Use that old PC Power Supply as a high current +3.3, +5 or +12 volt resource

Picture of Use that old PC Power Supply as a high current +3.3, +5 or +12 volt resource
When the power supply for my Belkin USB 4 way crossover went belly up I needed a +5 volt supply capable of outputting a minimum of 2 amps. An old 12 volt car battery charger could easily power an LM323 3 amp 5 volt IC but it would take a week to get it. The biggest wall-wort I had was .700 milliwatts.

Enter my old power supply from where I found it under the bed. In addition to providing me with a high current +5 volt supply it gave me a high current +3.3 and a +12 volt supply as well. All I needed to make it work was a load on the +5 PSU bar provided by my Belkin but any +5 volt resistive/inductive load, such as a case fan or a 10 ohm, 10 watt resistor, will do.

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Step 1: Okay, so you to have an old power supply but not a Belkin or other device in need of +5 volt power so now what?

Picture of Okay,  so you to have an old power supply but not a Belkin or other device in need of +5 volt power so now what?
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Try using an old but still working case fan. If you have the power supply you are bound to have one of these. Otherwise you can find them online for about $8 plus shipping. You can also use a 10 ohm, 10 watt resistor or maybe one that is even smaller for less than $1. To find out you'll have to experiment.

Why do you need a case fan or a resistor anyway? Well its because the circuit shuts down unless it has a load. How come? Its probably because the circuit can detect a no-load condition and is designed to shut down when it does. Most switched power supplies need a load in order to operate. When the Belkin or the power resistor or case fan is removed or if the output lines are shorted the circuit will shut down as the result of the internal circuit. One of the great things about using a PC power supply besides the high amperage available is that it will force you to reconnect the load or to remove the short circuit before it will restart after removing and reinserting the line power plug. That's a great safety feature that can save you lots of other trouble.

The case fan shown below is rated at .12 amps and 12 volts or 1.44 watts. .12 amps on the +5 volt bar represents a load of only .6 watts and apparently .6 watts is all you need to tell the PSU shutdown circuit a load is connected and there is no need to shut down the PSU. Now for the connections...

clothbot7 years ago
Only improvement I would suggest is replacing the 10-Ohm resistor with something more useful like a DC lamp and/or fan.
watermelon (author)  clothbot7 years ago
That's a great idea, however, not everyone already has a +5 volt load like my Belkin crossover and resistors are less than a dollar whereas 5 volt fans start at around $8 if you don't have one lying around. By wiring a bunch of 12 volt or 6 volt lamps in a series/parallel configuration it might also be possible to match the 5 volt and 10 ohm (or possibly less) minimum load requirement of the power supply.
Agreed! I think I only spent $2.00 for a 2-pack of 33-Ohm, 5W Sandbox resistors. I wouldn't suggest a lamp, as this would be a little higher heat source.. the ceramic casing of the sandbox resistors is more than enough to dissipate the heat.. Also, the 33-Ohm provides less drain from the full +5V punch.

NOPE! a lamp filament is very hot, but a proper lamp load won't necessarily produce a lot of heat, as the filament is enclosed inside the glass bulb. a 12V bulb fed with 5V won´t be too hot, and doubles as a pilot light. Actually, an automotive lightbulb wastes less power than a fixed resistor. Been there, done that. Amclaussen.

watermelon (author)  clothbot7 years ago
(removed by author or community request)
Urm... What 10 amp requirement?


P=V*I=V*V/R= 25/10=2.5W

If it works, there's nothing wrong with using an old PC case fan (normally hooked to 12V) at the lower 5V. Embed it in an improvised soldering fume-hood. ;-)
watermelon (author)  clothbot7 years ago
Although the +5v bar can can supply up to 20 amps it turns out that .288 amps provides sufficient load for the power supply to operate and may indicate that so long as the load is not zero only minimal load is required. I'll revise the instructable since in addition to providing something more useful than a toe warmer (burner) a case fan can be plugged right back in after switching the +12 volt fan connector pin with the +5v pin.
russ_hensel2 months ago

Just a note to let you know I have added this instructable to the collection:
Encyclopedia of ATX to Bench Power Supply Conversion
Take a look at about 70 different approaches to this project.

samurai12007 years ago
I am not understanding the purpose behind step 2. I understand what you did (removed two pins, moved +5v over, and left 1 ground), i just dont understand the purpose. Doesn't a case fan already have it's connector in the correct configuration to be used with a computer's PSU? This project is perfect, I'm glad i found it. I have an old 200W PSU i want to use in powering a 100W car speaker amplifier... takes 12 vdc.
You might want to go to a 300-Watt supply, minimum, if all you're looking for is +12V, and use ALL of the yellow wires (from both, the ATX connector & drive connectors) to the +12V terminal. remember, most car amps have a fairly big supply terminal, and usually wired directly to the car battery.

