Electronics > Beginners

Newbie ground / earth voltage difference question

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eenewbie:
If I hold the negative terminal of a 12v step-down transformer and touch the case of my computer, I get an electric shock. Why is this? I had been using my computer as an anti-static ground plane when working on said 12v device. Now I think it wasn't such a good idea!

Why are the negative terminal of the step down transformer and the computer case (presumably earth?) coupled like this?

JohnS_AZ:
If you can feel the shock it's certainly more than 12V. Have you put a meter across it?

PetrosA:
It could be that your step down transformer is passing through what should be the neutral from the mains, but if you don't have a polarized plug it could be passing the line voltage through. This shouldn't be the case if you live in N. America unless your outlet is wired backwards, but could easily be the case if you live in the EU where outlets are not wired for standard polarity. This won't affect a low voltage AC appliance since the neutral will be stepped down (AC flows both ways...) but if you touch the negative lead and a mains ground or neutral, you'll get the full mains voltage since the transformer can't act as the buffer anymore. I'd definitely measure the voltage on the negative lead before using the transformer any more.

tyblu:
the transformer is probably isolating, which is a good thing, but it means its potential relative to anything else floats. So keep grounding yourself!

GeoffS:
As you're in Australia, I suggest that you have a neutral problem as well a poor earth connection.

All power points have to be earthed so even with a bad neutral, there should be little if any voltage differential between neutral and earth,
Is your power cable a standard, earthed IEC type?

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