I picked up some toroidal transformers and was surprised to see that they have a 'Screen Pin' on the Primary side (none of the other toroidals I own have such a connection). Is this an Earth connection and is it optional? (particularly as the transformer is completely encased in plastic and will be used in a plastic enclosure


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hi,
Check out this link.
https://www.allaboutcircuits.com/technical-articles/transformer-isolation/
Interwinding screen
Clip:
If the transformer has a inter-winding screen, the screen should be connected to earth ground; also it's common practice to connect the transformer frame to earth ground as well.
E

 

Many thanks for your reply Eric. I read the article and yes, you have confirmed that the screen pin should be connected to earth ground. I have confirmed that there is no direct electrical connection between any of the pins on the transformer so I think I am quite comfortable that there is isolation from earth ground on the secondary side. Am I right in thinking that this is what seems to referred to as an 'electrostatic shield' in this article on the web? My reason for asking whether connection of this pin to earth was necessary is that I planning to replace the switch mode power supply in a somewhat dodgy chinese function generator with a linear power supply. The unit currently has only a live/neutral mains connector and I was expecting I could retain this when using a toroidal (although I guess I could replace the mains plug connector with a iec 3 pin connector). The most important aspect is maintaining isolation from earth ground on the secondary side.

 

Hi,
A good quality mains isolation transformer will have a grounded electrostatic shield between the primary and secondary windings.
E