Simple DIY inductance meter (L-meter)
In this article a simple inductance meter is presented. It comes in handy when winding your own coils or transformers as needed in switching power supplies. It becomes even necessary for winding flyback transformers where the air gap must be adjusted. The oscillator circuit presented here is not my own creation, it can be found on various web sites. My contribution is the PCB design, the additional range switch and the buzzer.
As mentioned above, the circuit was designed by an unknown author. I tried to optimise some part values with LTspice to increase the range of inductances for which the circuit oscillates, but didn't really succeed. If you want to have a try, download the LTspice simulation file here.
UsageThe measurement setup is shown in the title picture above. Connect the coil to the meter and measure the frequency at the output. This can be accomplished by a modern multimeter, an oscilloscope or a sound card in connection with a sound recorder/analyser. If there is no oscillation present, the inductance is probably out of the measurement range. Try to switch the range or use other values for the oscillator capacitor. If an oscillation is visible or audible (f < 16kHz) the inductance can be calculated according to the formula
where L is the inductance in Henry (H), C is the oscillator capacitance in Farad (F) and f is the oscillation frequency in Hertz (Hz). I've chosen the capacitors in the way they result in nice values when multiplied with 4π. This simplifies the calculation a little bit. The circuit is very tolerant to the supply voltage range. It works well with 5V.
LM311 data sheet
I guess the range of inductor values that oscillate is limited. The oscillator can also be sensitive to the comparator type and how the supply voltage is stabilised with capacitors.
What is the name of this type of oscilator?
You could use it to automatically center the resonant frequency of a LC circuit connected to the base of a transistor without having to manually calculate the impact of the base diffusion capacitance for example.
The other issue is the actual reactance of L & C at 7kHz If you modify the simulation to put L & C in parallel with a series resistance of 100k between them and the source that is closer to your real circuit. Then look at the voltage at the junction of R with L & C the voltage is tiny and that is probably the real reason for lack of oscillation.