SmaTrig 2.1

Simple DIY inductance meter (L-meter)

inductance measurement setup

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.

L-meter schematic

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 formula

L = 1/(4π2 C f2)

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.

DIY inductance meter close-up

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

Comments

LM311
I also want to know why the use of LM311, can it be replaced by an LM741?
What is the name of this type of oscilator?
#6 - Fabian - 01/07/2013 - 22:43
Question L Meter
Hello, looking the mathematical formula I have a question: making small changes this circuit could Capacimeter functions?
#5 - Mauricio Artunduaga - 10/31/2012 - 13:30
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.
#4 - seanvn - 08/09/2012 - 09:18
Why LM311
I wondered if someone can tell me why this circuit uses a comparator (LM311) as an oscillator instead of an op amp or something? Is it because op amps tend to have lower bandwidth, because a square wave output is desired or because it's easier to interface with an LC circuit or what?

Cheers
#3 - Matt - 04/15/2012 - 11:27
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 - 12/27/2011 - 00:54
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 - 02:08
I found tonight that my element(USI1614-12) only went up 1.2mH inductance from where I had it set from my computer aided method. I would say that using the inductance method a guy could get 90% or so of the sensitivity the unit is capable of. I was pretty amazed at how sensitive the inductance was to relatively small changes of armature positioning. I would say that this method would work best on elements that have their greatest sensitivity when centered.
#0 - TBasket - 06/18/2010 - 13:52
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