I am not happy with my last explanation, so am going to try to be clearer.

First of all, not all power supplies have a switch on the back.  For the 
ones that do, the switch is sort of a master switch that disconnects the 
whole unit from the current source.  Maybe you can turn it off when you go 
on vacation or during an electrical storm.  I don't know what else it is 
good for.

When the switch is on, or when there is no switch, the power supply is live, 
but is not supplying full voltage to all of its wires.  Some voltage is 
there, as you can see on motherboards with led's on them, which are lit up 
even when the computer is not running.

The on/off switch on the computer case front is not actually a switch so 
much as it is a push-button, like a doorbell.  When you push it, it tells 
the motherboard to boot up, and when you hold it down, (depending on the 
cmos setting) it tells the motherboard to shut the computer down.  The 
motherboard actually controls the power supply, not the case switch.

So, the power supply can be plugged in and the switch on, but the fan is not 
spinning, because it is alive but mostly hibernating.

When the motherboard tells the power supply to start up, it now powers up 
its main power circuits, but only if there is a load connected to it, a hard 
drive, for example.  Othewise, it will still just sit there, appearing dead. 
If it powers up, then the fan starts spinning.

You can run a crude, outside-the-box test of the power supply using a paper 
clip and hard drive.  Directions are at:

http://www.pcpower.com/support/ATX_troubleshoot.htm

This is not a sure-fire test as to whether or not  the power supply is good. 
I have had a power supply pass this test, but still not work.  But, if the 
power supply does not pass this test, then it is surely defective.

Be very careful when testing the power supply.  While the voltages are low, 
there can always be a high-voltage short....

Dean Kukral

----- Original Message ----- 
From: "Paul Hachmeyer" <[log in to unmask]>
Sent: Monday, October 30, 2006 11:34 AM
Subject: [PCBUILD] power supply question


I  recently purchased 2 Ultra power supplies from Fry's in sealed boxes, one 
a 400W one a 500W.  Should I be able to detect any voltage on the plugs 
without connecting them to a motherboard?  There is no power-on light on 
either unit, the fans do not spin up when I plug them in, the power cord is 
good.  I switched them both on/off with a working voltmeter connected to 
either 12v or 5v pins, no voltage detected on either supply.  Both set for 
110/120 volts as well.
Paul Hachmeyer

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