A bad switch on a PSU

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A bad switch on a PSU

Postposted on Tue Apr 30, 2013 3:44 pm

So I encountered a dead computer the other day and it won't turn on. Opened up the computer checked to make sure all the PSU connections and the front switch connection was good and still doesn't work. Disconnected all the connections from the PSU, plugged in a few 120mm fans to the molex connections and shorted green to ground and nothing happens. At this point I noticed that the toggle switch on the PSU felt mushy. PSU was out of warranty so I popped it open, and checked to see if the switch made a connection with a multi meter and it doesn't. The switch is dead.

So I disconnected one of the wires going to the switch and soldered it to the other one to bypass the switch, and now the PSU and computer works fine and is completely stable. Thought I wonder if bypassing the switch would be bad for some reason?

I just thought that was the weirdest thing ever to have a toggle switch on a PSU to die.

A lot of PSUs don't have these switches so... what are the switches for? Do they serve any practical purpose? I remember there were some other models of PSUs that had switches like this but was for switching 240v <-> 115v.
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Re: A bad switch on a PSU

Postposted on Tue Apr 30, 2013 4:21 pm

I've also wondered why they even put a toggle switch on PSUs. It sits right next to the power plug which is just as easily pulled out if you need to cut power (pulling the power chord out of the PSU is probably better anyway). Bypassing the switch shouldn't make a bit of difference as far as I know.
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Re: A bad switch on a PSU

Postposted on Tue Apr 30, 2013 4:45 pm

IIRC it's there as an option to terminate all voltage into the PSU (including the +5V sb) while allowing you to leave the PSU plugged in and providing a ground for ESD.

If my memory serves me right the very old A+ curriculum stated that AT systems should be serviced unplugged. ATX meanwhile should be serviced while plugged in and master switch off (and naturally with ESD equipment like wrist straps in both cases).

The above is no longer part of the A+ curriculum.
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Re: A bad switch on a PSU

Postposted on Tue Apr 30, 2013 10:12 pm

Ryu Connor wrote:IIRC it's there as an option to terminate all voltage into the PSU (including the +5V sb) while allowing you to leave the PSU plugged in and providing a ground for ESD.

Yup. It allows the mains power to be cut (so it is safe to mess with stuff that might be connected to the +5VSB rail, which is always powered even when the system is off), while maintaining an earth ground. If the PSU doesn't have a mains switch, you can accomplish essentially the same thing by using a switched power strip.
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