ian said:
The coils will have higher "resistance" (reactance) when AC current is
applied, if you can find a wall wart with AC output (possibly a dial-up
modem wall-wart) with approx' 12V output - hook it up with a dial or
indicator bulb in series, a gross difference in brightness between any two
identical windings would indicate a problem. You can also make use of your
meter on its AC voltage range for a more precise indication.
If you really do need an actual DC resistance measurement, the easiest way
is to apply a regulated (and known) current to the winding and then measure
the voltage developed, from there its a simple Ohm's law calculation: V/A=R.
This topic comes up frequently on so the regulars
have had plenty of practice giving advice.
For the DC measurement, if you have not a regulated current source, and
if your DVM can not measure current, try the following:
- buy a resistor between 1.5ohm to 15ohm, let's say R;
- put it in serial with the coil;
- put some DC voltage across the two resistors;
- measure the voltage around the coil (Vc) and around the resistor (Vr)
- compute coil resistance: Rc = (Vc)/(Vr)*R
Warning: depending the DC voltage and the value of R, you have to
correctly select the power of R. Do not put too much DV voltage. For
example, if R = 4.7ohm and if you apply 5V, then power =
4.7*(5/(1.5+4.7))^2 = 3 watts
Olivier