Adjustable bench power supply from computer PSU

[x88x]

Have you ever wanted a steady power source for electronics tinkering, but didn't want to spend hundreds of dollars on a specially made bench PSU? Me too! Do you have a bunch of old computer PSUs lying around gathering dust? Me too! Let's see if we can take care of both problems at once.

I forgot to take any 'before' pics, but just picture a normal, cheap ATX PSU. I picked it up on clearance at Microcenter a while ago; it's a "400w" PSU by iMicro. Needless to say, I wouldn't have trusted a PC to it, but it was ~$5, I think, so I grabbed it. Now I'm glad I did.

So, first the problems I ran into. With modern PSUs, even if you short out the 'power' pin, they will only power on into a lower-power standby mode. In order to bump it up into a full power mode you have to provide a load. Now, most places I read said you only needed a load on the 5V rail, but then I started finding some stating a need for a load on the 3.3V rail as well.. For some reason, the first time I tried hooking everything up a couple weeks ago it didn't work, but now it is...I'm just gonna assume I had something wrong before. I finally figured it out for sure by opening up an ATX PSU tester that I have.

Solution:
Acquire two <=10 Ohm resistors rated at >=10W. Place one between the 5V rail and ground, and the other between the 3.3V and ground. Now your PSU should start up.

Next problem:
The 5V resistor gets hot. YMMV depending on what power rating your resistor is and what physical style you got. I got special power resistors built into small aluminum heatsinks that have mounting brackets to mount to either the chassis or a better heatsink. Given the amount of heat the 5V resistor generated when I put a steady 3A load on the 12V rail, I chose to mount it to another heatsink.

Solution:
I have several old heatsinks from the Pentium and Athlon XP eras, so I had a couple options. I tried a dry run with one of the tiny Pentium heatsinks, and decided it was still getting too hot.

So, I broke out one of the Athlon XP heatsinks:

Now, that's a bit large, and honestly not all needed. Soo..let's see what we can do about that. I finally wised up about using my dremel to cut thick aluminum.. So this was the first time I used my new 7" metal cutting discs with my compound mitre saw.

Dry-mounted in the PSU housing. The resistors will be screwed to the heatsink base eventually, instead of just resting on top, but I have no screws the right size, so that will have to wait for the weekend. Between the resistors and the heatsink, I'm using small pieces of that dense heat-conductive-foam-stuff you find in high-temp electronics (if anyone knows what this stuff actually is, please speak up ). I scavenged quite a bit of it out of a trashed 50" Plasma TV I found last weekend (cracked screen, not worth repairing), so that was convenient timing.

More to come. Stay posted.

[blaze15301]

hey i have some smaller heat sinks if you want them just pm me.

[x88x]

Thanks, but nah. Overkill is always a good thing.

[Diamon]

Interesting. I was lucky enough to find a regulated PSU in my schools pile of old computer parts but I might build something like this in the future anyway

Will be following.

[x88x]

Ok, I got the structural stuff done for now, so I figured I'd take a break and throw up an update.

First off, I got screws to mount the resistors, drilled and tapped holes, and here we go:

Then, after much fitting, grinding, refitting, filling, and so on, I got I got the mount completed:

And together:

And mounted in the PSU:

Now I'm going to start on the wiring. I had a power switch in mind, but a thought occurred to me to check, and it turns out that a round switch that I had is exactly the right size to fill the hole where all the wires used to go out. So, that one it is.

[x88x]

So, I got the resistors and switch wired in:

Now, unfortunately, I used too small diameter heatshrink on the 5V resistor connection, so there is some bare contact showing (you can't really see it in the picture, but it's about 1/4". Now, I thought "eh, whatever, the aesthetics of them not matching will bug me, but it's just 5V, right? It won't be a problem." ...yeah... I ran it for a while under a decent load (those electric motors again), and the resistors never got more than maybe a degree or two above ambient. The heatsink is working great. The jumper wires I was using got pretty hot though..I don't think they're rated for a 3A load.. Unfortunately (or maybe fortunately, come to think of it), the last time I reached down to feel the heatsink while it was running...well, let's just say I found that 1/4" Since the tip of my finger was touching the heatsink, and the heatsink is grounded through the mount, I got a nice big jolt across the last two segments of my left index finger. Yeah, it's only 5V, but the 5V rail is rated to (supposedly) a max of 30A. ..yeah, I need to fix that gap..

[mDust]

I got a nice big jolt across the last two segments of my left index finger. Yeah, it's only 5V, but the 5V rail is rated to (supposedly) a max of 30A. ..yeah, I need to fix that gap..

WHY WERE YOUR HANDS IN THE PSU WITH THE POWER ON!?!?! LOL
Your skin should resist 5v, but wear a rubber glove anyway. Moist skin is likely the problem. And 30A...if only some of that finds its way through your torso you will die.

[msmrx57]

WHY WERE YOUR HANDS IN THE PSU WITH THE POWER ON!?!?!

What he said. Definitely be more careful about that. As far as checking the heat sink it won't cool in the 3 seconds it takes to un plug it.

[diluzio91]

x88x... hate to say it... so ill let red do it for me



other than that, nice work

[Technochicken]

Awesome project! I've been thinking about building one of these, as my only bench psu can only do 2 amps at 12 volts, and I have a pair of atx PSU's that do 27.

I've done something even stupider with a PSU. I once read something about fixing a PSU with a broken fuse temporarily by just soldering some wire over the gap. I decided to try this by bridging the gap with the pair of scissors on my Swiss Army knife. There was a loud "CRACK," a large spark shot up scarily close to my face, and the current melted the tips of the scissors and blew some small flecks of molten metal around.