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A have a transformer with a screen between the primary and secondary windings and I want to power an audio amplifier with it. My amplifier has no connection to earth, it is powered only by two wires. If I connect the screen only to GND of the power supply (between the filter capacitors) the garbage from the primary will pass to secondary ?

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It could be dangerous to use a power transformer that has an unearthed earth screen because if a fault develops, live could become connected to screen and, if the screen is not properly earthed, you have a dangerous situation. What do you know about this transformer other than what you are saying in your question?

Most transformers these days are designed to be "fail-safe" in that they use double insulation techniques to "guarantee" that this sort of fault is virtually impossible. Also, the use of an RCD (UK terminology) or a GFCI (N American) will tend to make the situation less dangerous but why take this risk?

Regarding the noise reduction properties of a screen, this is well-known to be a useful addition and this technique is still used in a lot of designs however, if that transformer is somewhat elderly you cannot rely on it being safe without the screen connected to earth.

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  • \$\begingroup\$ The transformer has just been made by a friend of mine who is really know what is doing. The insulation between primary and screen is very good. Anyway, I asked more theoretical if is some improvements if I connect the screen only to GND. \$\endgroup\$ – Marus Nebunu Feb 9 '17 at 19:32
  • \$\begingroup\$ If the screen is connected to unearthed local ground as soon as you connect a remote signal source whose signal (more-or-less) is grounded, you will create a big common mode AC voltage that might be coped with or might not. The common mode voltage comes from the live and neutral via the screen (by capacitance) transferring a common mode voltage that is approximately 50% of the AC line voltage - in effect this is your amplifier's 0 volt signal ref. point. You should be able to touch local ground (unearthed) and feel a slight tingle as it pushes about a milli amp through you. \$\endgroup\$ – Andy aka Feb 9 '17 at 19:47
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    \$\begingroup\$ You may even damage equipment that you connect to the input. Just think about what the screen is when it isn't earthed - it's aplate of a capacitor and the primary winding is the other plate. \$\endgroup\$ – Andy aka Feb 9 '17 at 19:53
  • \$\begingroup\$ OK, I gues I better leave it unconnected... Thanks. \$\endgroup\$ – Marus Nebunu Feb 9 '17 at 19:57
  • \$\begingroup\$ As long as you believe it to be safe - personally I'd double check my answer with the guy who wound it just to see what he says. \$\endgroup\$ – Andy aka Feb 9 '17 at 20:01

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