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
>
>
>
Visit our website regularly for FAQs,
articles, how-to's, tech tips and much more
http://freepctech.com
|