View Single Post
Old 15th Dec 2020, 7:23 pm   #12
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Advice on mains current sensing, safely

Current transformers don't mind a short on the secondary, but they object to being open-circuited.

I'd suggest a CT with permanent load consisting of a bridge rectifier feeding an electrolytic capacitor and your 24V relay coil.

If your TV takes 400W, that's about 1.6A, and the relay needs 50mA, then you need a current ratio of 32:1.

So, I'd suggest a 240V:9V mains transformer, (ratio 26.7), rated at about 20VA. Connect the bridge rectifier to the 240V winding. Connect the 9V winding in series with your TV. It's now working as a current transformer. And the insertion drop will be around 24V / 26.7 which is less than a volt.

And of course, you get your isolation. Twice, once in the transformer, once in the relay.
kalee20 is offline