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How to measure inductance of a coil without using LCR meter? Answered

I need to measure the inductance(L) of a certain coil I have wound.How do I do it experimentally without using LCR meter?

3 Replies

Jack A Lopez (author)2012-10-09

Put this inductor L in series with a resistor R and throw a square wave Vinput  at them.  Then measure the voltage across the resistor (which is proportional to the current through both L and R) using an oscilloscope.

Choose the R so that it is much larger than the actual copper resistance present in your inductor.  That is so you can just sort of ignore the small copper resistance of your inductor.   

Also if your inductor has some sort of core, like iron or ferrite, that can saturate, choosing big R can help to make I(t) small, hopefully smaller than the expected saturation current. 
If it is an air core inductor, don't worry about saturation.

Vinput is the square wave you put across L and R in series, so:

Vinput = L*(dI/dt) + R*I

And the output is the voltage across the resistor, which is R*I

R*I = Vinput - L*(dI/dt)

In the limit of frequencies much less than f0 = (1/2*pi)*(R/L), the voltage across the resistor  R*I  is much bigger than the voltage across the inductor  L*(dI/dt), and the output will look much like the input, like a square wave.

In the limit of  frequencies much greater than f0, the voltage across the inductor L*(dI/dt) is much larger than R*I, and  the output will look like a sawtooth, whose slopes are plus or minus (R/L)*(Vinmax-Vinmin)

At frequencies close to f0= (1/2*pi)*(R/L),  you should actually be able to see the shape of the exponential.   See:

The time constant τ, the amount of time it takes for the exponential to fall to (1/e) its previous value, is τ= (L/R)

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steveastrouk (author)2012-10-08

measure its impedance, apply a voltage sine wave of known frequency and voltage, measure the current through the inductor. Ignoring the coil resistance, or assuming R< Z= 2 x pi x F x L

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rickharris (author)2012-10-08

Possibly the easiest way at home would be to parallel it with a known capacitor, Feed the system with a variable frequency and check for resonance - Knowing the resonant frequency and the value of the capacitor you can calculate the inductance.

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