Strange voltage on floating chassis.

[Arx]

I just finished my gainclone and am noticing a really weird phenomenon. The chassis feels like it's got a slight voltage running through it when you brush your hand against it. It only happens when the power supply is connected (it's external), but i've checked and there are no shorts to chassis, and checked each terminal to chassis with my vom on the megaohm range.

Any guesses what could be the cause? I was thinking perhaps it was in the magnetic field of the transformers, but it doesn't seem to matter if I move it a couple feet away.

-Nick

[quasi]

If you can "feel" a voltage (apart from static electricity) then you have a potentially dangerous situation. You should check the voltage against an earth reference with your meter. It could be that you just need to earth your chassis or it could mean something else. So for peace of mind try to isolate what it could be.

Cheers

[Arx]

I measure a couple volts when I've got a source connected, and around 20vac with nothing connected.

what I'm really wondering about is where the voltage is coming from.
you can hardly feel it, it's most obvious if you lightly drag your knuckles across the chassis.I'm not particularily concerned. I haven't got the mains supply shorted to the chassis or anything. It's running off a pair of transformers which aren't in the chassis, so it's isolated there, and I'm sure there's nothing shorted (and besides that, what could short

the question is merely, how do you get 20vac in a floating chassis that isn't shorted to anything.

could it be rf? if so, it feels like it's modulated at 60hz, since I can definitely feel a buzz.

I can also feel it at 2v, assuming my meter's not lying, which I think it likely is.
I wouldn't have noticed it at that low level, but since I know it's there I can feel it if I'm trying.

-Nick

[quasi]

Most digital multimeters have too high an input impedance to accurately measure "stray voltages" because they are susceptible to RF and other electromagnetic inteference. Try putting a 47k resistor across the meter leads while you measure. This will give you a more accurate reading.

Cheers

[Arx]

Originally posted by quasi
This will give you a more accurate reading.

Unless it is HF noise which I'm feeling

Maybe if I can't figure it out otherwise I'll hook it to earth through a series resistor and cap and see if I can discern the frequency range.

[Nordic]

How is your starground connected to chassis?

[pwillard]

Yes, it sounds like you have your MAINS wired incorrectly. You might want to recheck your 3 wires coming from the wall plug and make sure they terminate at the correct locations in your case.

PW

[dnsey]

Could be a leaky mains transformer, but if you're using a type which can develop such symptoms, you should also use a grounded chassis.

[sangram]

The chassis should be shorted to mains ground. If you're not shorting *anything* to the chassis you should add a small tag on the back of the chassis and connect it to a good ground reference (such as the chassis of the CDplayer/preamp).

Sometimes a fully floating chassis may have some voltage on it due to induced voltage from a component somewhere else. Chips have a dielectric between them and the chassis. A bolt may be very close to a PCB track. There may be some voltage on the shield of an upstream component, with the sockets connected to chassis.

[ralphs99]

In countries with 220V mains...

For safety, a chassis made of metal or with metal parts containing components at mains potential must be protectively Earthed to the mains Earth. (with exceptions for double insulated equipment)

The green/yellow mains Earth wire must be strain relieved and securely connected to the chassis immediately it enters the chassis. The wire should be terminated in an eyelet by crimping and passed through a screw, nut and star washer combination. Make sure of good contact between the chassis and screw by removing any paint or insulating coatings on the chassis under the screw assembly.

You should also check that your mains outlet is Earthed. I have come across outlets and extension cords that have been mis-wired or are missing an Earth.

I'm not sure of the specific requirements for Canada, but following the European 220V requirements is good practice even when they are not strictly necessary eg the US market.

The cause of the symptoms you describe can be a number of things, most probably parasitic capacitances in the power transformer or Y-capacitors if a mains filter is employed. There are specific requirements for maximum allowable Earth leakage for various equipment types. If in doubt you can assume 0.5mA.

Cheers,
Ralph