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Testing power supply
eyemnatas Apr-28-02 12:28 PM
I work for a School and I have been getting alot of power supply problembs, due to the fact the school was built in the 50's and has some serious power issues, brown outs and over all failures in general. So because of this problem I have been getting alot of computers with what I believe to be power supply issues how can I verify weather or not it is a power supply ..... How do I test them with my multimeter?

1. RE: Testing power supply
Twinhead Apr-28-02 02:42 PM
In response to message 0
LAST EDITED ON Apr-28-02 AT 02:49 PM (EST)
Testing a PSU with a DigitalMultiMeter or an AalougeMultiMeter is only possible when there is a (old) working Mainboard connected to the PSU.
If it will start up, the PSU should be OK.
But if you insist on testing the PSU, here is some data:

Black is ALLWAYS the "Common"
ALL measurements are taken with the BLACK meterlead to a black PSU cable, and the red to the color who is beeig tested.
ALL cables of the same color are connected in Parrallel.
For instance: a red lead is ALLWAYS +5 Volt.
If you have a DMM, there is no problem testing.
If only an AMM, SET the Voltage switch on AC!
A Negative voltage is invisible on the scale of an AMM, and can damage it in the Process.
The highest voltage is + or - 12 Volt.
Set the meter at least to 20 Volts so you cannot damage it.
(Even on the + and - 12, the meter is not directly damaged.)

I assume ATX systems by default.
If an AT, the colors are the same Voltage, but the connectors are different.

See this URL for more details:

For the colors, go to this:


2. RE: Testing power supply
DJ Net2Infinity Apr-28-02 04:50 PM
In response to message 1
LAST EDITED ON Apr-28-02 AT 04:52 PM (EST)
You can test a ATX power supply without a motherboard, I do it all the time when troubleshooting. I am experienced with electronics ... and if you are not DO NOT TRY THIS.


Simply Jumper Pin 14 ( which is the green wire) to any of the ground pins which are 3,5,7,13,15,16,17 ( all which have a black wire). I use a paperclip to do so, just stick the paperclip into the connector like you would be pluging it into the motherboard.

You can then test your power supply as Twinhead mentioned above.

3. RE: Testing power supply
Twinhead Apr-29-02 08:02 AM
In response to message 2
LAST EDITED ON Apr-29-02 AT 08:03 AM (EST)
Hello Brian.

Your method of testing could be OK, but since there is no load, the voltages are even with a small defect in The PSU at their rated specs.

If a mainboard is connected, the load of the Chipset (And everything else on that board) will couse a good load to rally test the PSU.
For Instance:
If a +12V line is desighned to deliver 5 Amps, without load the voltage will be +12V.
With a connected board, a small defect in the + 12V ciquit will be seen.
For instance, If the +12V can only deliver a few hundred MiliAmps, where 5 Amps should be normal, you can see it dropping in Voltage.


4. RE: Testing power supply
lbyard Apr-29-02 12:23 PM
In response to message 3
A diagram for turning-on and ATX power supply is in our power supply FAQs at http://duxcw.com/faq/ps/ps.htm. Getting back to basics... the best way to troubleshoot these sorts of problems is with a know-good computer and a known-good power supply. Simply substitute before digging-out that multimeter (which I almost always never do). I'll betcha those computers are old and were bought from the lowest bidder, and they have cheap power supplies and motherboards, etc. in them (and the people who made the decisions are long gone). Larry

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