Has someone tried to shield mains transformers in order to decrease the interwinding capacitance?

I have a transformer (toroid, power is about 5 VA,) and interwinding capacitance is a problem.

As I understand, I should make a shield (use a foil, maybe) and then connect it to somewhere.

Where should I connect the shield?

My output circuit looks like this:

enter image description here

  • 1
    \$\begingroup\$ Hi, I upvoted your question, really nice question ! Why do you want to decrease the interwinding capacity ? What is the topology of your converter ? By interwingding capacitance, you mean the parasitic capacitance between the primary and the secondary ? Is this for a common mode noise problem ? Or a ripple output problem ? \$\endgroup\$
    – Jess
    Apr 17, 2020 at 8:30

1 Answer 1


Sure, shielded transformers are readily available. They're frequently used in medical equipment and other noise-sensitive applications.

In a schematic, it's usually denoted by adding a dashed line passing through the core. It's usually connected to circuit ground.

transformer symbol

(Image source)

If you are constructing such a transformer yourself, you need to be sure that the shield itself does not form a shorted turn — it must NOT connect to itself after wrapping around the core.

  • \$\begingroup\$ thanks for noting about a shirted turn - i lost it even though it is obvious. But i have no "Ground" (or i am wrong)... \$\endgroup\$ Apr 17, 2020 at 12:18
  • \$\begingroup\$ You haven't stated the nature of your "problem". But if capacitive currents through the transformer are causing issues for you, the shield gives you the opportunity to shunt them elsewhere. Where do you want them to go? \$\endgroup\$
    – Dave Tweed
    Apr 17, 2020 at 13:41

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