Use That Old PC Power Supply As a High Current +3.3, +5 or +12 Volt Resource





Introduction: Use That Old PC Power Supply As a High Current +3.3, +5 or +12 Volt Resource

About: I'm an Emu. As a young chick my parents use to feed me watermelon and I loved it so much everyone nick named me, you guessed it, watermelon. Now that I have moved away from home I rarely get to eat any water...

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.


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?

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...


Step 2: Using the Case Fan: the Original Connector Can Be Used With Slight Modification.

The connector that exits the power supply has a male connector with female pin sockets. When the key (rounded edges) are facing up the red +5 volt lead is on the right and the yellow +12 volt lead is on the left. The connector you want to modify is the female connector with the male pins, coming from the fan. Notice how the red +5 volt lead is now on the left and the yellow +12 volt lead is on the right with the key facing up as shown in the picture. What you want to do is to remove the +5 volt and +12 volt pins and insert the +12 lead in the +5 volt position. OUCH! Won't this set the fire alarm off and get me grounded for three weeks after school? Nope. Its that simple. To remove the pins use a pointed object like a sharp stick pin or dental pick to bend the pin tab locks inward so the pins will slip out of the connector. The tab locks can then be bent back into place and the +12 volt yellow lead pin inserted into the +5 volt pin socket. Now all you have to do is to plug this into one of the power connectors used to power the hard drives and your PSU will have its turn on load.


Step 3: Using the Resistor: Find the Two Output Connectors Marked P8 and P9

Both the P8 and P9 connector have ground connections but the ground connection on the P8 connector is what to use since most 10 ohm, 10 watt resistors have axial leads rather than radial although these are made too. The axial leads are not quit long enough to use the P9 connector by itself. Use the resistor to bridge one of the output grounds on the P8 connector and one of the output +5 volt power leads on the P9 connector. (see drawing)

The simplest way to do this (after being sure the power supply is unplugged) is to insert one of the bare resistor leads into one of the P8 ground pin sockets (the ones that have a black wire from the other side of the socket). Insert the bare resistor lead as far as it will go and then mark the wire at the point it exits the other side of the socket.

Remove the resistor and then cut the excess wire away so that after reinserting the wire it will not exit the other side. You can alternately bend the bare wire and cover with shrink wrap. I like to put in a few extra bends so when I heat the shrink wrap it grips the wire a little better. If you do it this way be sure the shrink wrap extends a little bit beyond the wire end. I do not suggest using electrical tape because the resistor gets too hot to hold and electrical tape has a tendency to unwrap when heated.

Do the same thing with the other lead coming from the resistor only insert it into one of the P8 +5 volt pin sockets (the one that has a red wire going into it from the other side).

(Notice that in the photograph the end of one of the bare resistor wires is sticking out of the back of the P8 socket that has not been cut away. Be sure to cut or cover this bare wire so that it either does not protrude or so that the protrution is insulated in some way. If you do not then you should be prepared for unexpected shutdowns.)


Step 4: Use One of the Hardrive Connectors to Tap the +5 and +12 Lines.

The hard drive connectors coming from the power supply are male so you have two options to complete this step when tapping the +5 and +12 voltages.

A. use a female connector and tap its leads, or

B. cut off the male connector and use the power supply leads.

I used an old female-to-male floppy drive connector, removed the individual pin connectors and used alligator clips to tap the +12 volt supply to make some pigment for model airplane paint.




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    i am not sure of the use of fan. today every PSU have a fan so anyway it will rotate it (the load needed)

    can i use 12 v DC fan istead of 10 ohm resistor

    Hi i need 12v adjust table 5amps to 15amps from my ATX power supply... what should i do?

    Hello I need 12v 5amp power supply from pc old smps. What i need to do?

    Only improvement I would suggest is replacing the 10-Ohm resistor with something more useful like a DC lamp and/or fan.

    3 replies

    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.

    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.

    2 replies

    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))

    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.

    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

    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.

    1 reply

    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.

    Could you use this to power the LED lights in a dollshouse?

    1 reply

    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