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Low Voltage Power Distribution?
thesp asks: "As I look around my apartment, I am continually struck by the plethora of high-voltage AC to low-voltage DC power adapters I use to power my various devices. At a recent estimate, around 30% of the power consumed in my house is via these adapters. From my laptop to my digital music player, and from my mobile telephone to my PDA, each device is down-converting its own power through its own adapter. Double this number to include my partner's devices. Many of these run hot, and are inconvenient to remove/replug to conserve power and outlets. Does Slashdot know of any moves to standardize power delivery to such devices, or of hobby/home-brew projects to distribute low-voltage power from a central power converter? Alternatively, are there reasons as to why this would not be a simple and effective solution to the proliferation of wall-warts."
"On closer examination, these adapters seem to fall into four major categories, 7V, 5V and 3V, with the most common being 5V. Despite this, each device uses a different DC plug configuration, which makes efficient use of adapters difficult. It seems to me that, just as AC power is standardised, portable electronics power requirements should be also be standardised, with a standard wall outlet and car outlet at, say, 5V, and a standard device cable and interface. Electronics manufacturers would save money on power adapters, and the consumer would have the cost of the converter written in to home construction or automobile construction costs. No longer would we have to lug 4 separate power adapters with us on an overnight business stay to power our various equipment."
Article is a dupe...original discussion can be found here, which amusingly enough, is itself a dupe of this discussion. Even more amusing is the fact that all of these submissions share the same editor.
Way to go, Cliff...a dupe hat trick. Zonk has nothing on you.
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1. You can't (simply) transform DC voltage to a different voltage. This can be done very efficiently with AC. The 120v to 5V (or whatever) in your power supply is done before the AC is rectified to DC.
2. Low voltage == High losses, esp. with DC.
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Actually, while there is no standard, I've definitely noticed that manufacturers are tending to use the same kind of plugs for the same voltages more and more. It seems like everything I've got that's 12VDC has the same plug, and I really mean everything, from my Radio Shack A/V transmitter unit, to the Intel webcam (from long long ago) that I connected to it, it's all the same size barrel connector. Obviously it's not standard but it's getting better.
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Let:
P = power dissipation
I = current
R = resistance
V = potential difference (voltage)
We know that power is a function of power and current. For direct current,
(1) P = V * I
By Ohm's Law,
(2) V = I * R
Therefore
(3) P = I ** R
So power dissipation is proportional to the square of the current. Given a requirement to deliver some arbitrary amount of usable power to the devices you have plugged in, by (1) you know that if you halve the voltage you must double the current to deliver the same amount of power. But, by (3) you also know that if you double the current you square the power dissipated by the resistance in the cabling. Hence if you step down from say 120V to 12V, you must deliver ten times the current and hence power losses are multiplied by a factor of 100.
This still wouldn't amount to much in reality as the sort of devices you're talking about are generally rated between 1-10W and therefore you're only delivering current on the order of an Ampere or two per device. Plus of course the resistance in your domestic cabling should be absolutely negligible.
However, it does explain why the power companies use high tension power lines (tens or hundreds of kilovolts) to transport electricity over long distances. Imagine the amount of current these things carry. When they step the voltage up by a factor of a thousand, the power loss due to resistance in the cables (and over hundreds of miles it'll be a lot) is reduced to a millionth of what it would be if transported at domestic voltage.
Depends on your current draw. Check out this table. Remember that by time you wire your entire house, there will be several hundred feet of wire.
There's a reason we feed houses with AC.
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If you have a cluster of devices of all the same voltage at the same location, then it would make sense to have a common power supply. Otherwise, it makes more sense to use a higher voltage for distribution purposes. The electric utility generally brings power down to your street in the 11kv to 14kv range, and a permanent transformer drops it down to the 120 to 240 volt range you get into your home. Distributing power at 240 volts would not even be considered beyond at most 100 to 200 meters. Every time the voltage goes up by 2, the distant can go up by 4 since the current is cut in half, which means the voltage drop is cut in half, which has even less effect on twice the voltage. When they run the voltage at 50 to 100 times as much, they can deliver power over substantial distances. Cutting voltage to 1/10 as much means you can deliver the same amount power to only 1/100 the distance.
