Minimalist inductance meter

millwood

a while back, e3po showed an inductance meter based on discrete oscillators, to measure inductance.

it is an excellent instruments but requires a scope / frequency meter.

I thought what if I have a bunch of oscillators already and how would I make that work?

well, here is my idea:



it is an inductance meter consisting of 4 parts + your inductor to be tested.

1 oscillator (V1);
1 led (D1);
1 resistor (R1);
1 variable capacitor (C1).
L1 is the inductor whose inductance you wanted to measure.

Here is how it works: V1 generates a pre-determined frequency. with L1 plugged in, L1/C1 forms a resonance tank that presents the highest impedance at its resonant frequency (1/(2*pi*sqrt(L1 * C1))). At that point, the current through the led is the lowest and the led will be dimmest (completely dark in my test).

In the example, you see a 1Mhz oscillator, and with C1 = 25pf, the current is the lowest. That means L1 must be 1/(2*pi*f^2) = 1.013mh, vs. 1mh in the simulation - pretty close.

with a given variable capacitor, the range of inductance measured is limited to 1/(Chi*(2*pi*f)^2) - 1/(Clo*(2*pi*f)^2), where f is the oscillator's frequency and Clo/Chi is the variable capacitor's lowest / highest capacitance.

For my variable capacitor, here is the range under various oscillators:


Xtal (Mhz)        Llo(uh)        Lhi(uh)
75.00          0.04                 0.90
25.00          0.34                 8.11
6.00                  5.86                140.72
2.00                  52.77       1,266.51

I already got the 75Mhz / 25Mhz oscillators and I can build two 6Mhz / 2Mhz oscillators with a hc132 + two crystals.

Throw in a 4way switch, you get an instrument that can measure inductance from 0.04uh to over 1mh. and potentially more.

not a bad effort.

millwood

two tips:

1) using an led as the current indicator isn't terribly sensitive. you could use a current meter - preferrably an analog one, or a voltage meter (with a diode + capacitor).
2) you may need to experiment a little bit with the leds. I tried a few and settled on a bright white led as it produces the most dramatic changes in brightness near resonance.
3) (ok, one more) the resistor value isn't critical. I happened to have a 3.3kohm resistor around and I am sure other values will work equally well.

enjoy.

e3po

millwood 发表于 2012-9-23 07:16
two tips:

1) using an led as the current indicator isn't terribly sensitive. you could use a curr ...

I think red leds are more sensitive and capable of emitting light in ultra low current.