Inductance Meter

Circuit :  Abu-Hafss, Pakistan Email  :

Description
Using just one fixed value inductor and a multimeter set to measure frequency, Abu-Hafss has devised this simple yet efficient inductance meter. Notes
An inductance meter becomes necessary when we have to make our own coils, chokes and transformers. For that we need to acquire expensive LC or LCR meter. How about building your own L-meter? You can find some websites offering micro-controller based complex circuits which give accurate results. Here a simple and perhaps the most cheapest Inductance Meter with quite accurate results is presented. The most interesting thing is that the circuit can be built within few minutes and can be powered-up with a single 1.5V cell. However, a frequency meter would be required to work-out the inductance.

The circuit is pretty straight forward wherein, two small signal general-purpose NPN transistors are cross-coupled to form a flip-flop oscillator. The values of R1 and R2 could be anything between 47 - 100R. The frequency of the oscillation is inversely proportional to the inductance and it can be calculated with the following formula:

 Frequency (kHz) = 50, 000 Inductance (µH)

CALIBRATION:
Initially the circuit has to be calibrated and for that we use a known inductor. In our case we use an inductor of exactly 100uH.

Putting the value of inductor (100uH) in the above formula, we get 500kHz.

Turn the SPDT switch SW to position [C], the circuit enters the CALIBRATION mode.
Power on the circuit and it will start oscillating.
Connect the frequency meter at point A and ground (or B and ground).
Adjust the POT until the meter reads 500kHz. Now the circuit is calibrated.

INDUCTANCE MEASUREMENT:
To measure the inductance of an unknown inductor, connect it across the two green alligator clips.
Turn the SW to [M] position for MEASUREMENT mode.
Power on the circuit and read the frequency with the frequency meter as mentioned above.
The above formula can also be written as:

 Inductance (µH) = 50, 000 Frequency (kHz)

Putting the value of frequency in this formula, the value of the inductor can be found.

PCB Layout
Abu-Hafss has also kindly created the PCB Layout for this project. This is provided in PDF format and both the bottom layer (coper tracks) and top layer containing the silk screen are available on the links below:

Copper

Silk Screen