Re: How to measure inductance indirectly?

"DaveC" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> I have several inductors I'd like to know the values of. I have no way to
> measure L directly, ie, with a meter.
>
> I've got a scope and signal generator. Is it as simple as hooking a known
> value of C in series with the L and sweeping the frequency range looking
for
> a peak in V (accross the inductor) and a drop in I?
>
> Thanks,
> --
> DaveC

If you are mesuring smps type inductors (or, for that matter, any form of
power inductor/transformer) then V=LdI/dt is the simplest way. All you need
is a scope and a couple of bits. Charge up a large cap to some voltage V,
then short your inductor across the cap while measuring current (a hall
effect DCCT is useful here, but so is a resistor. beware peak pulse power
and resistor ESL). If the energy stored in the cap 0.5CV^2 >> energy in L at
current of interest 0.5LI^2, then V remains pretty much constant,  and from
the slope of the current waveform, L=V/(dI/dt). If you dont have a
storage/digital scope, repetitively pulse the choke using a big FET. I
typically charge the caps with a current limited PSU & a series R. This
approach costs very little, takes a few seconds and gives a lot of
information - saturation current, and inductance-vs-current - iron powder &
step-gap chokes have very non-linear inductance, and the inductance at very
low current can be order(s) of magnitude higher than that at rated current,
leading to rubbish results from most inductance meters. Its easy to test a
choke at 5,000A this way too.

Terry