Computer power supply

Hi all, Read a post the other day about powering a 12v fan. A computer power supply was mentioned as a power source with a "trick" to get it to run. I want to use one for a touch screen but when I jump the green & black wires it won't turn on. Do I need to apply a load somewhere? What is the trick to using a pc power supply "out of the case"? Thanks, Tim

Tim Aberegg wrote:
  Quoted Text Here  
    To answer your question, you probably need a greater load on the +5 volt leg. Attach an old HD to the PS and see if that doesn't work. If that (the fan) is all you want to power up, I would use a +12 wall wart. They are smaller and more than sufficient for the fan.

  Quoted Text Here  
Here's a copy of the text of an article I posted 2 years ago in this newsgroup, describing how to test an ATX power supply. It pretty much answers your question (and a couple more that you are bound to have eventually). I do have one additional comment, concerning the minimum load on the 5 VDC bus. It seems that it varies from one supply to another, and I'm not sure if the specified "minimum load" is also what is required for the supply to turn on, or if it is just the recommended minimum load for proper operation. Whatever, many of the larger (400+ watt) supplies list minimum loads that range from 5 to 10 Amps for the 5 V bus. That should be taken into consideration!
Whatever, here's the original text:
There are two requirements for an ATX power supply to "act normally". One is the PS-ON lead (green wire) must be grounded (to a Black wire). The other is there must be a minimum load on the 5 volt line. I've looked around, and cannot handily find any specification on what the "minimum load" must be though. It seems that it is extremely small, and an old disk drive or cdrom will be enough. Another guess would be a 25 ohm 5 or 10 watt resistor, which would draw about 200ma of current across 5 vdc.
If you really want to test it, here's a proceedure, which assumes a 20-pin plug (the colors would be the same, but see below for a 24-pin plug pinout).
1) Plug AC in.
2) Measure pin 9 (Purple wire). Should show about 5.0 vdc.
3) Measure pin 14 (Green wire). Should show some voltage between 3.5 and 5 vdc. The exact voltage is not critical.
4) Unplug AC.
5) Put a load across the 5 vdc line. You can do this by using an old disk drive, cdrom, or a resistor.
6) Jumper pin 14 (Green wire) to ground (any Black wire).
7) Plug AC in.
8) A. The fan should run. B. All voltages should measure close to their nominal values (Note that -5v and -12v may be very poorly regulated).
Note that pin 14 is the PS-ON signal line, which is normally wired to the on/off switching circuit on the motherboard. It has a 1000 ohm pull-up resistor connected (internal to the PSU) to the +5v Standby line, and if there is no other connection it should probably read close to 5 volts. That voltage will cause the power supply to be "off". The nominal switching point for PS-ON is 0.8vdc, and standard operating voltages are less than 0.4 volts for "on" and more than 3.5 volts for "off".
Here is a list of leads on a 20 pin connector,
PIN COLOR NAME DESCRIPTION ------------------------------------ 1 Orange +3.3V Power, +3.3V 2 Orange +3.3V Power, +3.3V 3 Black GND Ground 4 Red +5V Power, +5V 5 Black GND Ground 6 Red +5V Power, +5V 7 Black GND Ground 8 Gray PWR-OK Power OK 9 Purple +5V VSB +5V VSB 10 Yellow +12V Power, +12V 11 Orange +3.3V Power, +3.3V 12 Blue -12V Power, -12V 13 Black GND Ground 14 Green PS-ON PS Remote on/off 15 Black GND Ground 16 Black GND Ground 17 Black GND Ground 18 White -5V Power, -5V 19 Red +5V Power, +5V 20 Red +5V Power, +5V
And this is a list of leads on a 24 pin connector,
PIN COLOR NAME DESCRIPTION ------------------------------------ 1 Orange +3.3V Power, +3.3V 2 Orange +3.3V Power, +3.3V 3 Black GND Ground 4 Red +5V Power, +5V 5 Black GND Ground 6 Red +5V Power, +5V 7 Black GND Ground 8 Gray PWR-OK Power OK 9 Purple +5V VSB +5V VSB 10 Yellow +12V Power, +12V 11 Yellow +12V Power, +12V 12 Orange +3.3V Power, +3.3V 13 Orange +3.3V Power, +3.3V 14 Blue -12V Power, -12V 15 Black GND Ground 16 Green PS-ON PS Remote on/off 17 Black GND Ground 18 Black GND Ground 19 Black GND Ground 20 White -5V Power, -5V 21 Red +5V Power, +5V 22 Red +5V Power, +5V 23 Red +5V Power, +5V 24 Black GND Ground
The -5 VDC line may not exist.
The +5 VSB supply is Standby Power, which supplys parts of the motherboard which are always powered up (to allow options like "wake-on ..." to work).
The PWR-OK line is at ~5 VDC if the AC input and the +5V and +12V lines are within specifications. If either the +5V or the +12V line falls to below the specified voltage tolerance or if AC is lost for more than one power cycle interval, then PWR-OK, will drop to ~0 VDC. (If it helps any, if AC is lost, PWR-OK is supposed to drop at least 1 ms before the +5V and +12V lines go below specified voltage tolerances!)

Floyd L. Davidson wrote: <snip of excellent guide to getting a computer power supply to work on the bench>
One little thing to watch for is that the 12 volt rails have their own regulators but often aren't included in the feedback loop controlling the main switching regulator.
So this not only means that you have to have a minimum load on the controlling output (normally, but not always only, the main 5V rail) to get the 12V rail to produce significant output, it also means that you may need a much higher than minimum load on the 5V rail to get the full output on the 12V rail, without instability. Some of the better supplies do take feedback from all the output rails, so don't need any load on any particular rail, as long as one rail at least is loaded.
Now it is normally fairly easy to identify the feedback trace from the 5V main supply output to the switch-mode regulator IC. Simply cutting that trace and connecting it to a potential divider across the unregulated input to the 12V (normally linear) regulator, means that the 12V will give full power under all conditions - often without needing any 5V load at all.
The basic PC power supply can be easily modded to produce a whole range of different voltages, with fold-back and over-current protection, variable voltage and current limits etc. They are now so cheap that many people often use them as the starting point for many applications. Far, far cheaper than starting from scratch..

  Quoted Text Here  
That is *exactly* the kind of information that *I've* been looking for!
(And I'm not snipping a word of it here, just so that this message will show up in my archive, and the next time someone asks I'll have this for a reference too.)
Thanks for posting.
  Quoted Text Here  

  Quoted Text Here  
Thank you both! Excellent info. It worked too! Hooked up a HDD and the touch screen to provide enough load as the HDD was not enough! Thanks again. Tim