 volts in civilised countries. If you could find 200 disposable AA batteries {or 250 rechargeables}, and wire them in series so as to get 300V, you could plug this straight into your computer and it would work.
The problem with cheap diodes is basically that they take awhile to stop conducting. Something like a 1N4007 is apparently fine at 50Hz mains frequency, but by the time you get up to 1kHz it's hardly rectifying at all. If manufacturers would just spend a bit more money on decent quality rectifier diodes, there would not be a problem. And users would end up spending less in the long term, because everytime a diode is not shutting off on time, energy is being wasted -- and PSUs would also last much longer because these poor-quality diodes are slowly damaging the capacitor and fuse.
Je fume. Tu fumes. Nous fûmes!
It was my understanding that the power supply converted ac current to dc for the pc's use - so rather then having 10 servers with 10 power inverters you have 1 big power inverter and 10 servers w/ no power supplies....
"Everyone knows the alternating vs. direct current wars ended with Thomas Edison and Nikola Tesla." Well, I wasn't there at the time, but I think it was Charles Proteus Steinmetz who ended the DC vs. AC debate when he applied Complex Number mathematics to calculate the amount of load flowing over an AC line, which then gave power generators the ability to determine how to charge for power, something they did not know how to do up until that time for AC power. Steinmetz Wiki Steinmetz Google Complex Numbers
A hand up and a foot on every chest...
AC vs DC
Tesla vs Edison
Tah-MAY-toe vs Tah-MAH-toe
Linux vs Unix vs Windows
Just let me mention that Vi really is better than Emacs.
Feel free to discuss amongst yourselves....
He was not correct then, nor now.
Sure, in some special cases DC is better at the *device*. But even then, i challenge you to not notice how the power got to the building in the first place. It came via AC and thru transformers. And for good reason. AC is more efficent for electric power transfer.
---- Booth was a patriot ----
The Edison/Tesla one was about long distance transmission of power, and AC is still the winner there.
TTL logic has to run on DC, so you have to convert the supplied AC to DC. This is just recognizing that instead of converting it individually in each of dozens or hundreds (or more) machines, that it is more efficient and reliable to have one (and perhaps a redundant standby) converter providing DC to the same machines.
You could just run your datacenter off the 30 tons of TNT.
30 tons of TNT is an effective way to remove any viruses from the datacenter all at once!
At one point, the article mentions "Sun's Bechtolsheim is unconvinced of the merits of DC, though. The crux of his argument is that DC requires two conversions: one from outside AC to 48-volt DC for distribution within the building, and a second, within servers, from 48 volts to 12 volts."
Why can't the in-building distribution be 12 volts? Is it because there is too much loss across several meters of distance, so 12V would not make it to each PC? Too many PC's in parallel connected to the source DC would drop the voltage too much?
And, another statement says, of loose bars ""We have been involved in a number of cases where one joint failed catastrophically," he said. "The explosion kicked out the entire power distribution system. It wasn't maintained, because everything was packed in so tight that it wasn't accessible."
Why would 48 volt bars cause an explosion?
That's all...
Remember the death of Archimedes!
Anyway, the respondant claimed:
Check your history!
Edison died nearly penniless too.
The account which I read described how he ran across some iron-rich sand on a beach, and it gave him the idea to try a new mining technique where the ore would be extracted from non-ore material by dropping the sand past magnets. The idea was a good one (and is still used) but the site he chose to build his iron mine turned out to be almost completely lacking in iron ore. The iron ore in the sand on the beach had apparently washed up from some other source.
Maybe he wouldn't have been desperate enough to try such a risky thing if he had been ALLOWED to sell AC power. I'm sure he could see the advantages of AC for power transmission but Edison didn't have the patents for that, and you can bet that Westinghouse wasn't going to license the technology at a reasonable price to their chief competitor.
So Tesla got ripped off by Westinghouse because he wasn't business savvy and they got ownership of the patents. Then Edison, even though he was somewhat business savvy, got shut out by Westinghouse because they owned the patents. In both cases, patent law helped business-people who didn't invent anything get rich while the real inventors lost out. Shouldn't we remember that the patent system was set up in order to encourage invention?
