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Measuring Inductance?

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1/30/1998 10:36 AM

Measuring Inductance?

Regarding my question on were to find a 650mH inductor for my sad sounding wah, Paul Francis Perry suggested I use a multi meter with "Bells+Whistles" to measure the inductance of inductors I might already have or when making my own. This would be great, however........ how do I measure inductance with a multi meter? I would love to find or wind my own inductors, so can I do this.....or is this a much more comlex test, like with a scope for example.
And Q????????
Thanks, Derrick.

1/30/1998 10:48 AM
jason	This won't help with measuring them but Mike Fuller of Fulltone sells his inductor(suppose to be made like the old fasal inductors). go to www.fulltone.com jason

1/31/1998 9:38 AM

I can think of one way you may be able to measure an inductor without a scope. However you will need the ability to sweep either the test frequency or the capacitance you attach across the inductor. The capacitance can be a bit of a problem but if you have a frequency generator you can try something like this:

Connect the inductor and a known value capacitor in parallel. Connect one end to ground. Connect the frequency generator to the other end of the two through a large resistor, start with 100K. Connect the anode of the diode to the top of the inductor capacitor pair and the cathode of the diode to a <1uF capacitor. Don't make it too big or the response time will be too slow. This is just a peak dectector and as long as the meter has a sufficiently high input impedance, you should be able to get away with a pretty low value.

From the cathode of the diode you connect your multimeter and measure DC voltage. Crank the level of the frequency generator until you get a reasonable level showing on the meter. Start at a low frequency, record the level, increment the frequency, record the level, etc. You should see a rise in voltage to a peak and then a falling off of voltage as you continue up in frequency. The peak will be the resonance frequency and you can calculate the inductance using: F=1/(2*PI*SQR(L*C)).

The large value resistor feeding the test circuit is important because this affects the Q. The larger the value, the higher the Q, and the more dramatic the change. If you need more level in order to measure the rectified level either: use a germanium diode or lower the resistance of this value. The lower it gets the lower the Q will be but you can still calculate the Q of the inductor once you've measured the resonance curve.

I'm typing this in off the top of my head before finishing my first cup of coffee so there's bound to be errors. Can anyone see a reason why this wouldn't work?

--johng

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