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Unisolated chassis interface

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dac

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I am sure that someone will find this dumb. I have a circuit that is grounded via a bridge rectifier to my mains supply. Thus it is in fact live and an earthed circuit. If you were to connect these grounds together one or both will fuse. What I need to do is pass a video signal from the earthed one (PC) to the live one (TV without this facility). I have found the connections on both circuits but I need a way of safely connecting them. I have though of using some kind of transformer though I can’t seem to find such. Would it work if I made use of two capacitors (one on ground and one on the signal) to interface these devices or does someone know of a proper way to do this.

Very much appreciated.
 

Nigel Goodwin

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Most Helpful Member
dac said:
I am sure that someone will find this dumb. I have a circuit that is grounded via a bridge rectifier to my mains supply. Thus it is in fact live and an earthed circuit. If you were to connect these grounds together one or both will fuse. What I need to do is pass a video signal from the earthed one (PC) to the live one (TV without this facility). I have found the connections on both circuits but I need a way of safely connecting them. I have though of using some kind of transformer though I can’t seem to find such. Would it work if I made use of two capacitors (one on ground and one on the signal) to interface these devices or does someone know of a proper way to do this.

Very much appreciated.

There are basically two ways it's done, firstly you could add a mains isolation transformer in the TV's mains supply - this would be the simplest way!.

Secondly you can isolate the input itself, this is usually done with hi-speed opto-couplers - a number of Hitachi TV sets used to do it like this.

I've just been and dug an old Hitachi manual out (circa 1990), here's a scan of the video input circuit - you do (obviously!) need to provide an independent isolated PSU for the isolated side of the circuit - the Hitachi TV did this with a very small transformer fed off the LOPTX at 15.625KHz.
 

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