Topband: measuring transformer core inductance

[Rick Karlquist]

Robert Kavanagh said:
>
> Just one point to add to what Nick and Tom have said, if by any chance
> the  application for your transformer has a d.c. flowing in one of the
> windings,  then the situation changes drastically. The inductance

It's worse than that.  If the transformer EVER had DC flowing in
its lifetime, it can have permanently reduced permeability, because it has
been partially magnetized, like the old magnetic core memory in primitive
computers.  1 ampere turn is usually enough to make a
substantial change, and I have seen effects from a few tenths of
an ampere turn.  (This generally applies to non-gapped ferrite cores.
Cores with an air gap, and powdered iron cores (which inherently
have an air gap) are basically immune to this).

If you damage a core with DC, you can heat it to the Curie temperature
to demagnetize it.

Rick N6RK