I had knocking around the idea of running two small machines in 1 case, which I believe one could easily do
assuming the powersupply was say twice what was needed, well maybe only one could be hooked up
to soft power, etc. I guess one could remove the power supply of one machine, split the power supply leads,
(create two of everything, and run 1 set of wires into an adjacent machine ).. The next step would be
for someone to make a device, a super powersupply, which runs 3 or machines, perhaps with each machine having
its powerspupply replaced with what's basically a box (of the same formfactor ) , and maybe some circuits that
provide the soft power, atx , etc. in other words each machine thinks it has its own atx power supply, but in fact
they're sharing 1 mega sized powersupply.
Telcos have been doing this for almost a hundred years, increased reliability, lowered risk to personel, lower cost. Where's everyone else been?!
Also back then cheap power transistors useful for converting DC-DC voltages were not available. In fact transistors hadn't even been invented yet. Changing AC voltages was relatively trivial, involving just coiling of wire.
I've been through the NEBS gauntlet a number of times. They take your equipment, put a gas burner under it and ignite it intentionally. When the flame is removed, the equipment must self-extinguish in accordance with the NEBS rules. Here's a snippet from a Verizon testing document -
Then they take another piece of your equipment (from your manufacturing line, no less) and hit it with overvoltage conditions simulating lightning and power-line contacts. Only devices that are specifically designated as fuses may open-circuit. If a PCB trace peels up, you fail. If an IC opens up, you fail.
NEBS certification requires flogging several pieces of produciton gear to death in horrible conditions. Going through the process is tough, and it teaches you just how much you don't know about real world failure scenarios.
Telecom's have been doing this for years... I have several rack mount systems that I assume were doing some sort of text or message routing via serial that were replaced with newer equipment - these are all running off DC power supplies. (I have about 20 of these systems, they were acquired through a friend who works for a very large telecommunications company)
The systems I've seen typically consist of racks full of DC powered servers booting off CF cards (The ones I have are k6-2 350's with 32Meg of ram and 32M flash readers and cards using IDE adapters) loaded with serial controllers.
Not all that suprising really...
Wisest is he who knows he does not know.
-Eric
SJW: Someone who has run out of real oppression, and has to fake it.
Any electrical engineer will tell you that politics not common sense determined our current current. ;)
Transpose Tesla and Edison, and you'll have it right.
Tesla invented AC for fuck's sake.
I guess the fact that as a child I want to "Westinghouse middle school" which was right next door to the "George Westinghouse Museum" caused me to learn about this before most other geeks did.
LK
"Hi. This is my friend, Jack Shit, and you don't know him." - Lord Kano
he also exploited scare tactics: at county fairs he plugged a dc generator into a pen of sheep w/o harm, then plugged in an alternator: fried mutton;-) but that gave states the idea to execute prisoners w/ ac...
;-);-);-)
back then electrocution wasn't a word, so they used the name of the alternator's manufacturer, giving rise to their famous ad line:
"you can be sure if it's westinghoused"
Wasn't this discussed already [slashdot.org]?
Good switching power supplies utilize both PWM mode and PFM mode (and PFM burst mode) to maximize. PFM mode is "Pulse Frequency Modulation", which is a bit of a misleading name. It really means that the width of the pulse remains constant and the time between them changes. This is used when power draw is very low compared to max output.
In your review of DC converters, I think you missed flying (or switched) capacitors? This system is also good only for very low current applications and only can step up, but is fairly efficient and requires no inductor. Nearly everyone owns a device that uses one of these (all their computers) because they are used to generate the voltage needed in an RS-232 serial port without +12 and -12 (or indeed any negative) supplies.
Not complaining with your argument overall. I think it would be efficient to have a DC distribution system in a house. It would have to be high voltage (like 50V) to overcome line losses, but it would make it possible to use an efficient full-wave rectifier for all your DC needs and save some number of components at each device location. I cannot really see using DC for traditional motor (any device that produces motion or does any serious amount of actual work) applications, AC is great for that.
http://lkml.org/lkml/2005/8/20/95
Getting a shock from DC is much worse than AC.. AC tends to kick you away while DC welds your skin to the conductor and keeps you in the circuit.
The low frequency AC is optimized for AC motors.
The classic telecom reason, I believe, is that since the government required the old Bell monopoly to offer 100% reliability of phone service, everything had to be able to run on batteries during power outages. So they chose 48VDC as a practical voltage that doesn't require excessively big wires for large power transfer (like 12V would), yet isn't too dangerous (like 120V or 240V would be), doesn't require an excessive number of cells in each backup battery, etc. So from a data center perspective these reasons still make sense don't they? Data centers also need battery backup, and it's a hell of a lot easier to just have the batteries in the circuit all the time, on float charge, and therefore instantly available to run the servers when the AC power fails, than it is to continuously be converting AC to DC (to charge the batteries) then back to AC (to run the server power supplies) then back to DC again (inside the computers).
