ATX psu dc output connected to earth?

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mb82

223 posts

Master Geek

#183671 26-Oct-2015 10:32

What is the effect of one of the dc outputs of an atx psu (+ 3v, 5v 12v) coming into contact with the exterior casing of the atx psu?

Zeon

3916 posts

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#1413907 26-Oct-2015 11:38

Please don't rely on this as I am no expert.

My understanding of DC is that a circuit will need to be completed for current to flow. For AC that circuit can be with the Earth but with DC it needs to be with the correct terminal to provide the difference?

I don't think its a good situation. If the PC is broken and causing this I would be inclined to replace it. Also remember there are capictors within a PSU and they can shock you.

I have been shocked a few times and its not good!

Speedtest 2019-10-14

richms

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#1413911 26-Oct-2015 12:16

The 0v on an atx power supply is grounded to the case of it. If the 3.3 or whatever touches the case, it will shut down.

If one of the gutless -ve outlets touches the case, it will blow up that output because it doesnt seem to protect them. Dont ask how I know that ;)

Richard rich.ms

Aredwood

3885 posts

Uber Geek

#1414050 26-Oct-2015 17:41

The -VE outputs are generated from a flyback winding wound on the main smoothing inductor. Which is simply run through a diode and filter circuit. Which is why they don't output much power. And why the voltage often sags on them.

mb82

223 posts

Master Geek

#1414290 27-Oct-2015 07:36

richms: The 0v on an atx power supply is grounded to the case of it. If the 3.3 or whatever touches the case, it will shut down.

If one of the gutless -ve outlets touches the case, it will blow up that output because it doesnt seem to protect them. Dont ask how I know that ;)

Ok thanks. I am building a bench supply where a gutted atx psu case (used to contain the testing circuitry) will sit directly on top of another atx psu which feeds it. Was wondering if it would be better to insulate or shield the top psu case used for the testing circuitry from the bottom psu case used for supplying the power. I guess it is better to shield it to avoid the chance frying the supply psu. So if one of the dc lines in the case for the test circuit touch the case the supply psu will still work.

I am not modifying the bottom supply case.