Ground Loop Problem!!

You have a serious problem. Please get professional help.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

power

chassis

causing

I suspect capacitive coupling between the primary and secondary circuits of the power transformer. If the transformer is not shielded this leakage capacitance couples common-mode signals to the secondary.

Euthymios Kappos

Capacitive coupling in a working transformer is only going to be a few pF, certainly much less than 1nF. At 120V 60Hz, a 100pF capacitor will pass

5uA, if my math is correct. That's not enough to blow transistors (as the OP claims is happening). If he's getting that kind of current between the chasses of two mains-powered devices, then as Jim said, he's got a serious problem and he should seek the help of someone who's got test equipment and knows how to use it.

leakage

and

Mr OP doesn't describe his set up too well, oh dear......

I think....

C = Eo.Er.A/S

Take a small switch mode transformer with a width of 25mm and a winding length of 25mm for an area of 25mm x 25mm.

A = 6.25E-4 M^2

Eo = 8.85E-12 Something or another

Er = oh..... call it one

S = 1.5E-4 M

S is the winding separation. 3 layers of 2mil (mixed units!!) yellow stickey stuff.

C = 8.85E-12 x 6.25E-4/1.5E-4 C = 37pF

Turn 120VAC into 170VDC for the supply. Switch at 100KHz and ignore the harmonics. That's about 4mA.

Not totally insignificant...... but, sort of, not enough to blow up transistors.

Unless;

It's an SMPS with feedback from secondary to primary via some thing, probably an opto-coupler. If you don't pay attention to a bit of judicious capacitive/resistive coupling between primary and secondary then the resulting common mode hash is likely to send the feedback loop to apesville and things might well go bang.

If you rolled you own then you'd make sure it wouldn't.

Then you'd run it up and see what the nasties were. Couple primary to secondary wiv a bit of Class Y. Preferably, primary to ground and then back from ground up to secondary.... using two of them and taking care of regulatory requirements. Depends on where you live.

See how it rings.... wiv your 20MHz analog(ue) scope, do some sums.... add a bit of appropriate R in series with the Y to settle things down.

If you've bought an El-Cheapo from wing-tong-tiddle-aye-po then life might become a bit harder.

There you go. I do know about EMC type stuff, next question.

DNA