Inductance Meter
Here is a simple device that will provide a ball-park inductance measurement. It is based on an AC voltage (60Hz), dropped across a test inductor…a series R-L circuit. Essentially, a voltage divider between a known value resistance that is in series with the inductor under test. It is quick and easy to use; you can read the inductance directly off of a DVM, with the proper ratios.
It consists of a 24VAC TFM and 3 common resistor values: 1K, 10K and 100KΩ;
(a TFM output voltage of 26.526V is ideal).
On the × 100mH division scale, 1KΩ resistor, the AC voltmeter will read 100mH per volt and accurate to 500mH or 5.00VAC DVM reading…connecting a 200mH choke results in a 2.00VAC reading.
The × 1 scale uses a 10KΩ resistor and reads 1H/volt sensitivity up to 5H.
The × 10 scale with a 100KΩ resistor, 10H/volt and accurate up to 50H.
XL = 2πfL, XL will be 377Ω per H @ 60Hz.
The number
377 is a constant for the product of
2×π×60Hz...it’s a magic number, one to remember, next to your favorite sequential number.
Extra notes:
When scaling for lower inductance ranges, use a lower voltage TFM (12.6VAC).
The range resistor values correspond to direct-readings on a DVM.
On the ×10 scale: a 1VAC reading equals 10H with an impedance of 3K77Ω per volt.
On the ×1 scale: a 1VAC reading equals 1H with an impedance of 377Ω per volt.
On the × 0.1 scale: a 1VAC reading equals 100mH with an impedance of 37.7Ω per volt.
Due to the simplicity of this adapter, the Q-factor is not taken into consideration. Most true bridge LCR meters will take this factor into consideration. Most low value inductors will have a low “Q” at 60Hz and will measure a higher than true reading.
