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Constructing A Low-Power 2U Wireless Rack-Box
adelayde writes "Recently we decided to build ourselves a custom rack-mountable box that we could use as a web and DNS caching proxy and which would offer flexible wireless networking facilities and have an uniterruptible power supply. The result was a 2U rack-box with dual wireless networks built upon a low-power Via EPIA MiniITX motherboard. The box has two wireless networks built in with external antenna connectors, locking switches on the front to avoid tampering, a battery to give at least 20 mins of autonomous operation, a low wattage power supply and most importantly lots of blue LEDs :)"
...and most importantly lots of blue LEDs
You may have meant that as a joke, but blue LEDs suck quite a lot more power than red or green ones.
When you care about power consumption, rather than coolness, come back and ask again.
http://mirror.us.psand.net/plinth/
So let's use this http://freecache.org/http://flakey.info/plinth/ link instead!
Important info:
http://www.lifeaftertheoilcrash.net
http://dieoff.org/synopsis.htm
http://www.peakoil.net
Then how about a mirror?
http://hackish.org/~rufus/flakey.info/plinth/
Incase it is needed: http://fluky.org/slashdot/flakey.info/plinth/index .html
Their site seems to be slowing quite fast:)
I'm a college degreed Electrical Engineer with over 30 years of experience, including teaching electronics at the college level for three years. That regulator circuit looks quite familiar. As long as the heatsink is sufficient for the heat dissipation in the LM317, there should be no problem. Since this is powering only the wireless bridge, the current drain at 7 volts should be modest and it only needs to drop 5 volts across the regulator IC. The total power dissipation spread across both of the 1/2 watt resistors is only 85 milliwatts so no trouble there either.
If you are referring to the capacitor voltage ratings, the only requirement there is that the voltage rating of the cap be more than the voltage actually applied to it. For example a 35 volt cap is just fine with 6 volts across it. It could even be a 1,000 volt rated cap with no ill effects.
"Do the Right Thing. It will gratify some people and astound the rest." - Mark Twain
"Do the Right Thing. It will gratify some people and astound the rest." - Mark Twain
I'm curious as to what their definition of "low power" is. Low, compared to a power-sucking P4 or Athlon, maybe, but probably not very low by low power standards.
I've just set up a similar system as an home file server (no wireless, though, and I've added a cheap DVDROM drive), and my box is sucking up around 55-60W, idle. That's measured via an actual wattmeter connected to the power cord, and not by multiplying V*A.
On second thought, maybe a soekris board and a 2.5" disk drive might have been a better solution (less RAM and CPU, which would probably be fine for an home fileserver, but the power usage would probably be in the 10-20W range).
I find that UPS battery being too naked next to the powersupply... It rather be protected by itself in a cage because battery's can do strange things sometimes (I can tell :S)
..
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if that thing explodes in your rack you can throw away the hard drive, maybe the mainboard and wireless bridge
so far for a rendundant/autonome system
although a beowolf cluster of blue leds? hmmmmz...
--- I am known for the ones who want to find me on the net. Is that a privacy risk or a privilege? One might wonder..
They used both a DC-DC supply and a LM317 converter. The DC-DC supply was a pre-made thing that takes in something like 12V and supplies what you need for an ATX motherboard, 12V, 5V, 3.3V, -3.3V, etc. Lots of low-power and small form factor computers use them, as they dissipate less heat and are smaller than normal AC-DC switching supplies.
Then they used a LM317 linear regulator to provide 7V from 12V for the wireless bridge. The bridge's power supply was rated for 1.4 amps. A 5V drop over 1.4 amps is 7 Watts wasted as heat. Not impressive in something that's supposed to be low power. With their 20 DegC/watt heatsink, that is a 140 deg C temp rise from ambient to the TO-220 case. Easily out of spec. The wireless bridge surely doesn't draw the full 1.4A in operation, that's what saves them from their regulator melting.
They probably should have seen if their bridge would have run from the 5V line or if the bridge's power supply could handle 12V. They probably didn't even need the 7V regulator.
If they did need one, it would have been much better to use a switching regulator that would be around 95% efficient instead of their 58% efficient linear regulator. TI powertrends makes integrated switching regulator modules that would work perfectly. I used one for to power a digital camera off a 12V deep cycle battery so it could take photos on a timer for several days straight.
I also have to wonder why they used locking switches instead of just getting a case with a locking front panel. I've got a rack of such 2U cases at work, they're not hard to come by.
The switch from the 2nd hard drive's power was unecessary too, you can turn the drive off with software.
AHEM:
a nd
http://www.archive.org/web/freecache.php
http://www.archive.org/about/faqs.php
Freecache only stores files > 5 megs, and ONLY stores what you tell it to. Linking freecache-style to an index page will only cache that index (IF it's >5megs), not the whole site.
What is it with all the useless freecache links lately?
You wrote that "They probably should have seen if their bridge would have run from the 5V line or if the bridge's power supply could handle 12V. They probably didn't even need the 7V regulator."
While that can work for a while on the test bench, it only takes a modest power line ripple or age stress to make a system that is running that far off its designated voltage fail intermittently, or permanently. It's much safer to wire in the linear supply: also, in some cases, adding a bit of load on your primary power supply can help reduce its ripple or over-voltages quite a lot, so they may wind up better off doing this than you realize.
The milliamp rating of the part bears no direct relationship to the need for heatsinking. The heatsink requirements are related to the amount of power that the regulator IC needs to dissipate to stay within rated operating temperature limits. The power dissipation is given by Pd=(Vin-Vout)*I. Where I is the current drawn through the regulator. For example, in this case being discussed, the voltage drop is roughly 5 volts across the IC and if the current was 1 amp, then the power being dissipated would be 5 watts. Given, from the data sheet, that the junction to ambient thermal resistance is 50 degrees C per watt, the junction temp rise above ambient for 5 watts would be 250 degrees C which is far in excess of the junction maximum operating temp of 125 degrees C. So in that example a heatsink, which reduces the junction to ambient thermal resistance, is clearly a necessity. See LM317 data sheet
Don't feel bad, though, it's a common misconception that has caused a lot of burned fingers. You are right that the heatsink can't hurt, even if not required, since lower temperatures are ralated to lower MTBF (Mean TIme Between Failures.)
"Do the Right Thing. It will gratify some people and astound the rest." - Mark Twain
"Do the Right Thing. It will gratify some people and astound the rest." - Mark Twain
when I got a patent on them for my corporate employer in the 70's using all discrete parts - no IC's. It's pretty easy today to build a small one with relatively few parts, but you have to look out for the noise that they produce. Here's a link to an application note for one that would work in this case: LM2575 ap note (warning, 26 page pdf) I still like linear regulators for a lot of applications unless efficiency is paramount.
"Do the Right Thing. It will gratify some people and astound the rest." - Mark Twain
"Do the Right Thing. It will gratify some people and astound the rest." - Mark Twain