Inductance, a parameter that determines the amount of voltage induced from a change of current, can be measured in a variety of ways. You can use a signal generator, a digital multimeter, an inductance capacitance (LC) meter or even build your own inductance meter with a kit. The least expensive and easiest methods use an AC voltmeter, a frequency generator and a simple test circuit that includes just a resistor. These methods, which are considered very accurate, require that you use a formula. The formula permits you to calculate the inductance based on the signal generator frequency and the value of the resistor used.
Construct the test circuit. Connect one lead of a precision (1 per cent) 1000 ohm resistor to one lead of the inductor or coil you want to measure. Connect the resistor's other lead to the positive terminal of the signal generator. Connect the unconnected lead of the inductor or coil to the negative terminal of the frequency generator. Connect the positive terminal of the AC voltmeter to the point where the inductor and resistor leads are tied together.
Tune the signal generator waveform. Set the signal generator such that it will produce a sine wave with a RMS (root mean square) voltage level of 1 Volt. Vary the frequency of the signal generator until the voltage on the AC voltmeter is equal to 0.5 volts, or one-half the input voltage signal. Make a note of this frequency.
Calculate the inductance. Divide the value of the resistor used (1000 ohms) in the test circuit by the product of 6.28 times the frequency (in Hertz) obtained in Step 2. The resulting value will be in Henries, which is the unit for inductance.
The frequency of the sine wave applied to obtain a half-voltage reading will depend on the value of the inductor and the inductor's DC resistance. The frequency needed could range from as low as 1 cycle per second to several billion cycles per second. Most inductors range in value from 1 millionth of a henry (a micro Henry) to several henries. In general, the larger the physical size of the inductor the higher the value of its inductance. For a 1 micro Henry inductor, and a test circuit resistor of 1000 ohms, the frequency needed to adjust the inductor voltage to one-half of the AC signal voltage is obtained by dividing the resistor value (1000) by the product of 6.28 and the inductor value in Henries(0.000001). Using these values, the signal generator frequency needed would be approximately 159,000,000 Hertz or 159 megahertz. For a 1 Henry inductor, the frequency needed would be approximately 159 Hertz. For more accurate inductance measurement and calculations, measure the resistance of the resistor. Alternately you can use a resistor with a precision rating of 1 per cent or less. Consider that the resistance of the inductor will effect the accuracy of the calculation The higher the value of the test circuit's resistor in relation to the inductor's resistance the more accurate the calculation will be. Inductors often have resistance values less than 50 ohms.
Novices should take an electronics safety course or basic electronics training before attempting to work on electronic devices, especially inductors. Large inductors can be especially dangerous because they can store high levels of electronic charge. When working with electronic equipment, it is always best to work under the supervision of an experienced electronics technician or electronics engineer. Never assume that an electronic circuit or wire is safe to touch. Even after the power is disconnected, circuits can store electrical charge at levels that can be fatal.