Incandescent lights actually work better at lower voltage, especially for bulbs of lower wattage. Normally a low wattage bulb requires greater resistance in the filament. That means the filament must be longer and/or thinner. That means it is more prone to mechanical shock damage. It also has to run at a lower temperature, producing a more orange light (which in some cases is what is desired). The lower temperature wastes power since more is emitted as infrared instead of usable light. By changing the bulb design to a low voltage like 12 volts, the same power level can have a shorter, thicker, hotter filament, which can run more efficiently, even making up for the loss involved in having a transformer converting the voltage.
The reason I mention low voltage lights is to point out that they are rather standard at 12 volts (a few use 24 volts), yet transformers are generally located close to where the lights are, rather than in a central location which would require the power be distributed in low voltage form. If a central low voltage source were practical, low voltage lighting would be the first to use it. But with very few exceptions, they don't do it this way.
I once considered running lots of stuff in my house on lo
now we need to go OSS in diesel cars
... according to the chart, a #10 (6mm^2) wire (which, while by A/C standards is huge, isn't really that big at all) will get you 216 feet at 10 amps.
You might note that that applies at 120 Volts, not 12V - at the lower voltage your #10 gets you a whopping 22 feet. For 200 feet at 12V you need 1/0 gauge wire, which is ten times the cross-section, and three-and-a-half times the diameter...
Again, not huge in real-world terms, but bigger than you imagine...
I think the biggest pitfall is making sure you don't deliver too much (or little) current to the devices you plug in. It would be very bad to deliver 10 amps to a device which is expecting 300 milliamps, or 300mA to a device expecting 2A.
And thus your power source would be a fixed-voltage source, not a fixed current one. Technically, only raw components need to be protected from over-current situations - any (properly-designed) circuit should account for the max current going across any component within it, and prevent it from going overspec. Consider that most wall-warts do not limit the current being drawn from it - draw enough current, and you'll simply make the adapter overheat and melt down.
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DC does not require any larger conductors than AC does, for the same voltage and current. You must be assuming low voltage in reference to DC.
Three phase is only marginally better than single phase for converting AC to DC. And unless the power supply is a very complex and expensive type, it will result in a high level of harmonics and a low power factor on the AC source due to the rectification cycles. On a large scale this could also overload the neutral conductor.
Three phase is generally good for motors only above the 1 horsepower level. Many home appliances would not benefit from it. A few might (the big ones), but not all areas get three phase power, so the dominant appliance products use single phase power.
now we need to go OSS in diesel cars
Check out the specs on telco equipment. A lot of them run on 48 vdc with special 48vdc power supplies. You can get a lot of networking gear that come with 48vdc power supplies. I think there are probably computers that have the same ability.
Of course this is all pretty expensive since it's intended for telco companies.
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A question of DC vs AC for electricity distribution was the subject of conflict between Edison an Tesla in 19th century. You can read more on that:
/ >
<URL:http://en.wikipedia.org/wiki/War_of_Currents
Yes there is. A 1200W microwave draws 10 amps at 120V. At 12V it would draw 100A. You have any idea how thick the wire has to be to handle 100A?
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Edison, for some unknown reason, hated Tesla and tried to kill his ideas of AC power distribution. He apparently had the (AC-powered) electric chair created as a PR stunt so that people would know that AC power was being used to kill people -- but it turned out to be relatively difficult to reliably kill people with AC power because an AC charge turns out to be an impromptu defibrulator, so you essentially have to cook your victim.
DC on the other hand, causes the heart to go into a constricted mode which is harder to recover from. I was taught to always handle AC with one hond only, if at all possible (to avoid a possible circuit across the heart).
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