Inductance measurement

[catalin gramada]

Hello. I did a jig to determine the inductance of a coil as follow : using a 600 ohm output signal generator and a dual channel scope, I conected a 10k resistor series with the coil with the coil to the ground . Then applying signal to series circuit I dialled the frequency till I found exactly half voltage over the coil. Then I used a reactance calculator to found the value of the coil in respect with its 10k reactance for those frequency. (The DC resistance of the coil is negligible 20ohm around) . I may determine the value very accurate but I wonder how precise this method could be please ? Thx.

[mozz]

LCR meter is much easier.

[catalin gramada]

LCR meter is much easier.

Is the second inductor I build in my life, I don't pay a bridge for that...

[Helmholtz]

The idea behind the method is correct: Find the impedance of the coil and calculate its inductance from the impedance.

But inductance is reactive and its voltage is 90° out of phase with the voltage across the resistor.
Means that resistor and inductance impedances and voltages don't add linearly.
E.g.: If there is 1V across the resistor and 1V across the inductor, the voltage across the series wiring is not 2V but only 1.41V.

So you need to measure the voltages across resistor and inductor separately and vary frequency until these voltages are equal.

This said, finding the "correct" inductance with a high permeability core inductor can be tricky (even with an LCR meter), as inductance varies with frequency and measuring current.

What is the purpose/application of the inductor?

[catalin gramada]

What is the purpose/application of the inductor?

Hey. Thanks. I trying to build sort of mid control circuit as Ampeg did and is only reasonably sized electromagnetic core I found it in my shelves (1"×1"). The core is versatile enough: get around 150mH open, 400mH gapped , 1.6H interleaved - by my determinations. I will tailor the coil to final value when I.ll be sure by my measurements.
We're not talking about a power circuit but a measurement where the current is almost null...

[Helmholtz]

We're not talking about a power circuit but a measurement where the current is almost null...

What I meant to say is that measuring current and frequency should be close to the conditions in the final application.

If you need something around 400mH to 500mH, a typical wah-wah replacement inductor should work fine.

[catalin gramada]

I tuned in series circuit with a capacitor with a precise determined value and found by circuit resonance freq the initial result was pretty accurate, so no lcr bridge need it..