measuring inductance

Posted on Feb 7, 2014

Pick whichever circuit is better for you, but the one using the LM339 is better. Both the capacitors are 1uf metalized film, but anything that is non polar will work. It will need to be very close to 2 uF though. You can not use a capacitor that marks which connection is ground. One thing you may notice is that the LM741 is geared for analog computing. This means that it requires a negative voltage on it’s V- pin. If you don’t have a power supply that offers this, use two AA batteries to go 3v below ground as shown. The LM339 doesn’t need this and there is no problem inputting a negative voltage. Remember that the LC circuit will vary above and below ground.

measuring inductance
Click here to download the full size of the above Circuit.

Since our wave is a true sinusoidal wave, it spends equal time above zero volts and below zero volts. This means that the comparator will turn it into a square wave with a duty of 50%, and pulseIn(pin, HIGH, 5000); will measure the time in microseconds elapsed from rising edge to falling edge. This measurement can then be doubled to get the period and the inverse of the period is the frequency. Since the circuit is resonating, this frequency is the resonating frequency.

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