Differential high voltage measurement using a transformer

I have questions about the limitations of using a mains-rated power transformer (e.g. a small 6VAC 1VA PCB mount transformer) as a cheap differential probe, so that waveforms can be safely observed on an oscilloscope.

My questions refer to this schematic, which is not intended as a real circuit but to allow me to refer to the various elements:

^{simulate this circuit – Schematic created using CircuitLab}

Vin1 and Vin2 are any two points in a mains-powered circuit across which the differential measurement is desired. R1 represents a standard scope input impedance. For now let's ignore R2, C2, C1, R4, and R5.

My understanding is that with an ideal transformer, this is a safe way to accurately display a typical mains voltage waveform. However, a real transformer will have some common mode leakage represented here by C1. This should mean that the transformer's "CMRR" (as it were) deteriorates as higher and higher frequency content appears across it.

1. 
   
   

   My main question is: will this common mode issue present a danger to an oscilloscope when probing a circuit like a phase angle controller with fast edges? Will the high voltage, high frequency content be capactively coupled straight across the transformer, resulting in a short via the shell of the scope's BNC connector?

   
   


2. 
   

   If this coupling could be a problem, would the addition of a simple filter like R2 C2 suffice to solve it, assuming that the user did not mind the loss in measurement bandwidth?

   
   


3. 
   

   Are resistors like R4 and R5 a sufficient way to minimize loading on the circuit being measured?

   
   

Answer

Firstly, you cannot expect ideal transformer operation unless large secondary current is drawn; you must make a LOW resistance load on the secondary to accomplish this (the transformer core otherwise sees large magnetic field fluctuations, and that causes distortion of the waveform).

There is no significant danger from common mode signal leakthrough, if the transformer is intended for AC power (the 'ground' pin is just as likely to give you glitches when lightning strikes nearby). Oscilloscope probe capacitance is comparable to (or greater than) the expected C1 capacitance. It may matter WHICH pin of the secondary is grounded, however; experiment with a function generator connected common-mode to the primary winding wires. C1 might be on the order of 50 pF. The secondary winding and load impedance, unless it is VERY high frequency, will be much lower and will dominate.

If you expect lots of feedthrough, though, there are isolation transformer designs which include shielding, and it is possible to ground the magnetic core of some common transformers (which will have a similar effect).

Filtering is usually a bad practice in measurement; most oscilloscopes handle transient overages gracefully.

R4 and R5 will reduce loading, but will also (slightly) filter the signal (more than you think, because at high frequencies, the transformer gets lossy). On the primary side, it wouldn't hurt to put a common-mode choke (since you only want the difference signal).

^{simulate this circuit – Schematic created using CircuitLab}