However you still cannot get away from some conversions because switching power supplies are so much more efficient than linear regulators. So the PSU inside the PC is just as complex - there's no way around it. Yes, you could use separate bus bars for 5V, 3.3V etc. but the currents become larger and there can be problems with noise coupling from one system to another. Computers need very well filtered, quiet 5V and 3.3V. So the multi-voltage external supply may be more efficient but not more reliable.
I am running some DC power at home, and one more advantage for me is being able to use solar panels to directly charge the batteries rather than relying 100% on grid power. Again it is easier to use DC since that's what the solar panels put out. This would also be an ideal thing to do at a data center. The government ought to do something to encourage it (the way some European countries are). (But my system is 24VDC. Maybe I will have to switch to 48V if that kind of PSU becomes abundant and affordable.)
I believe the article makes an oversimplification by stating that AC is better for long-distance power transmission. Rather, it's easier to generate AC power (no rectifiers are needed), easier to switch (because the arc when the switch opens is much easier to extinguish - current flow actually stops for a short period of time, and the arc goes out), easier to run a motor from AC (no commutator), and easier to do voltage conversions (you only need a transformer). For really high-power long-distance transmission lines (like between states) they use very high-voltage DC because it is in fact more efficient. But I'm not sure how they do the conversion from DC back to AC in that case (would guess it's just a rotary converter - a motor running a generator). The losses from doing the conversion on both ends are acceptable only when they are less than the losses that would occur in such a long transmission line.
Losses are especially bad in AC transmission lines when the power factor is not correct, because while currents which are out-of-phase with the generated voltage waveform are expressed using imaginary numbers, in fact they are very real currents, and they cause increased heating losses in the transmission line. So the power companies switch large capacitors in and out of the circuit to try to keep the current and voltage in phase. (And they would appreciate if every device on the grid was power-factor-corrected, but this doesn't happen, mostly because motors are inherently inductive, and motors are the largest consumer of electricity. Sometimes they at least manage to persuade large industrial customers to manage their own capacitor bank, to correct for the inductance of their own motors, and give them a discount in exchange.)
For the same reason that power companies don't just route 120v everywhere. To provide the same power at a lower voltage, you need more current. More current over a wire with finite resistance causes more power loss.
telco has been run off "CO battery" since the first wire was laid, and the present form is a series of "rectifier" power converters powered by AC and/or standby generators, and battery banks, AND-connected through bus bars into a common 48 volt nominal positive-ground service distribution of many thousands of amperes. you lose AC, the batteries hold it all over until the generators kick in. generators run out of fuel, you have a problem within a couple of hours.
there are periodic outages in the telephone galaxy of remote offices that are not staffed, or where design screwed up and the battery capacity has not been upgraded. but the last big ones were new york in 911, san fransisco in the world series earthquake, and the gulf coast in katrina and rita.
it works pretty well. it's proven, and there are tons of servers availiable for 48 volt DC because of the telco systems management connection. mostly unix boxes, I might add, because of that close tie through history and requirements of massive unconditional uptime.
if this is supposed to be a new economy, how come they still want my old fashioned money?
But no one ever got fired for choosing AC.
Codswallow! Their are a ton of problems with pure Direct Current. One it is not as safe, proof (caveat true for houses built after ~1945 in major areas of the US).: The power source you are next, where it purely a conductive DC lead? You'd have at least major static discharge. AC On the other hand doesn't have this particular issue.
On the other hand what is true that the article misses entirely is that a highbrid system is even better. Induce the current directly, store it in a capcitor, and use AC techniques for sheilding. Even Tesla concieded this was far better then the pure AC pure DC source (Tesla the magician from Borders Book) BS
Edison lost because he couldn't send his DC long distance.
Long distance needs high voltage, and you can't deliver that
to the customer. In Edison's day the only way to rasie or
lower DC voltages was the use of rotating converters, really
a motor and a generator on the same shaft, often sharing the
same field and maybe the same armature. The NY subway system
usedrotating converters to get DC for their trains from AC
mains. These gizmos are not as efficient as a transformer
so a DC distribution system would burn more coal than an AC
one. So Edison also lost on price.
