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 ?
Electrical – It’s enough to connect the screen of a transformer only at GND and not to earth
transformer
Related Solutions
Electronic – What use is the copper foil between the primary and secondary windings of a transformer
It's there to reduce the coupling of high-frequency noise from the AC line, switching regulator harmonics, or other 'unclean' energy into the secondary winding. The shield won't improve the magnetic coupling but it won't hurt it either, and energy at higher frequencies that would have been transmitted via interwinding capacitance will be attenuated.
Part of a defense-in-depth EMI control strategy, basically.
What you may be getting confused about is the "ideal transformer" in the equivalent circuit. You should not regard it as having any magnetic qualities at all. Try and see it like this: -
Whatever voltage you have on the input to the ideal transformer, \$V_P\$ is converted to \$V_S\$ on the output of this "theoretical" and perfect device. It converts power in to power out without loss or degradation such that: -
\$V_P\cdot I_P = V_S\cdot I_S\$
The ratio of \$V_P\$ to \$V_S\$ happens to be also called the turns ratio and almost quite literally it is on an unloaded transformer because there will be no volt-drop across R3 and L3 and only the tiniest of volt-drops on R1 and L1.
This means you now have a relatively easy way of constructing scenarios of load effects and recognizing the volt drops across the leakage components that are present.
The equivalent circuit of the transformer is really quite good once you accept that the ideal power converter is "untouchable" and should just be regarded as a black box. For instance you can measure both R1 and R3 and, by shorting the secondary you can get a pretty good idea what L3 and L1 are. With open circuit secondary you can measure the current into the primary and get a pretty good idea what \$L_M\$ is too.
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Best Answer
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.