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Capacitance between two windings in the isolation transformer, does it matter ?

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BravoV:
The context here is mainly at the isolation transformer used for supplying from the mains power into the DUT that will be probed using an oscilloscope.

For example my 1000 VA transformer has 1250 pF measured between pri vs sec windings.

When or where this capacitance will become a factor that worth a consideration regarding the measurement results, again, in the context of measuring the dut powered from this transformer using a scope.

An explanation + example cases will be really helpful, thanks in advance.

AlfBaz:
Hi Bravo
I'm not quite sure what you mean by your question
If you are asking wether the isolation is compromised, you are better off measuring the insulation resistance between primary and secondary with an insulation tester.
1250pf equates to 2.5Gohms reactance so I don't think that will be of any concern.

As for it affecting any measurements there are other losses and parasitics that will swamp that capacitance into insignificance, not to mention the inductive coupling of the 2 coils

SeanB:
It will appear as a 1n capacitor connected to the mains. this current will be trying to flow through any connection you have to ground, so it will generate a current to mains earth via any probe you connect that has grounded connection at the meter end. Just keep this current in mind when using it, it may or may not be significant depending on application. If you are probing the primary of a switching power supply ( only reason that you will be using an isolation transformer in most cases) it just means you can only connect to the negative of the mains capacitor, and will have to measure from there, you will not be able to eg probe the gate drive for a high side switch easily. If you connect the ground to the high side switch something will blow up.

AlfBaz:
Sean, what am I missing? how do you blow stuff up with a hand full of pico amps? Or is it something to do with creating a ground reference with the oscilloscope's pwr supply and this leakage current?

SeanB:
Consider the impedance of this 1n capacitor if you apply 20kHz to it, at a voltage of 300V peak to peak. What current will flow in the scope lead. 40mA or more flowing through the probe lead will have a rather big influence on the display, especially as most probe leads are quite resistive.

Connected to the power supply capacitor the current will be at 100/120Hz and will be quite small, though it will be noisy, as it is going to have diode switching edges on it.

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