Emery's Projects
Inductance Meter
Building your own instruments to measure
electrical quantities and parameters of various components can
be a satisfying and worthwhile passtime. An inductance meter,
for example.
There are a great variety of circuits posted online, some are
wonderfully simple but perhaps less than ideal. They may not,
for example, account for coil resistance or self capacitance.
Each approach will have its pros and cons. An experimenter will
experiment...This is what I invite you to do:
How it works:
-The unknown inductor is inserted into the oscillator circuit
which then self adjusts to produce a precise stabilized
amplitude sinusoid. To achieve this, the oscillator amplitude is
fed back to control a negative resistance within the tank
circuit.
-The output is integrated twice in succession.
-The resulting waveform is demodulated and averaged out to give
a reading proportional to the inductance being measured.
Further details:
L1 is the inductance to be measured.
R13 sets the oscillator output amplitude, which should be at 400
mv peak.
The circuit uses a "home made" Led-Cadmium sulfid optocoupler
consisting of a single Led illuminating two separate photocells.
One of the cells is used to linearize the response, the other
for controlling the oscillator.
The low frequency noise components tend to be greatly amplified
in the process of integration, therefore a high pass filter is
an absolute must before demodulation.
High pass filters 1 and 2 are identical except for an offset
adjustment in filter 1. The purpose of filter 1 is to eliminate
noise while filter 2 assures that the phase conditions for
demodulation are correct.
Potentiometer R33 of the demodulator is meant to offset the U6
output, it should be adjusted to obtain a clean half wave
rectified signal. U6 output is meant to be as large an amplitude
as possible, without driving the LM318 into saturation.
Saturating it will result in unwanted phase shifts.
Finally, a digital voltmeter is connected to the averaged out
demodulated signal.
The circuit below will measure inductances up to about 20
milihenries. Meaningful readings can be taken even at the
milivolt level. All op amps used here were TL072-s except for op
amp U6 which is an LM318. The integrators can be scaled to any
custom range desired. The circuits here are capable of accurate
readings down to 10 microhenries.
I also include a clip showing the actual demodulated output and
control voltage levels. The measurement consecutively displays
outputs for 10, 20 and 100 uh inductors. The last measurement is
then repeated with a 200 ohm resistor put in series with the
inductor. Note that the output stays the same while the control
voltage settles at a much higher value.
