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. .

Usage

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.

Download

lmeter_brd_600dpi.png - PCB layout
lmeter.sch - Eeagle schematic
lmeter.brd - Eeagle board

Links

http://www.national.com/ds/LM/LM311.pdf
LM311 data sheet

Select Language English German Greek Portuguese (BR) Russian

Comments (7)

To be honest, I have no idea what the reason could be. My circuit oscillated with frequencies predicted with eagle, roughly (it's while ago).

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.

#7 - Luk - 06/09/2016 - 17:54

I just put one of these together tonight, and it appears NOT to oscillate at the frequency given by the resonance of the LC network. The inductor does affect the frequency, but not in any useful way. As another poster wrote below, this is just a relaxation oscillator (which seems to oscillate at about 1.5Hz in the absence of the LC network).

#6 - Stefan - 06/09/2016 - 02:20

LM311 Question

I think the author chose an lm311 as it is much faster than most common opamps and thuerefore suitable to build an oscillator that operates at frequencies above the audio range.

#5 - Gordon Robertson - 11/04/2015 - 19:37

R6

10K should be fine

#4 - Luk - 07/05/2014 - 10:54

Numerical Optimizer for LTSpice IV

I have written a numerical optimizer for LTSpice IV. It automatically adjusts component values to optimize circuit performance objectives you set:
http://www.evospice.site88.net/
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.

#3 - seanvn - 08/09/2012 - 07:18

Q-factor

Just had a quick play with your LTSpice file. If your inductor really has a resistance of 0.2 Ohm the problem is the Q of the circuit. For the series LCR circuit in that file, the resonant freq is about 7117Hz but the Q is only 1.12 - not very selective. Q = 2*PI*Fo*L/R for the series LCR (or (1/R)*sqrt(L/C) ). If you reduce R you will see the peak in the amplitude of the current in the circuit get sharper and the slope of the phase trace get steeper (as Q increases). Reducing C also increases Q but Fo increases too.
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.

#2 - David - 12/26/2011 - 23:54

Q-factor

Just had a quick play with your LTSpice file. If your inductor really has a resistance of 0.2 Ohm the problem is the Q of the circuit. For the series LCR circuit in that file, the resonant freq is about 7117Hz but the Q is only 1.12 - not high enough for oscillation. Q = 2*PI*Fo*L/R for the series LCR (or (1/R)*sqrt(L/C) ). If you reduce R you will see the peak in the amplitude of the current in the circuit get sharper and the slope of the phase trace get steeper (as Q increases). Reducing C also increases Q but Fo increases too.

#1 - David - 12/26/2011 - 01:08

Name

E-mail (Will not appear online)

Homepage

Title

Comment

To prevent automated Bots form spamming, please enter the text you see in the image below in the appropriate input box. Your comment will only be submitted if the strings match. Please ensure that your browser supports and accepts cookies, or your comment cannot be verified correctly. »