PSU Question 3

[RayWilson]

Hi,

How do you test a computers PSU with a digital multimeter? We done this at college but my mind is a bit hazy. I'm also reluctant to just barrow ahead. We have plenty of these PSUs lying about but are sure that some are faulty. We understand how the digital multimeter works and want to get a reading of the voltage output of these PSUs. Where do you put the red and black probes in the power connectors???

Thanks

R

 

[dinosnake]

That's a more complex question than you think. There are 2 types of power supplies, AT & ATX. AT supplies 5V & 12V; ATX supplies 5V, 12V & 3.3V. But for a power supply to pass muster it must also supply the required current, not just the voltage. And of course, as you learned in school, to measure current a valid load must be applied across the circuit. And to finish it all off a ATX supply is remote sensing, so it must get signals across valid pins before it even turns on.

+5V is red, +12V is yellow on the drive connectors. For ATX specs see:

http://www.enhanceusa.com/documents/ATX12V.pdf

Your mileage may vary...

 

[edfair]

And there are minus voltages involved.
For AT, with board plugged in, black lead of VM to black leads from PSU, red should be +5v, balance should be +/- 5 or 12 except pin 2 which is a power good signal that may float somewhere above 2 volts. Ed Fair
unixstuff@juno.com
Any advice I give is my best judgement based on my interpretation of the facts you supply. Help increase my knowledge by providing some feedback, good or bad, on any advice I have given.

 

[craigey]

Raywilson I just stumbled across this power supply load resistor, which as dinosnake pointed out, would be required to measure the voltage on the PSU's. Check out the link below. (it's quite long)!

[URL unfurl="true"]http://www.cyberguys.com/cgi-bin/sgin0101.exe?UID=2002062303514390&GEN6=00&GEN9=5CG01&HKW=5CG01&T1=113+0282&UREQA=1&UREQB=2&UREQC=3&UREQD=4[/URL]

Cheers
Hope you find it useful

 

[hotfusion]

I've heard that it can damage this type of PSU if you operate it without a load - that may or may not be true, but beware of simply switching it on to test the voltages. My suggestions are what I would try myself. If incorrect, I welcome corrections to my rather limited knowledge. Andy.

 

[RayWilson]

Thanks guys - these posts are inciteful!

Ray

 

[GrafSpee]

Here's how to turn it on w/out a mobo.

http://www.exclusivepc.com/Guides/guide.asp?guide=TurnonaPSUwoMotherboard