I understood both your explanations and I appreciate the info. Tested my 
old and 2 new power supplies with a jumper as indicated, all 3 provided 
power to spin up 2 hard drives.  Also powered up 2 cd/dvd drives 
successfully.  Thanks for the help.
Paul Hachmeyer  

Dean Kukral wrote:

>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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