ACShielded Xformer Mystery Wire

Apr 07, 2007 3 Replies

Hi all,



I found a few lower power shielded transformers at a flea market, and they do 120 to 40 CT VAC, so perfect for small bipolar power supplies.. They're steel-shelled with 4 mounting screws on the bottom and the transformer is potted into the shell, so I can't get a look at it without destroying it.



There's 6 wires coming out of it, the 5 identified and a mysterious grey one which is insulated from all the others ( inf. ohms) but reads



5 different voltages ranging from 2.8 - 65 VAC when measured against each of the other ones. This one and all the others are insulatd from the case, so its not a case ground wire.

So:


  1. Could this wire connect to an internal shield to minimize capacitive coupling or is it something else?


  1. If the shield, are these voltage readings normal for such a connection?

  2. If the shield, should I connect this wire directly to AC earth ground, or to chassis ground?

I'll just ignore it if I can't figure it out, but if it is some kind of shielding it would be nice to take advantage of it properly.



Thanks for any help.


That appears to be the case.

Normalish I expect. There'll be some capacitive coupling.

Yes.

Both ideally.

Connect it to ground.

Graham

Sometimes it is just another winding with one end unconnected and is a shield. Ground it.

If it is an electrostatic shield, the coupling would be electrostatic. You do not say how you measured the voltage. My guess is that you used a relatively high impedance meter. 100pF will have a reactance in the neighborhood of 25Mohm. With that, I would expect considerable voltage readings. Put about 100kohm in parallel with your meter to see if that changes your reading.

Bill

-- Fermez le Bush--about two years to go.

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