Most supplies are rated by the amperage of the lower voltages (+5V, +3.3V).. the +12V or the -12V are usually down fairly low, as most don't use them as much as the older XT/AT systems did. (RS-232C? Never heard of it! (chuckle))
watermelon (author)  samurai12007 years ago
You need to place a load on the +5 v line, not the +12 v line. Since computer fans are powered by brushless DC motors where speed is a function of voltage a lower voltage applied to the computer fan means only that it will run slower and consume less energy as a bonus to providing a load on the +5 v line.
Sandisk1duo6 years ago
I keep tripping my house's circuit breakers....
If it's tripping the breaker, but not the internal fuse of the supply (unless it's been bypassed.) I'd check the HV side of the supply for shorts across either the 200V capacitors (Leave the supply off for at least a minute, then check 1st, voltage across the caps, then, if near 10V (minimum! ) short them, then test for resistance (1K ohm range) across each. Most have built-in bleed resistors, but I imagine one may be shorted, or one of the caps themselves may be shorted.. As anyone will tell you, dealing with the high voltage side is dangerous, if not LETHAL! Exercise caution when opening the supply, and anywhere near those caps.. Already got into an argument in another instructable where someone converted a supply, where they claimed there were _NO_ caps with that high a voltage rating. (yet, the schematic they provided for proof, had 2x 220uF 200V caps on the high voltage side.) Also, don't always short the Pwr_Good & +5Vstby, I did this with one supply I was testing, and it blew-up the switching IC inside the lower-voltages side. (ah, the smell of burning bakelite!)
my friend once had a power supply that did that. same formm factor. it was a different brand though. i would suggest finding a different one
steveeeee4 years ago
I turned a computer PSU into a bench power supply a while back, after reading a few instructables from people who had done the same. Some PSUs have a built-in load on the 5V rail so you don't need to add your own. Just measure resistance between GND and +5V to check this, and if you get a reading try firing up the PSU unloaded and see if it will stay running. If you do add a load resistor, not that the 10W 10ohm resistor will get quite hot. Mine measured over 50°C after a few minutes. I hacked up an old northbridge heatsink and stuck it on three sides of the resistor using thermal tape, which made a huge difference. It's also taped to the inside of the metal case and in the direct airflow from the fan and barely gets warrm to the touch now. Though having read this guide, I should try hooking the internal 120mm fan up to the 5V rail instead. Damn thing makes too much noise on 12V anyway, and it would free up the resistor for something else.
I've gone a little higher on the load resistor value, with mine.. A 300W supply I converted, had a 33-Ohm, 5W carbon film (yes, film. I would've expected it to burn out faster.) resistor across the +5V, in the stock supply.. Another 160W Dell supply (No -5V, (white wire was labeled PS-Fan-Out, so I'm guessing tachometer for the supply fan.) , BIG fan on bottom of case (which is now facing up, along with the power posts.), I put a 33-Ohm, 5W sandbox resister on a spared +5V & GND wire).. runs without problem, and the resistor stays pretty cool, even with no other load on the supply. I've found a few other supplies, which have thermal control of the cooling fan, which can make them a lot quieter. (the fan runs at a lower voltage, unless the main heatsink inside gets warmer.) That should be a relatively easy circuit to whip-up. would require a thermistor, and a simple darlington-transistor circuit to increase voltage from the +12V to the fan as the thermistor got warmer.
TBirdy20104 years ago
Could you use this to power the LED lights in a dollshouse?
Sure can, I'm guessing your LED's run at 5V's, which on most ATX PSU's is the most stable voltage with the most current. I believe mine is 30amps for the 5V output, it's a 300W PSU
toelle4 years ago
I use these all the time for my projects :) They are great, supplies you with all the voltages you need, most likely has built in short circuit protection, and they are easy to get. Btw you can get 24V out of these too :) All you need to do is connect the +12 and -12 to whatever you need to supply with 24V. Blue wire on ATX connector is -12V
cant get very much at 24v. maybe .8amp, but you can tie two together in series to get decent 24v power, just have to isolate the dc ground to case in them.
Okay cool. Didn't know you could do that :)
.. But 0.8amps is mostly enough for what I use it for anyway..
ianai204 years ago
can you give me a project?
santy225 years ago
id tap that +5 volt line. that made my day.
you said googles must be worn when making this pigment (last pic) its goggles, not googles lol ... :-P
watermelon (author)  !Andrew_Modder!7 years ago
Thanks, fixed it.
is googles a webkinz? yes it is right? just thinking.
maker127 years ago
it looks evil!
locofocos7 years ago
Christmas tree lights, maybe?
bmohr7 years ago
Awesome, a great way to use those old power supplies and have a ready and reasonably reliable source of voltage for projects. One question what exactly are you doing in the last picture? Are you corroding a copper pipe?
watermelon (author)  bmohr7 years ago
We in the DIY, fixer-up, gee-wiz, look-mon-no-hands excitement business would NEVER refer to anything we do as "corroding". The proper term to use when refering to this highly specialized and constructive activity is by its true title "oxidizing". Yes, we are oxidizing the copper pipe. Remember this. it may be very important to you someday.
Well thanks for clearing that right up! Are you going to do an instructable on how to use copper sulfate to as paint pigment? And of course please accept my apologies for maligning your highly specialized oxidation process. :-)
I like =)) Would be useful for hydrolysis, but would probably eat up the electrodes in a few minutes, unless you were using platinum... Nice work!
watermelon (author)  T3h_Muffinator7 years ago
Carbon is in general the non-reactive anode(+) of choice since platinum is not only expensive but can act as a catalyst by itself. In this application, however, the byproduct of the reaction and the product are one in the same.
Oh, I was always under the impression that platinum just didn't oxidize (or deteriorate) as easily as the carbon rods.
watermelon (author)  T3h_Muffinator7 years ago
Depending on what you are doing platinum may not for other reasons be the best idea. I beleive hydrogen and oxygen in the presence of platinum which will catalyse the reaction can more easily explode. Click here to check with a scientist on this.