Today we have ignitrons, SCR's and other solid state switches
that can convert DC to AC and back again so DC-DC voltage
converters are almost as efficient as transformers. Enough so
that DC power lines are now practical (at least for very long
runs with very high voltage).
Sorry, that should have been 80%+ effeciency, NOT 60.
Not to self, use preview button more often.
Slashdot gets worse every day... Pipedot: News for nerds, without the corporate slant
We evaluated several rackfuls of HP blades in my previous contract. Each rackful came with redundant 12KW 48VDC power supplies (Don't get your wedding ring caught in THOSE power cables.) (We're talking some serious interlocking thus.)
Anyway, even with only 10 or 20 blades in each rack the increased density led to such increased power consumption that there wasn't anyplace we could put the racks such that the existing ventilation system could deal with it.
They're retrofitting the whole room with APCC InfraStruXure(TM) stuff now so they can direct BTUs and power exactly where they need it. The point being that it's density not total power that constrains you. Modern switching power supplies are pretty efficient. Moving all the equipment to DC only bottlenecks the system somewhere else.
Give a man a fish and you have fed him for today. Teach a man to fish, and he'll say "WHERE'S MY FISH, YOU IDIOT?"
My notebook computer makes it seem so trivial... If my datacenter was full of notebook computers instead of rack-mount servers... It would be silent, have no heat problems, have skinny little DC cables for power, use much less room... I'm just getting started...
...I know it. What's going to happen is that the proponents of AC power will use DC power supplies from server racks to power electric chairs and prove how poorly it works. No one will ever want to use DC again after that kind of fiasco. As the adage goes: "Those who don't know history are doomed to repeat it". You never know though... this could go the way of "second system syndrome". ;P
-"...bad old ideas look confusingly fresh when they are packaged as technology" - Jaron Lanier (Digital Maoism on Edge.o
If you've ever been in a telco data centre or central office you've likely seen vast amounts of DC powered equipment. I was in a Bell Mobility test lab where they had "battery backup" for some equipment: car batteries in a big plastic tub (like a Rubbermaid storage container.) At a different telco (Aliant) we had to get special permission to use AC power for some servers we were having installed, as everything else was run with DC.
The downside? The hardware costs a premium, especially if you're working in the PC/x86 world. At the time you had very little choice in server hardware if you wanted to go DC. At least, if you wanted it to run Windows!
And one small fault on "wacking great DC supply" will send the whole server room west. Way to go. Excitement for all. Have a contest for the "most imaginative explication for the boss". The winner is the only "expert" who doesn't get hung up by the heels and beaten with rubber hoses. Yep.
How many beans make five, anyhow ?
In Korea only old people run on DC, while in Soviet Russia .... Sorry. Couldn't resist it. I'll try to do better - honest !
How many beans make five, anyhow ?
Yes there is.
How many beans make five, anyhow ?
Oh? You mean like Altos computers? Funny, I don't remember them being all that big! ;-)
Ok, wise guys, Altos 1086 / 2086 were still single CPU systems with multiple slot style 10" X 12" phenolic boards of which ONE was CPU, 2 were RAM, 1 or more serial ports, etc. I just had to get my meetoo.
I'll think of a really good SIG just before I die.
The Altoses were in the same class as PDP-11 mini-computers.
:-)
No, I do not mean Altos.
Javascript + Nintendo DSi = DSiCade
See www.NextekPower.com. They have a gateway that takes power from DC sources (like Solar PV), and supplies DC loads like DC Fluroescent lights, servers. When there's not enough sun, it takes power from the grid and converts that to DC. HIGHLY efficient.
Hmpf... what year do you live in?
We have started looking at equipment from the networking people (routers, switches, ...) back in 1998 and decided for migration to 48VDC in 2000. We have decomissioned last AC powered server in December last year, so we now have only a couple of weird components for VoD and audio streaming left on AC. Everything else is on DC.
We have cut down on heat, skipped on upgrade of air conditioning, and we at least KNOW that all our equipment happens to be properly connected to in-house UPS. Because it can not be connected in any other way. All in all it had been a real money saver in last 5 years, so I'm surprised that not everybody in charge of more than 20 servers is doing it.
You know, Common Sense(TM) Just Works!(TM). In case of 48VDC, we have seen the common sense years ago, and we were not the only one. HP representative (we buy their servers almost exclusively) commented back in 2000 that it seems that everybody is already doing it.
Counter-rotating.
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....