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How do you test a power supply under load?

by apchar / March 27, 2008 3:42 AM PDT

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?

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Compare with Tom's.
by R. Proffitt Forum moderator / March 27, 2008 3:51 AM PDT
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thats a fancy rig. too fancy.
by apchar / March 27, 2008 4:02 AM PDT
In reply to: Compare with Tom's.

Toms test rig looks nice but I'm not going to shell out that kind of cash for a rig I'd only use once in a blue moon. His write up says the rig uses resistors but little detail is given. I'm using resistors too.

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You didn't detail it either.
by R. Proffitt Forum moderator / March 27, 2008 4:07 AM PDT

That 2 ohm would be some 6 amperes. I saw one rig where they tried that on just one connector. Try using 1 ampere per connection to remove the loss from wire and connectors. Or open up the PSU and measure directly on the PCB.

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loss should be minimal
by apchar / March 27, 2008 4:15 AM PDT

The wires in this thing are at least 18 Ga. The loss at 6A should be negligible. The 3.3V line is rated for 32A.

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Just sharing.
by R. Proffitt Forum moderator / March 27, 2008 5:38 AM PDT
In reply to: loss should be minimal

I have an oscilloscope and a hot iron (and more.)

So my advice is to not drop such a load on one wire and connector. The last time I checked it was only good for 2 amps per.

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There are 3 3.3V rails
by apchar / March 27, 2008 6:44 AM PDT
In reply to: Just sharing.

There are only 3 3.3V rails to carry 32A. That's 10.7A each. I'm only trying to pull 4.4A. I ohm'd them out and they're all connected internally.

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The stock molex connector is not rated for that much.
by R. Proffitt Forum moderator / March 27, 2008 8:18 AM PDT
In reply to: There are 3 3.3V rails

You are over 2x it's rating so spread that load to at least 3 leads for plus and the ground (6 connections total.)

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stock molex is good to about 9-12A
by apchar / March 28, 2008 3:25 AM PDT

Stock molex connectors are good to between 9-12A per terminal. I'm only pulling 4.4 and only for a few seconds.

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Just checked the specs again.
by R. Proffitt Forum moderator / March 28, 2008 3:45 AM PDT

It's either you are over or the PSU is toast.

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have you tried a power supply tester?
by ramarc / March 27, 2008 4:34 AM PDT

if all you need is a thumbs up/down, a $20 unit will do the trick.

antec's: http://www.antec.com/us/productDetails.php?ProdID=77203

this one's gotten good reviews:

if you really want to test the output quality of a power supply under various loads, you need something like a SunMoon SM-268 or SM-8800. but that's professional equipment and i don't think you want to speed $5K+ to test a $50 power supply.

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It looks like they just check unloaded lines
by apchar / March 27, 2008 6:38 AM PDT

Looking at the specs/features for all but the SunMoon devices these testers only verify the unloaded voltages. I can (and did) do that with a multimeter. The PSU looked fine unloaded. It's when I threw a load on that the voltages dropped so much.

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sun moon's can apply and monitor load
by ramarc / March 27, 2008 8:34 AM PDT
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SunMoon device is might pricey.
by apchar / March 28, 2008 3:29 AM PDT

Is there anything in these SunMoon things besides some power resistors, switches, and multimeters? I couldn't afford one anyway. I should be able to accomplish the same thing with what I have. I just don't know if there's anything more to it than just hooking resistors across the pins for each rail and shorting the PSON pin to ground.

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you can buy a
by Noobie Tuesday / March 28, 2008 12:13 AM PDT

Digital multimeter that includes and RS232 hookup and electrical monitoring software

sears has one or you can buy a tesla they are more pricey but the electronics in the multimeter are a higher grade

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I'm using a multimeter
by apchar / March 28, 2008 3:30 AM PDT
In reply to: you can buy a

I'm using a fluke multimeter. It's a very accurate DMM.

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