Isolation Transformer Mystery Voltages

Question

I have a surplus TRIAD N-59M isolation transformer I'd like to install on my home test bench.

I was doing some sanity checks on it and came across some unexplainable (to me) AC voltages between the output and the frame.

First off I did resistance checks between between all connections and frame, and as expected only the primary and secondary windings show continuity with themselves. No continuity to the frame.

However when connected to mains with/without a load (light bulb), I read AC voltage between the output and the frame:

• Output term 1 reads 83VAC to frame
• Output term 2 reads 30vac to frame

I did not test if I could actually draw any current with these connections to the frame.

Should I consider the isolation transformer junk?

Answer 1

What you see is kind of expected based on what you measure and how you measure it.

Isolation transformer output is isolated. Output has no reference to anything, so it floats.

And because it has a high impedance, it is easily affected by stray capacitances.

Your multimeter also has high impedance inputs, in the order of 10 megaohms, so it does not much load the measured circuit.

Therefore the stray capacitances in the transformer make the output wiggle based on input voltage.

For measuring leakage currents, typically you would not measure the voltage directly with a high impedance multimeter, but over a 1500 ohm resistor. Service technicians use 1500 ohms and about 150nF in parallel.