So I have this unmarked, unknown transformer (probably from a microwave oven so it is pretty beefy) and I thought that a safe way to determine the primary from the secondary windings would be to put 1 volt of dc connected to one of the windings and interrupt it with a switch. Then I put my multimeter across the other windings and tapped the switch pretty quickly (say 5 times a second). My thinking was that I would either see an increase or decrease from 1 volt at the meter and know the ratio of turns in the two coils.
Two things seem to be wrong with this idea:
Firstly, the experiment seemed to step-down the voltage regardless of which side of the transformer I connected it to.
Secondly, at the time I didn't know about "snubber networks" so I did not include one. I've just now come to understand a little about "inductive kick" and realize that though it was probably too transient for my meter to register, it is possible I was generating voltages on the secondary upwards to the kilo-volt range and reverse voltages on the primary in the mega-volt range. I say this because I simulated this with Lt-spice and though I know I didn't account for capacitive coupling between windings, core losses, wire resistance, etc. the simulation shows some pretty nasty voltages. I'm sure there must have been some bad arcing going on in the switch which dissipated the bulk of the energy; however, could it have damaged the multimeter, and if I had hooked it to an oscilloscope capable of detecting such a signal, would that have damaged it?
Here is the Spice schematic:
And the output: