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Had some trouble with an ATX computer not powering up -- realized the power switch does not directly switch the 115 VAC -- wrote to the lvlug and tried some things, the following summarizes what I learned.

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On Monday 02 December 2002 10:21 am, Linc Fessenden wrote: > ATX uses a switched power supply. Before trying the power supply on
> a machine you know works, simply unplug the power jumper from the MB
> and use a paperclip to quickly short the 2 pins together. If the
> system starts then, you have a bad front panel switch. My best guess
> though is that you have a defective powersupply. I have noticed that
> atx powersupplies, specifically the cheap ones, go bad rather quickly
> as opposed to the old at PS's, many of which are still running smile

Thanks, Linc! I got it working (I didn't fix it, I just got it working). Did things like pulling the connector off the motherboard and putting it back (in the course of measuring the voltage across the switch) and pulling the main power connector off the motherboard and putting it back. Then I tried the system and it worked.

For anybody that's very interested:

The voltage across the "power" switch is 4 VDC (with the AC cord plugged in -- with the cord unplugged it quickly "decays", but to 2 volts, not 0 -- maybe I've found a source of free energy (or maybe the 3 Volt battery is in the circuit ;-)).

The voltmeter I used is fairly low impedance (20,000 ohms per volt), and just measuring the voltage across the switch was enough to make the power supply "change state" (i.e., to on if it was off, to off if it was on).

I then checked the voltage on the power supply connector for the motherboard (while not connected to the motherboard) -- looks like something in the power supply delivers 5 volts on one of the pins even when the power supply is off, another pin seems to carry the 4 volt "sensing" voltage used to detect the on-off switch. Interesting -- almost wish there was a problem that forced me to tear into the power supply. (Not really!)

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Contributors

  • () RandyKramer - 02 Dec 2002
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Topic revision: r1 - 2002-12-02 - RandyKramer
 
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