Adjustable bench power supply from computer PSU

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. :D

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. :D

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.. :whistler: 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 :D ). 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. :D

More to come. Stay posted.

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

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

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.

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

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.

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" :whistler: 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. :hurt: Yeah, it's only 5V, but the 5V rail is rated to (supposedly) a max of 30A. :eek: ..yeah, I need to fix that gap..

Quote:

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Originally Posted by x88x

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

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WHY WERE YOUR HANDS IN THE PSU WITH THE POWER ON!?!?! LOL:D
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.

Quote:

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Originally Posted by mDust

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

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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.

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



other than that, nice work

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.

Hahaha, yeah...I'll be taking more precautions in the future, including actually wearing the nitrile gloves that I have a box of in the closet...and not reaching towards live wires. :P

IDK about 30A necessarily killing me though...I've had house run through my right hand and (presumably) out my feet before...though come to think of it, that may have had a different effect if it had been my left hand... Ah well, I was young, and kids heal fast, right?

BTW, I've also since redone the connection with larger diameter heatshrink, so there isn't a bare spot anymore...but still.

Quote:

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Originally Posted by x88x

IDK about 30A necessarily killing me though...I've had house run through my right hand and (presumably) out my feet before...though come to think of it, that may have had a different effect if it had been my left hand... Ah well, I was young, and kids heal fast, right?

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10-60mA through the heart or brain is enough to kill you...blood, being mostly water, is an ideal conductor leading throughout your entire body to and from your heart. Taking a full 30 amps for any amount of time would likely fry every bit of flesh and organ it passed through...might as well be struck by lightning.:dead:

Quote:

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Originally Posted by Technochicken

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.

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Heheh, I remember once when I was messing around with a house-current clock/timer thing, I accidentally touched one of the contacts with a butterfly knife I was holding..steel blade, brass handle..not much in the was in insulation there. :P I ate a small chunk out of the blade (fortunately the back of the blade), and gave me a not-insignificant shock. ...also knocked out the circuit breaker.. :whistler:

Quote:

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Originally Posted by mDust

Taking a full 30 amps for any amount of time would likely fry every bit of flesh and organ it passed through...might as well be struck by lightning.:dead:

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That would explain why parts of my finger were numb for several minutes...and one point on the tip still hurts occasionally when I'm typing..presumably where I touched the heatsink and the current came out. Gotta love high current levels. :P

Love them...but respect them too!:D

Quote:

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Originally Posted by 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.

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Stupid, stupider, stupidest -_-

Nice job on the bench PSU! I should get unlazy and do one myself, Although mine are way older and can't get 30A off of it but shouldn't need that hopefully!

Getting the 30A would be hard due to V= I (amps) * R (resistance)
so assuming 5V and your body should have a decent resistance you shouldn't get too bad a shock but being DC it does hurt a bit more. Capacitors you have to watch for those bastard can be rated at 500V obviously your resistance doesn't change but the amps will increase! And that can kill you.

Well, it's done-ish for now. I'm waiting on some banana plugs coming from Mouser to finish it off, but for now it'll do. I made some heatshrink shrouds that can be pulled on and off fairly easily, but not so easily that they'll fall off on their own.

I'll eventually have plugs on the top, status LEDs, individual switches and fuses, and an adjustable 12V line, and so on, but for now wires sticking out the ventilation holes will have to do. :P

EDIT:
Yeah, I know...blue isn't standard. I accidentally cut off too many of the 3.3V lines and had to add some of my own.

Nice Job.

Props for the HF Hobby knife set in the background!!

Actually I got that for something like $3 on clearance at Microcenter. It does seem about HF quality though...I've broken 5 blades already... :whistler:

Nice job so far ,,
yeah I have to done one of these a while back they come in very handy,
and yeah getting a belt from a capacitor is nice either.
Will you be putting banana plugs on it, for various voltages and a rheostat too

Yup, once I get some plugs (...which I still haven't gotten around to ordering...), I'll be adding a ground post or two, a 3.3V post, a 5V post, a 12V post, and an adjustable 0-12V post, hopefully with a readout...I'm still looking into that.

Thread revival! Mwahahahaha! It lives!!!!

So I never did end up putting plugs on this one, but I made another one. :D

This one is a little different.. :twisted:

First off, I'm not starting with a random garbage PSU...I need to actually have dependable, high-current supply from this one, so I'm starting with this:

PC Power & Cooling Silencer 500W.

Well, that's what it was like a couple years ago. Then it got cables cropped for MightyMite...which apparently I don't have any pictures of...

And here it is now! :D

Like I said, this one needs to be able to supply high current levels, so the plugs are fed with four wires instead of one.

I have that same PSU running my desktop, and it's great. 35 amps on the 12 volt rail is excellent for a 500 watt PSU.

Heheh, yup. That's why it was sacrificed. :D Well, that and the fact that the way I had modified the cables made it useless in most computer cases... That 35A rail will serve it very nicely in its new life as a battery charger PSU. :D

Let's see if we can start a car with that...
lol

I'm guessing probably not...though maybe if it were a small, low compression, engine. My car battery charger has a 'jump-start' setting that puts out 75A.

You've inspired me to do this with a spare PC Power and Cooling Silencer 470 I have. I'm going to try to follow this guide, for more voltage options.
http://www.wikihow.com/Convert-a-Com...b-Power-Supply

that's not a bad guide tech - I referenced that a few times when building Power House :D

Cool. Just keep in mind that the current limits on the - rails are different than the current limits on the + rails. Also, YMMV, but that guide only says to put a load resistor on the 5V rail and I found that I needed one on the 3.3V rail as well.

Very nice and useful project, thanks for all the hard work. I will build a bench PS based on this.

Where is the variable voltage portion, I must have missed it.

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Originally Posted by Swankrg

Where is the variable voltage portion, I must have missed it.

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I never did end up doing that bit. For very low current levels, you can get away with just hooking up a potentiometer, but if you want to do it properly, I would suggest something along the lines of the below link. Just beef up the mosfet to the needed level, and put the load hookups where he has the motor.

http://pcbheaven.com/circuitpages/PW...r_using_a_555/