isolated probe

[electros6]

hi guys,
           while designing my fly-back converter I want to see the signal in the both side of the transformer. Since my analog scope has common ground i am not able to see  both sides of the transformer since the ground short both sides of the transformer. please tell me how to do such measurement.

[PChi]

An alternative to isolated probes which used to be expensive is to isolate the mains using an Isolation transformer.

[David Hess]

The usually solution is to use a differential probe.  Most oscilloscopes support an add and inverter mode which allows two channels to be used this way but the performance is not as good as a dedicated differential probe.  An isolation transformer as PChi mentions is another alternative.

Galvanically isolated voltage probes also exist as well as oscilloscopes with galvanically isolated vertical inputs.

[tszaboo]

hi guys,
           while designing my fly-back converter I want to see the signal in the both side of the transformer. Since my analog scope has common ground i am not able to see  both sides of the transformer since the ground short both sides of the transformer. please tell me how to do such measurement.

Why not? Unless you have other connections, it is not a problem for a flyback to short the grounds together. Do you have a voltage difference there, coming from somewhere else from the system? Otherwise you can just short it, and measure it.

[Richard Head]

Tektronix P5200 probes.

[David Hess]

Tektronix P5200 probes.

The cost is what drove me to get into 7000 series oscilloscopes.  For less than the cost of a P5200, I have 2 high bandwidth differential amplifiers in the form of a pair of 7A13s and a number of mainframes to use them in.

If you can do without the Tektronix name and some of the performance, a Pintek DP-25 is a lot cheaper:

http://www.pintek.com.tw/
http://www.globalmediapro.com/dc/CKG7/Test-and-Measurement/Test-Probes/Differential-Probes/

[Jeroen3]

Just disconnect the earth tab inside the scope, they (should) all have transformers* which will automatically make them isolated to a certain level.
The only drawback is that the scope ground becomes a small (in EU ~100Vac) charged capacitor (few pF) relative to your circuit. Which will discharge at connection.
Compare it with an esd discharge, on improper circuits your microcontroller can reset.
My fluke 87 had a low enough resistance to slowly discharge that capacitance if measuring Earth against floating Scope Earth.

I don't recommend this as permanent solution though...

^{*mine had a large toroidal}

[David Hess]

It is usually better to isolate the device being tested from ground than the test equipment and the oscilloscope capacitance to ground is going to be a lot higher than a few picofarads.  In addition, that does not isolate the probe grounds from each other and if you have one of the oscilloscopes which includes isolated probe grounds like the Tektronix TPS2000 series, then you are worrying about nothing because it can make this measurement directly.

[BravoV]

When it comes to the isolated probing, each method has it's advantages and trade offs, click HERE and download the Tektronix "Fundamental of Floating Measurements and Isolated Input Oscilloscopes".

Imo the best but the most expensive method  is to use an isolated scope, still it has it's trade offs  . But you can use each scope's channel as in your case since each channels are isolated each other.

Example illustration from Tektronix isolated scope design.



Example what you can do with an isolated scope, watch the different hook points at both probes ground clips, you just can't do this with ordinary scope, even using an isolated transformer or floated scope/dut.