# Use PSU as replace for broken source +12Vdc 3.33A?

I need 12Vdc 3.33A source power for Silicon Graphics 1600sw display, I have two ATX computer power supply. Is it possible to use these PSU as replace for broken 12Vdc 3.33A adapter ?

UPDATE: I connected the 12v 35w halogen lamp for provide a small load to +12 VDC(Yellow) and Ground(Black), then I turn on the PSU by shorting the PS-ON#(Green) and Ground(Black), DCV show about 11,43 V, so I connected the monitor and it works!

It should be possible and not too complicated. There are many guides explaining the process. Here's one.

UPDATE:
Some computer PSUs may already have needed load installed and can provide stable voltages out of the box, but you can't rely on that.

The instructable recommends use of $10\mbox{ }\Omega\mbox{, 10 W}$ resistors for $\mbox{+5 V, +12 V}$ and $\mbox{-12 V}$. The $\mbox{12 V}$ resistors are going to be dissipating around $\mbox{14,4 W}$, so my recommendation is to get some bigger resistors, just in case.

The other option would be to use resistors with higher resistance for the 12 V lines, for example $22\mbox{ }\Omega$. They'll use less current, but I think that it won't be a major problem because the screen will use considerable amount of power too.

This isn't mentioned in the instructable, but I'd put some $10\mbox{ }\Omega$ or $22\mbox{ }\Omega$ resistors on the $\mbox{+3.3 V}$ line too.

Another part that has been omitted is how to locate the power supply rails inside the power supply itself.

Here you can find the color code for computer supply wires. Just follow the wires inside the case of the supply and see where they end. The wires you're interested in are yellow for $\mbox{+12 V}$, orange for $\mbox{+3.3 V}$ and red for $\mbox{+5 V}$.

Don't forget the LEDs for gray and purple wires. You can use tool like http://ledcalc.com/ to get the correct resistor for the LED.

• can you explain the process over here? – Kellenjb May 23 '11 at 13:42
• @Kellenjb Indeed. I seem to be getting it wrong these days. – AndrejaKo May 23 '11 at 16:03
• Excellent edit! I TeXified the rest of your numbers to try to make it look more consistent. – Kevin Vermeer May 23 '11 at 17:18

The biggest problem with using the 12v rail on the PSU is that it is designed to pump out a lot of current, and as a result wont provide stable power at lower currents. However, 3.33A is a pretty sizable amount of current and will probably work out just fine.

I am not sure about details of the graphics display, but I would recommend you look at the datasheet to see if there are any specifics on the amount of voltage swing it is able to handle.

If you want to double check your PSU before you plug it into your display, you could get yourself a high wattage resistor, plug it into the PSU, and then measure the voltage. 12v/3.33a = 3.6 Ohms at 39.96 Watts. On second thought, that is a lot of watts to put in a resistor and you will probably have a hard time easily finding one. You might have better luck finding an 8 Ohm 18+ watt resistor.

The other issue at hand is being able to turn on the PSU. PSU's are designed to be turned on by the motherboard, and so you will need to fake the PSU. The way that you do this is by shorting the PS_ON# pin to a COM pin. Usually the PS_ON# wire is green and all of the COM pins are black. It will probably be beneficial to put a switch here instead of a wire so that you can easily switch it on and off.

You also mentioned that you have two PSUs. I am not sure if you were thinking about using both together, or just saying you have some extra. Just in case you were thinking about it, I would not try connecting the two together. Depending on the design of the PSUs they can end up fighting each other some, and bad things happen when electronics fight.

One other problem I've seen with switching PSUs is subtle high voltage spikes in the output. The quick and easy way to deal with this is a neon lamp across the output. If it lights even dimly, you HAD a problem and are dealing with it. I've seen this on many PSUs.

An easy way to test it would be automotive headlights at similar wattage. These are much easier to obtain than power resistors of that size. Get a replacement headlamp for your car that you will eventually use or something like that.