(copied from http://www.dos4ever.com/flyback/flyback.html#ind2)

When you want to start experimenting with boost or fly-back converters it is good idea to buy at least one inductor with known parameters that may act as a kind of reference device for the inductors or transformers that you make yourself. I use a 100µH inductor with about 0.2ohm series resistance capable of handling several Amps of current. It is especially designed for SMP applications. The circuit depicted in Fig. testbenchsch allows you to compare “an unknown” inductor (or transformer) with the reference inductor. <figure testbenchsch> <caption>Circuit diagram of the inductor test bench }</caption> </figure>

The circuit is designed to test the inductor as closely as possible under conditions that occur in the boost converter presented in the last section or in the fly-back converter to be presented in one of the next sections. Basically, the circuit is little more than the inductor which is connected to the 12V power supply by transistor T1. The current through the inductor is measured by the small series resistor R4. A voltage drop of 100mV over R4 corresponds to a current of approximately 1A. When the transistor is opened, the inductor can dump its energy in diode D3. Since the voltage drop over the diode is only 0.6V, it will take about 12/0.6=20 times as long for the current to drop to zero (remember I=(V.t)/L). This is the reason why the gate of the transistor is driven with a highly asymmetric signal generated by the oscillator around N1-N6. The transistor on-time is determined by C1 and R1+R2. R2 is set so that the transistor on-time is equal to the transistor on-time in the converter under normal load. The transistor off-time is determined by C1 and R3 and about a factor 20 longer than the on-time.

<figure testbenchpic> <caption>The inductor test bench circuit (left) and a measurement off the reference inductor (right).</caption> </figure>

In Fig. testbenchpic (right) you find a measurement of the reference inductor. We find that with a supply voltage of 12V the current through the inductor reaches a value of I=V/R=0.361/0.11=3.28A in 27.1µs. Since I=(V/L)t we find for the inductance L=(V/I)t=(12/3.28)27.1=97.6µH. Not bad! At a little bit higher current we observe a sharp increase in the current through the inductor. This is the point where the ferrite saturates. The inductor should not be used beyond this point.

You may now want to try different inductors e.g. inductors salvaged from old (computer) power supplies. Switch S1 make it easy to compare these inductors with the reference inductor. Another important parameter to watch is the current consumption of the test-bench. An increase in switching losses in the inductor core is reflected by an increase in power consumption.

An alternative simple and quick way to measure the inductance of an unknown inductor can be found here.