This circuit synchronized by the power grid can be used as a shield. We found it in an 80's Electronic Engineering. R1 can typically be 4k7.
This configuration that appears several times with variations on this site is from old American...
This configuration (somewhat critical, as the imbalance of characteristics between transformers can cause...
This circuit is from a Popular Electronics of the 90's. Any power MOSFET can be used. The circuit...