It depends on the power and frequency requirements?
I'd be looking for 200KHz maximum and minimum 20KHz. I have read that I need to consider the minimum frequency I want to pass through the primary. So I do the calculations
Impedance = inductance x 2 x Π x F to get the minimum inductance
I'm not sure of the power requirements I'd need. I was thinking the current was the main determinant of gauge size, too thin and its a fuse. So 0.5A and 50mA would be the maximums i'd need to work with.
I was hoping to use this transformer with an audio amp output 4 ohms, with the above calculation it looks like I need inductance minimum of around 20uH. So this one with 10uH primary and 1900uH secondary looked ok. Until I pulled it apart. Now the wires look too thin to be of much use. I've probably missed something else, I can kind of tell its there but I'm not sure what it is! I won't be able to test it for a few days as I'm waiting on a power supply.
I have taken a pic of the transformers I have. They are impedance matching transformers for a piezo buzzer of some kind, rated at up to 125KHz. The datasheet doesn't mention the power requirements. It simply says, 125KHz, 2.2mH minimum, Q = 80 and turns ration 1:20.
It seems to me like you're not aware of the enamelled copper wire that is used for winding. Thin layer of enamel is painted on the copper wire so that they're normally isolated on each layer of winding, so an explicit separation between primary and secondary is not actually necessary, but they do.
I wasn't aware of that no! It's very clever, and very simple. In this case it is I think just plain wire, could be wrong, i've taken a pic.
Thanks for your comments.