Power Supply

Will a power supply run while not installed in the comp. I have a comp that was plugged up and the power supply would not run..not even the fan...thinking the power supply was bad i got another supply and tried it out of the case and it would not run...not even the fan..i have power to the supply ...go figure......any ideas anyone??

 

Did you have the power supply connected to the motherboard ? As a half educated guess, I'd say it should work if it was at least plugged into the board.

 

I'm going to change my user name to Dumb_ButtLOL

 

Okay...so the power supply is maybe good...is there a way to test it other than with a motherboard? If the supply is good and i have no power...does that mean that the mobo is bad??.....Thank you for your patience

 

I suppose you'd need a circuit diagram of the power supply. When the power button is pressed when a pc is switched on, it must activate the power supply. So if you knew which of the wires from the power supply to the motherboard you needed to connect together, you could test it. BUT if you don't, then best not to mess with it.

* EDIT - Just been reading up on Pc power supplies. They contain dangerour voltages when connected to the mains, even if they appear to be off, with no fan spinning. Be careful.

If you want to look up more details about power supplies and wiring connectors, here's a link

http://www.pcguide.com/ref/power/sup/parts.htm

 

It can be done but unless you have experience reading diagrams and working with electricity it can be very dangerous as noted by 12quid.

 

:major Here's a read on the subject. But, as already mentioned, don't try this at home if you're not sure of what you're messing with. Best of luck.

http://help.lockergnome.com/windows2/Testing-ATX-power-supplies-ftopict450696.html

 

:major Hey, eye...you would get a chuckle out of this...I have been an electronics/electrical guy for 29 years, and I still feel the power occasionally.

 

LOL I know exactly what you mean thats why I let the guys getting paid for it do it unless I have no other choice

 

It depends on the design of the power supply, but I would not expect it to run.

They're switching power supplies. To oversimplify: they generate the required DC voltages by rectifying the AC input, and switching the rectifier output on and off extremely rapidly in such a way as to generate the required output voltage into the design load. If there is no load attached, the switching circuitry can't keep the output voltage down to the design output level. Some will switch themselves off to avoid damage to the unit.

 

:major Hey, Rob. I thought the idea was that, once the motherboard passed the POST, the BIOS turned on a switch and enabled the power supply outputs. I thought that was part of the ATX power supply spec. Maybe you can educate me a bit more about it. Anyway, I would say that is why dead's power supply wasn't powering up.

 

:major Uh, sorry....Morgan, not dead.

 

Well, maybe -- sort of. In other words, you've got a part of it.

The problem with your scenario is that the BIOS needs power to run the POST. That power comes from the PSU over the 20- (or 24-) pin motherboard connection. The POST checks peripherals like hard drives; they need power to be able to respond to the checks run by the POST. In other words, the PSU's outputs have to be live to run the POST.

I think what you're thinking of is the "power-good" signal. It predates the ATX spec, and maybe even the XT spec. (Does anybody remember those days?) The PSU has circuitry that monitors the voltage on its outputs; when those stabilize after turn-on, with voltages at the appropriate levels, the PSU switches on the "power-good" signal. The motherboard receives that signal, and starts the CPU. If the power-good signal doesn't come up, the system does not start. In a healthy system, the power-good signal will come up in less than a second after power is turned on.

I don't know about the ATX spec, but in the early days it was not unknown to build a reset switch into a system that didn't have one by wiring a momentary-close switch into the "power-good" line. Closing the switch shunted the voltage on the line to ground, killing the power-good signal. The CPU would stop when the power-good signal was lost, and would promptly re-set itself when the power-good signal returned.

Morgan, you might want to check the voltage on the power-good signal line. It's Pin 8 in motherboard connector in the ATX spec -- usually gray, (Pin 5 in some Dell configurations, usually orange) and the voltage should be between 3 and 6 volts. Nominal voltage is 5 volts. Check the voltage while the PSU has everything connected by poking the voltage probe into the back of the motherboard connection plug.

The reference I've used for the pin-outs and colour coding above is Upgrading and Repairing PCs, 17th ed., by Scott Mueller (Que Books), Chapter 19: Power Supplies