BH curte tracer.

[gearheaddruid]

I found this circuit while doing some research to figure out a bunch of unmarked toroid cores. Thought I would pass it on. You do need to wind the turns on the core. The secondary can be any size as it is not handling any current. The primary will need to be heavy enough to support saturation current.
More turns = less current but higher voltage. The resistor in series with the pri,ary limits current and will need to be chosen for the core size.

[gearheaddruid]

That should be CURVE. This can be done at all frequencies using a generator and power amp.

[jackinnj]

EDN mag has had a number of DIY BH Tracer articles -- EDN PDF

and a thread on DIYAUDIO:
B-H Curve Tracer

[Sch3mat1c]

For circuit values (A_L, Phi_max) I just hook up an inductor to a MOSFET, drive it with my function generator and measure current.

As the circuit suggests, Phi is the integral of EMF (EMF detected with a secondary, so you don't get error from the primary's DCR), and MMF is just the applied current. A single transistor method, as I use, assumes constant EMF (when DCR is negligible), and reads I directly. It allows I = 0, but not B = 0, so a core with high remenance will give silly results (a mag amp core shows up as fractional uH/t^2, highly nonlinear, while a small-signal AC test gives a realistic and consistent value of several uH/t^2).

Tim