# Inductance measuring jig.

I built this circuit while studying 1/2CV^2 = 1/2LI^2
The transistors I had were RCA brand 1B05 and the diode is a 2000 volt fast Trr diode.
Set the current by adjusting Vcc.
I used a single function generator to get a ttl square wave to allow current through the inductor under test, and simultaneously, the triangle wave, adjusted on the front panel, dumped the voltage for each cycle.
You size the unlabeled capacitor to get a reasonable Vpeak when the current stops. It was .1uf@200V when I used this circuit.

By using a scope to measure the current through the 10 ohm resistor and the Vpeak at the capacitor, you can calculate the size of the inductor.

Not elegant, but functional, and it can be cobbled together with junk box parts.

Let's pretend you have a millihenry (but you don't know that). Install a 0.1 uf capacitor and increase Vcc to about 0.51 volts (assuming 0.3 volts for the transistor). That puts the current at 1ma. You can tell because the voltage at the 10 ohm resistor is 10mv on the 'scope. Run the test and find 0.0316 volts increase over the 0.51 volts of Vcc offset. The measurement is buried in noise. Jack the Vcc up to get 10 ma, o.1V on the 10 ohm resistor. Now Vcc is about 2.4 volts. and Vout is 1 volt above Vcc. Measurable, but weak.
Change the cap to 0.01 uf, stay at 10 ma and you get 10 volts in addition to the 2.4 volts of Vcc. Now you have something that seems useful.
Change the cap to 1000 pf and you get 100 volts above Vcc. Definitely useful.  • cmartinez

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