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Stray capacitances in transformers

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I measured the stray capacitance of a few transformers. This goes from 20pF (3VA) to 300pF (180VA). A good transformer with separate chambers (30VA) has 40pF only. Does anyone know in which range the measured values are, where the transformers have shielding between primary and secondary?

For large mains transformers, Fluke state that with a shield between primary/secondary one can get to 1pF or below. 

> The guard shield also surrounds the secondary winding of the DVM power transformer utilizing a foil type shield with the secondary winding wound completely separately from the primary windings and other secondary windings that feed cligital circuits outside of the guard. By doing this, the capacitance from the guarded secondary winding to the primary winding and the transformer core can be kept typically below one picofarad, This causes a reduction in the common mode current.

p98, Fluke Calibration Philosophy In Practice

If you're looking for low stray capacitance transformers, some of the toroidal transformers used for audio applications are quite low.  Some have electrostatic shields to achieve low stray capacitance.

The Ultra Isolation mains trsf can acheve 0.001 pF in shielded guarded measurement.

Use a trsf with an internal Faraday shield if you need low capacitance.

That afftects ONLY CMRR not DMRR.


I always wonder where the best place to ground the shield is, since both the primary and secondary are coupled to it.


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