Isolating chassis ground on scope

[OldTechie]

Years ago (early-mid 80s or thereabouts), I saw a design idea in one of the trade magazines (Electronics, EDN, Electronics Design, etc.) that described a circuit using a full wave bridge rectifier that was wired into an electrical outlet that effectively broke the connection between AC neutral and safety ground to an oscilloscope. The circuit "floated" the scope's chassis (otherwise connected to safety ground through the power cord) by one or two diode drops from the neutral connection. This arrangement broke the "ground loop" and provided effective common mode noise reduction while maintaining the scope chassis close to safety ground in case of problem.

Does anyone remember this article, or have a copy of it? I've tried to remember the hookup, but
just can't seem to get it right.

Thanks,
Dave M

 

[Sceadwian]

What you posted doesn't make sense to me, it's impossible to have the scope floating and still have a ground reference, it's one or the other can't be both. Bridge rectifier doesn't make sense either because that can't create electrical isolation. If you just want common mode noise reduction all you need is a large choke.

 

[crutschow]

I don't have the article, but they apparently removed the safety ground wire from the plug and added a bridge rectifier between the wire and the plug ground pin. To get two diode voltage drops in either direction, short the output of the bridge with a wire and connect the two bridge AC inputs between the wire and the pin. That will isolate the chassis for any voltage below about ±1.2V. To maintain safety in case of an AC short to chassis you should use at least a 50A bridge rectifier.

 

[Sceadwian]

Sounds human safe.

 

[OldTechie]

Yeah, that might be it. Think I had it drawn out that way a couple of times, but seemed wrong. IGuess didn't follow the current flow all the way through.
Thanks for helping me remember