top-quark.blog
Switched Power Supply in the 400 Watt Range
Fri, 06/10/2005, 21:26Skydiving. Running with bulls. Playing inside high voltage power supplies. Except for the first two, I've done 'em all now.
I needed a power supply. Problem is that commercial supplies are a specialty item, and are overbuilt for what I need. The wonderful world of consumer electronics offers a fine solution, ripe for hacking. A casual stroll down the cutthroat world of pricewatch shows about a bajillion cheap PC power supplies. PC supplies offer a buncha different useful voltages; 3.3, 5, 12, -5, -12, etc... The problem, though, is that they need a load to function correctly. I never knew how to handle this.
Then...
I came across an interesting website recently. It's not terrifically detailed, though the author does disclaim that it's not a HOWTO (ten bucks says he's just covering his ass). I also found this website, which is useful in conjunction.
Disclaimer: Don't do this. I'm a junior Electrical & Computer engineering major, and I still wake up at night, in a cold sweat. Chances are that you'll shock the shit out of yourself, and I really, really don't like court. I'm not even going to tell you to be careful; just don't do it.
I went to pricewatch and bought a 400 watt power supply for $10 shipped. Since my PC supply's too loud, and I think the bearings are going to die, I was gonna hack that. Well, as it turns out, the $10 supply was louder, with even dodgier bearings, so I just kept my PC supply where it was.
So, battle plan:
-Keep one drive cable, for computer accessory testing/dev.
-Four terminals: Ground, 3.3, 5, and 12. (black, orange, red, yellow wires)
-Two wires each, for MAXIMUM POWER (well, to preclude any melting)
-Rewire power switch
-Wire and attach load resistor
*giggle*
In the lower-right, you can sorta see some of the dire warnings of fire, brimstone, and firestone tires.
The innards. I want to make some sort of "diversity" joke here, but it'll probably reflect poorly on me in the future.
Note the BigAss(TM) capacitors.
After a little pruning, it's starting to look a little more civilized. Here I've isolated the wires I'll be using into operational clusters.
The power switch (bottom black box on the rear panel) needed a bit of research.
Power control on an ATX supply is actually two-tiered. Not only is there (sometimes) the master switch, controlling the 120v connection, but the motherboard additionally controls the supply though the green wire on the ATX connector. What I did here was bypass the switch's 120V control, to instead use the switch to short the (thin) green wire to ground.
I could have just put in another SPST switch to short the green wire to ground, but I felt this was a cleaner setup.
The 10 ohm, 10 watt sandbar load resistor, across 5V and ground wires. It does a great job, and I'm proud of it. Note the zip ties, as recommended by the pseudo-HOWTO. It's ghetto, but effective. A note for posterity: Thermal compound's messy stuff.
The cover. Spent a little time in CAD, made a drilling template. Center punch with a nail, then drill 1/4" holes for the binding posts. Note that 12-5=7; a fact that's handy for making your computer case fans quieter.
Maybe I'm stupid or something, but I couldn't figure out how fiber washers were supposed to isolate the screw portion of the binding posts from contacting the metal chassis. So I epoxied them in place. Here I baked them under my desklamp, because I'm gangster like that.
The finished product. I'll probably put in an LED power indicator lamp in the future, but as noted on my reference website, the fan is a good indication.
Results? Pretty nice, actually. Positive across the board.
- Voltages on 3.3, 5, and 12 volt lines are 3.39, 5.12, and 12.26 volts, respectively. That's 2.7, 2.4, and 2.2 percent deviation, for the mathematically impaired. I'm not doing anything analytical, so that's more than fine.
- Power dissipated by the load resistor is, I think, about 2.5 Watts. It doesn't get hot at all.
- Cost: $10 for the power supply. $8.30 for the other parts at Ratshack: Silicone thermal compound, at $1.99, a 2-pack of resistors for $1.79, and $3.99 for a 4-pack of posts. The last item makes me feel victimized, but that's what you get for patronizing your local consumerized techie-sellout chain. So, in total, about $18 for a 400 watt power supply, with some supplies that can be reused for another project.
- I didn't die.