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Power supply Tricks – Turn on a power supply without a computer!

So today I am going over a very useful trick that many do not know about. That is how to turn on a computer power supply unit (PSU) without a computer… Please read my disclaimer about this at the bottom.

I call this “jumping” a PSU, or a “jumped” PSU. Much like if your car was not turning on, you would jump it (with a nother car or high amp car battery charger). I’m not sure if there is a correct term for this, but I’m using it.

First off, why do I want to do this?

Generally for testing things I like to use a jumped PSU to test with. This will include everyhting from fans, to optical drives, to case lighting. You cant really used a jumped PSU on motherboard; so everything that absolutely requires a motherboard, you will not be able to test (video card, processors, ram, etc…).

IF you are into automotive stuff, a computer PSU can provide switched 12V DC power, much like a car battery. a computer PSU is smaller and lighter than a car battery, and you do not need to worry about charging the battery and can leave it on as long as you have AC power running.

The other thing was once up-on a time when I did not have as much money, and needed a larger power supply for the pump in my water cooling system, I actually used two power supplies at once in my system. Yup, you heard it right, two power supplies ghetto-rigged to turn on at the same time in one computer. This is where have an abundance of spare computers and parts came in handy, because the mod required a second power cable and a second PSU.

Again with water cooling, I needed to both test a loop and bleed my system for air. If you have done water cooling you know how important it is to do a leak test; and how shoudl not do this with a running computer. Instead I used a jumped PSU to provide power to the pump to leak test; this allowed me to assemble the loop fully onto my computer while it was off.

So how do we do it?

Fairly simple (I mean I was able to do it!). It requires shorting one of the wires on the main ATX power connector (this is the large 20 or 24 pin connector, it will have two rows of pins and is generally the largest connector).

If you have an older PSU that does not have the large ATX power conenctor, chances are you are using an old 10+ year old power supply, or possibly something proprietary… I have seen Dell power supply which have two of these large connectors, and I have seen some small form factor PC’s have some really odd connectors.

What you are looking for on the main ATX power connector is pin is pin 16 (or 14 for 20 pin connectors).  if you were looking directly in the connector (as if your eyes were the connector on the mother board and you were about to power your face), position the connector so it is horizontal and the clip is on top. This will then be the 5th pin on the top left; if you look at the wire for this pin, it will be a different color than all the other wires in the connector (usually green or purple).

This is the signal circuit. To activate the circuit you will need to short it out to the ground. The ground will be a black wire (there should be several to chose from, this could also be the 6th pin from the top right from the above description).

MAKE SURE THE PSU IS NOT PLUGGED IN. Now you can either cut the ground and signal wire and use electrical tap to connect them, use wire and shove them into the end of the connector, or what I like to do is use a bent paper clip and use electrical tape to avoid shocks. What ever you do, just please be careful, I doubt this will kill anyone but lets not try.

Now plug it in! you will probably have a fan in the PSU that will begin to spin. If it seems like your PSU is not powering on check to make sure you have the correct connector and pins. Also check the black rocker switch beside the power cable that its in the correct position (usually there is a “-” and “o”; “-” should be depressed). If it still doesn’t work, you may have a dead PSU (you probably got this PSU free or it came from a dead computer).

Some things to note about this “mod”:

This is a switched PSU. You should by definition not destroy the PSU if there is no load on it (however the fan should be inducing a small load anyways); but you may get unreliable voltage on lines that do not have other loads on them. You can either rig up some resistors and leds to do this, or some fans. I would make sure you have a load on both the 5V and 12V lines (these are red and yellow on the 4pin molex connectors).

I also want to note that you should test the voltages with a multimeter to ensure they are what you expect. “Dead” computer may have died because a component was over or under volted, but the power supply still works (sort of). 10% should be a good enough tolerance, but its always best to have it close as possible.

If you know about relays and how they work, you can probably find a cheap 10 dollar automotive relay and wire it to the 12V on one (main) PSU as a signal for the relay, and use the switch on the relay to close the circuit (that youve shorted) on the second psu, and you will then have two two PSU’s that will turn on at the sane time. You could do this with more PSU if you wish; my only suggestion to have all the secondary PSU’s attached to the main PSU (do not daisy chain them as relays can be slow, the PSU at the end could turn on much later than the first). Don’t get turning signal relays that turn on and off constantly, you will want a relay for like a fuel pump that will continually stay on all the time while signal is applied.

Disclaimer: Do this at your own risk. I will feel bad if you break whatever you plan on using with this mod, or if you hurt yourself or others, but I take ABSOLUTELY NO RESPONSIBILITY for what you do. I also provide no warranties that this will work (it will if you follow my directions, but I still won’t provide any warranties). What you do outside of my arms reach at the time, will not be on me, ever.



  1. Daniel on Friday 16, 2010

    great post, thanks for sharing

    • fyfester on Friday 16, 2010

      No problem, this comes in handy all the time for me. Outside of testing water cooling loops, I often will test case fans, case lighting, and use it to power drives that I will use with a USB adapter I have.