power supply - Measuring ferrite core parameters

I have large amounts of inductors and transformers from power supplies or other undocumented sources. They are of course completely unmarked and I have no idea what kind of ferrite material they are made of but sometimes I would like to use them in one-off power supplies or other experiments since ferrite cores are quite expensive to buy in low quantities.

Do you know of any good methods to determine the important characteristics of the cores? \$A_L\$ is quite easy (I just wind \$N = 10\$ turns and measure the inductance using \$L = N² \cdot A_L\$) but what about \$B_\rm{max}\$ or maximum useful frequency? Are there other parameters I should have in mind?

Basically I like to know all the good tricks you know for determining suitability of cores of unknown origin. :)

Answer

B_max: There is no universal definition about the exact point above which an inductor is called saturated. Many modern ferrites saturate around 300 mT. Useful values tend to vary between the points when the inductor has lost 10 % to 33 % of its original inductance. Here's a practical test useful for finding a reasonable \$I_\rm{max}\$ for a given inductor: You apply a square voltage across the inductor and monitor the current while doing so. You will observe a current waveform that starts from zero in a linear way as soon as the voltage is applied. You can calculate the inductance using \$L = U * \frac{dt}{dI}\$. You will observe an increased rate of rise once you get to the point of saturation. \$I_\rm{max}\$ is reached when the rate of rise just starts to get bigger.

Maximum useful frequency and other parameters: Most interesting parameters (including frequency) have to do with core losses. It's quite hard to find out about them without being able to compare an unknown core to a known sample. When I was designing transformers for switching power supplies, we would test different cores and just see how hot they behaved at different currents, with or without being driven into saturation, at different temperatures and at different frequencies. It was a lot of experimentation and trial and error, and we made samples with different numbers of windings, different air gaps and various ways of layer construction.