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.
Usage
The 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 formulawhere 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.
Download
lmeter_brd_600dpi.png - PCB layout
lmeter.sch - Eeagle schematic
lmeter.brd - Eeagle board
lmeter.asc - LTSpice simulation file
Links
http://www.national.com/ds/LM/LM311.pdf
LM311 data sheet
