Newbie ground / earth voltage difference question

[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?

[Zero999]

The neutral should not be connected to the negative side of the power supply so it shouldn't matter which way the live and neutral are connected. If the neutral is connected to the negative side of the power supply then it's a fault with the PSU, not the mains wiring.

Normally I'd say it's leakage across the Y capacitors but with PC power supplies the 0V is normally connected to earth or will be at least capacitively coupled to earth. It sounds like you need to check your earth bonding. Check the resistance between the PC's metal case to the plug's earth pin, it should be near 0 Ohms, then check the resistance from the 0V side of the power supply to the metal case, again it should be near 0 Ohm. If possible do these measurements with the motherboard and PSU in place.

You could check the resistance between the 0V rail and the live and neutral, it should be very high, 1M at least. When you do this, connect the live and neutral together and measure between them and the 0V rail.

[PetrosA]

The OP stated that this is a step down transformer - he didn't mention anything about a power supply. There are step down transformers with one (neutral) side of both windings bonded for polarity controlled connections. If the feed polarity is reversed then the bonded side will pass through the mains voltage to the "low voltage" side. The low voltage load will operate correctly, but the low voltage side will have one terminal with mains voltage available.

GeoffS could also be correct. If there is a high resistance connection in the mains neutral there could be neutral load seeking a path back to the mains transformer through the ground system - if the two are bonded together in Australia.

[Zero999]

Oh I see what you mean, I thought he was talking about a PC power supply but it could be an autotransformer, there again he said negative which implies it's a DC power supply.

If it's autotransformer it's not classified as SELV, is not safe to touch and should be treated in the same manner as the mains because even if the polarity is correct, it's possible for the winding to burn out leaving the secondary floating at mains voltage, the neutral could be broken or even disconnected before the phase when the plug is removed from the socket. All connections to the secondary side should be insulated and any metal enclosure should be earthed or double insulated as is the case with the mains.

[eenewbie]

Yeah it is a DC step-down transformer (a typical wall-wart). There was a 0.1v DC difference between the transformer and the PC case... aside from that I haven't figured it out yet. I've also noticed the metal shell of some equipment, like a rechargeable DVD player, has a strange sensation to the touch. It feels like a 50hz hum but there's no electric shock - it's just a subtle vibration I can feel when I brush my finger over the metal casing.

Edit: I just plugged in the transformer again and now I don't get any electric shock whatsoever. How odd. Does this back up tyblu's isolation / floating comment?

[GeoffS]

Edit: I just plugged in the transformer again and now I don't get any electric shock whatsoever. How odd. Does this back up tyblu's isolation / floating comment?

Was it plugged into the same outlet as before?

[eenewbie]

Yes.

[Zero999]

Yeah it is a DC step-down transformer (a typical wall-wart). There was a 0.1v DC difference between the transformer and the PC case... aside from that I haven't figured it out yet. I've also noticed the metal shell of some equipment, like a rechargeable DVD player, has a strange sensation to the touch. It feels like a 50hz hum but there's no electric shock - it's just a subtle vibration I can feel when I brush my finger over the metal casing.

I bet the wall-wart doesn't weigh much compared to the older mains adaptors.

Then it's not a transformer but a switched mode power supply like I said before. A transformer only converts one AC voltage to another, for DC a rectifier is required. Older mains adaptors consisted of a transformer with a rectifier and filter capacitor connected to the secondary. Most modern mains adaptors use a switched mode power supply, consisting off: a rectifier and smoothing capacitor (AC to DC or the same peak voltage as the mains, 325VDC for 230VAC), an oscillator (DC to PWM squarewave AC), a small lightweight transformer (AC to AC of a lower voltage), a rectifier and smoothing capacitor (AC to DC) and a feedback circuit to alter the duty cycle of the oscillator to keep the DC voltage constant.

The advantage of an SMPS is it's more efficient, weighs less and the voltage regulation is better.

Unfortunately there's lots of high frequency AC noise on the DC side which is capacitively coupled via the transformer. To get round this there's a small capacitor connected between the low voltage DC side and high voltage DC side to divert the noice back to the DC ground. The problem with this is a small amount of current from the mains leaks from the AC side to the DC side. The current is too low to hurt you but you can just about feel it.

Here's a simplified schematic of an SMPS, the controller and MOSFET convert the DC to AC. Y1 is the capacitor which gets rid of most of the high frequency noise but leaks some of the mains onto the DC side.

https://www.eevblog.com/forum/index.php?topic=1997.msg27677#msg27677