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Which is the best secondary connection to clean the main line power from EMI, line noise and HF harmonics by using an isolation 1:1 230/230VAC transformer with balanced outputs:

1) A floating (C1 and C2 are enough)

2) A to C to ground via a small capacitor

3) A to B ground directly

After having determined that connection for EMI suppression purpose ONLY, how to connect D ground output just for safety purpose (D to A, D to B, D to C, D to nothing)?

FYI: the transformer is multi shielded and each shield is connected to B ground. It has also a copper belly belt, currently floating.


I'd not expect a transformer to eliminate harmonics. Thus additional L+C+L filtering seems needed.

Is this a TOPAZ transformer?

  • \$\begingroup\$ Not a TOPAZ, but should have same purpose. As mine has balanced outputs I want to use the middle leg to diminish noise. \$\endgroup\$ – Gianluca G May 21 '19 at 12:35

If, the EMI, line noise and HF harmonics are common mode AND the transformer screen is good, then you won't need output capacitors and point A can connect to point B optionally.

If, the EMI, line noise and HF harmonics are differential then any potential reduction at the output is due to leakage inductance in the transformer and, given that you haven't specified this then it's difficult to make any statement about how good it will be and whether there might be some circumstances where the noise at the output gets worse (due to resonances with the added output capacitors).

If in doubt, get hold of a free simulation tool, model the transformer and work this problem out yourself.

  • \$\begingroup\$ @ Andy I'm not sure why the down-vote. I view this as a crucial question for system fidelity and precision measurement, in typical trash-laden environments, and is of high interest to me. If the only degrees-of-freedom are the capacitors, is a good solution possible? \$\endgroup\$ – analogsystemsrf May 21 '19 at 5:05
  • \$\begingroup\$ @analogsystemsrf I don't understand the down vote either! \$\endgroup\$ – Andy aka May 21 '19 at 8:29
  • \$\begingroup\$ Just checked that solution 3 is against safety! So the only solution available is 1 or 2, but do they have any effect? \$\endgroup\$ – Gianluca G May 21 '19 at 12:38
  • \$\begingroup\$ "Do they have any effect?" - what part of my answer do you not understand? \$\endgroup\$ – Andy aka May 21 '19 at 12:43

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