ACShielded Xformer Mystery Wire

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?

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

3. 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.

 

[Eeyore]

nope@noway.net wrote:

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

That appears to be the case.

or is it something else?

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

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

3. If the shield, should I connect this wire directly to AC earth
ground,

Yes.

or to chassis ground?

Both ideally.

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.

Connect it to ground.

Graham

 

[Homer J Simpson]

<nope@noway.net> wrote in message
news:5blf13p0q6bmcoooej4s2gns9dep3dkjf6@4ax.com...

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

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

 

[Salmon Egg]

On 4/7/07 10:32 AM, in article 5blf13p0q6bmcoooej4s2gns9dep3dkjf6@4ax.com,
"nope@noway.net" <nope@noway.net> wrote:

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?

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

3. 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.
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.