Magnetic coil failure

Question

I got some very small coil (OD<1cm). They are made of copper wires at 46AWG. By applying a changing voltage(sine wave), I can get a very small magnetic field (such I could attract some small magnets).

The matter is the coil fail very easily. I have checked that there is no mechanical damage (they are wrapped and protected from any mechanical damage). Also the heat dissipated is very small (at least not noticeable from touch).

It is found that the resistance of damage coil is much larger than than those good coils (Mohm vs ohm). I suppose it would be some wire damage inside...So, what would cause the breakage? Thermal? the frequency of sine wave (stressing on the thin wire)?

Answer 1

It can be a localised spike in temperature, as Ignacio says in the comments, that easily melts such thin wire.

MOhm for a small inductor coil means completely broken, certainly if it used to be Ohms.

It can be that the increased temperature was from using an inefficient frequency, but, since you aren't referring to any current known or observed, I'd say it's over-current.

If you look in this PDF you can see that for simple chassis wiring or a bundle for power transmission the current limit for AWG40 is already in the tens of miliAmps, and they just don't say for enything tinier. I would estimate the limit for AWG46 will be around 25mA or so, if not lower.

And that's only for a couple of wires together for a coil you need to lower that, usually by at least 30%, depending a bit on the coil shape and build-up.