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