I am confused about a problem I am having measuring the output of a full wave diode bridge rectifier driven by a function generator. I am only getting half of the output as if it where a half wave rectifier. This is only a problem when I hook up my o-scope.

This is just an experiment. I don't plan on driving any sort of real load.

Now the obvious answer here was that the both the generator and the scope where referenced to earth ground. I tried to solve this problem by floating the function generator through a 1:1 isolation transformer. I also used an adapter to prevent earth ground from being connected through. I confirmed there was no continuity between the ground shields of the BNC connectors of the generator and o-scope yet I got the same result.

At this point curiosity/frustration caused me to put caution to the wind and I tried to drive the rectifier directly through a toroidal transformer I had salvaged from some strip lighting that was rated for line voltage with 12V out. It worked as expected. I had both sides of the rectified signal.

So what is the difference between using this toroidal transformer and driving it from a function generator that I have floated using an isolation transformer.

I just ordered a caltest differential probe. I figure it will be a useful piece of gear to have one way or the other. Im hoping this might help to float the ground of the probe.


I cant seem to reproduce the problem today. So I guess the problem was between the chair and oscilloscope. Sorry...

Just for the sake of completeness this was the setup:

Wall -> Powerstat 3PN1168 Variac -> Triad N-59MG Isolation Transformer -> (adapter to lift earth ground) -> SDG2042X Waveform Generator -> Diode Rectifier

Wall -> Rigol DS2202A -> RP3300A 10x Probe -> Diode Rectifier.

Lifting the earth ground with an adapter is necessary. Perhaps I was somehow missing this yesterday in swapping between different configurations. It does add a significant amount of 60Hz noise to the output of the function generator regardless if the output is turned on or not.

  • \$\begingroup\$ I don't see any reason why the setup you described wouldn't have worked as you expected, so there might be some important detail you didn't think to describe. Pictures of your setup might help. Also, another (safer) option than isolating the line input of your function generator would be to place a transformer on the output of the function generator (if the waveform is suitable). \$\endgroup\$ – Kevin Reid Jul 5 '20 at 18:44
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    \$\begingroup\$ First things first: Do not float devices with isolation transformer, and especially, do not lift the ground pin so it's unconnected. Unless you know what you are doing, of course, but given the circumstances, I don't think this is the case. The obvious thing to try is to use differential measurement with two probes, did you try that? You don't need a special probe for that. Were the scope and function generator the only devices, and not connected anywhere else? Do you know how the isolation transformer breaks or connects the earth/ground on output? And how did you connect the bridge rectifier? \$\endgroup\$ – Justme Jul 5 '20 at 19:19
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    \$\begingroup\$ Did you check if the isolation transformer actually isolates? If your isolation transformer has a grounded outlet on it, the ground pin may still go to earth. \$\endgroup\$ – Spehro Pefhany Jul 5 '20 at 20:36
  • \$\begingroup\$ @digitaladdictions: This is a similar question. Maybe the answers there will help. \$\endgroup\$ – JRE Jul 5 '20 at 21:39

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