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Testing an ATX12V2.0 power supply before connecting it to the system board.

Posted on 2010-08-26
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Last Modified: 2013-12-11
I'm testing a new ATX12V2.0 power supply before connecting it to the system board. I'm getting low/no voltage on the 12V2 rail of the 24 pin molex connector and no voltage on the 4 pin molex connectors. Am I missing something or does it sound like the power supply is bad? This is probably a ridiculous question, but I have to check - the 24 pin connector doesn't have to be connected to the system board in order to read voltage from the 4 pin connectors, does it?
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Question by:Concentus
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jamietoner earned 167 total points
ID: 33538270
It doesn't have to be connected but it has to be on. PSU testers will turn it on when testing but lack the ability to test under load(which is a common PSU failure). If you are using a multi-meter to test to turn it on short pin 16(green wire ) to a black ground wire. If that's what you are doing then the power supply is probably bad.
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_ earned 166 total points
ID: 33538646
What he said.   : )
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by:nobus
nobus earned 167 total points
ID: 33539359
you can test all the voltages as well on the connector, here the links for 20 and 24-pin :
http://pinouts.ru/Power/atxpower_pinout.shtml
http://pinouts.ru/Power/atx_v2_pinout.shtml
and yes - it must be running to measure; i usually connect an (old) disk or cd drive to give some load..
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Author Comment

by:Concentus
ID: 33544756
Thank you for the responses. Am I understanding right - I'm probably not getting any voltage on the 4 pin molex connectors (usually supplying power to the hdd, dvd/cd drives, ...) because there's no load? If I plug the 24 pin connector into the motherboard, the 4 pin connectors will "turn on"?
Probably, I should have mentioned how I'm measuring voltage. I'm using a Manhattan power supply tester (with dummy lights and digital readout) and a multimeter. When I plug the power supply tester into the 24 pin connector, I get LL on the 12V2 rail and persistent beeps (the other digital readings look good). When I plug the tester into the 4 pin molex connectors, I get nothing - no lights or anything. If I measure the voltage on the 4 pin connectors with the multimeter, I get no potential between any of the pins, or between any of the pins and ground.
These readings I've been taking before installing the power supply.
When you say it must be running or it must be turned on, do you mean it has to have a load? I have it plugged in to the surge protector - which is powered on. It sounds like I have to install it and connect the 24 pin connector and turn the PC on to have voltage on the 4 pin connectors.
I apologize for the long question. This is a new situation for me and I don't want to plug a faulty power supply in to the motherboard. I really appreciate your help with this.
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Expert Comment

by:jamietoner
ID: 33545042
The PSU tester should be turning on the power supply. You don't need load to get a reading on the molex connectors, it just needs to be on. It sounds like you have a bad power supply. Whats the make and model of the power supply?
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Author Comment

by:Concentus
ID: 33545133
jamietoner, I will check the make and model this afternoon and let you know. Thanks.
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by:Concentus
ID: 33546933
The maker of the power supply is High Power and the model is SI-A400m2 (it's an ATX12V2.0).
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by:nobus
ID: 33548468
no - that is not what i said,  i usually connect an (old) disk or cd drive to give some load.. because unloaded circuits can give false readings
by running - i mean it must be switched on - normally you can see the fan running then. as Jamie said : "to turn it on short pin 16(green wire ) to a black ground wire"
BUT since "I get LL on the 12V2 rail " and the fan runs on 12 V, you see nothing at all

if you have no 12 V - the PS is bad !
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Author Comment

by:Concentus
ID: 33554068
I'm not sure how it all worked out, but...
I plugged the power supply in (there's no switch on it like some have). The fan wasn't spinning. I plugged the 24 pin connector into the tester and saw the 12V2 reading LL as I mentioned. If I remember correctly, the fan was spinning then. I googled "12V2 LL" and found a site or two (including frozencpu.com) said it didn't necessarily mean the power supply was bad. So I tried plugging one of the 4 pin molex connectors into the 5 pin receptacles on the tester (Manhattan 101530) while the 24 pin connector was still plugged in, thinking that the reason I wasn't getting 5V on the 4 pin connectors was load related. Apparently, that was it. The power tester 5V light came on.
So I installed the power supply and everything worked fine.
Thanks for everyone's help.
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Author Closing Comment

by:Concentus
ID: 33554079
Thanks for all the help. I didn't quite understand at first. I appreciate the confirmations and clarifications.
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by:_
ID: 33554885
Unexpected, but Thank you.   : )
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