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Old 03-27-2008, 01:54 PM   #1 (permalink)
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Question Tried to test PSU with resistors and got strange results

About 2 months ago my PC croaked. It would start up and immediately shut down. The CPU had been running hot so I replaced it. No help. So I yanked the Antec SP-500 power supply to test it. I shorted pins 14-15 to get it to turn on.
With no load the voltages are what they're supposed to be. So I tried loading it down with some big (>100W) resistors from work. Below is the setup
Pins 14-15 shorted
2 ohm resistor across pins 10-15 (the +12V line)
.75 ohm resistor across pins 1-15 (the 3.3V line)
.75 ohm resistor across pins 6-15 (the +5V line)
A 10 ohm resistor across the 12V line on the 4 pin mobo power connector.
A fluke multimeter to measure the voltages.

I didn' t load the -12V line since the spec says the minimum current draw is 0 A.
All the other lines should be drawing more than their minimum (.5 A) and much less than their maximums.
When I fire it up the 3.3V line goes down to 1.3 V, the 12V line drops to 10.6V, and the 5V line drops to 3V. So I figured the power supply was dead. Just to verify my results I applied the loads to another power supply that I know is OK and got about the same result. So there must be something wrong with my setup. What could be wrong?
Thanks,
Apchar
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Old 03-27-2008, 02:04 PM   #2 (permalink)
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Default Re: Tried to test PSU with resistors and got strange results

Why did you feel the need to use resistors?

Test the power supply rails with everything hooked up and under load, either gamming or prime 95.
Tolerence is 5% +/-
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Old 03-27-2008, 02:10 PM   #3 (permalink)
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Default Re: Tried to test PSU with resistors and got strange results

I had to use resistors. I can't test it with the motherboard since the PC won't power up. I don't have any other load.
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Old 03-27-2008, 02:26 PM   #4 (permalink)
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Default Re: Tried to test PSU with resistors and got strange results

Did you ever consider the power supply not to be the problem?

Tried a window Repair?

Does it even POST?

What error message if any?

Tried booting with mobo outside of case?
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Old 03-27-2008, 04:27 PM   #5 (permalink)
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Default Re: Tried to test PSU with resistors and got strange results

I can't do a window repair if it won't boot.
It doesn't have time to POST.
There is no error message.
I have not tried booting with the mobo outside the case.
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Old 03-27-2008, 04:43 PM   #6 (permalink)
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Default Re: Tried to test PSU with resistors and got strange results

Try different RAM, in different slots.
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Old 03-28-2008, 01:36 PM   #7 (permalink)
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Question Re: Tried to test PSU with resistors and got strange results

Swapping RAM wouldn't explain the peculiar behavior of the PSU. I'd like to better understand these things. Is there anything more to testing a PSU under load than throwing some power resistors across the rails, shorting PSO pin to ground, and watching the voltages on a DMM? Do the loads have to brought up in some sequence? Do one of these pins operate as a sense?
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Old 03-28-2008, 01:58 PM   #8 (permalink)
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Default Re: Tried to test PSU with resistors and got strange results

There is no problem with the power supplies, the rails were the way they were because it was not under load, you had very little to nothing attached to it when you tested it.

So now, you said you tried different RAM and in different slots and you said you tried different power supplies, and you also said you tried POSTing with mobo outside of the case.

So now your next step, assuming you did everything you said, is to try a different motherboard.
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