How to measure small inductance?

[lumanauw]

How to measure small inductance? About 0,5uH or 2,5uH?
I tried to measure these with DMM, it is missleading. One time it reads 2uH. The next measurement 4uH, never steady. Try other DMM, also not convincing. For mH values, they are OK.

[XEAGLEKEEPER]

You need a LCR meter. There is also a way to measure inductance with a simple oscillator circuit and a O'scope or a freq.counter and a calculator.

[djk]

You need a frequency up around 100Khz and a test setup to null your lead inductance.

A handheld will probably not be of use.

[lumanauw]

I've tried a couple of LCR (handheld, DJK ), but no use, not consistent readings.

Using CLIO it reads "out of range"

I've got audio oscilator up to 1Mhz. Also got scope. How to do it?

From equation R=2.pi.f.L, I will need about 16Mhz signal for 1uH to get 100ohm, I don't have that high oscilator.

[XEAGLEKEEPER]

The formula would be L = XL divided by (2 x pi x freq ). inductive reactance formula

[XEAGLEKEEPER]

This may be of some help
michaelgellis.tripod.com/68hc11/ind.html

[lumanauw]

Hi, XEAGLEKEEPER

Thanks for the link I will try that method.
The opamp CA3100T, can it be replaced by NE5532 or other common opamps?

Looking at the datasheet, CA3100T is 38Mhz opamp with -3dB at 110khz.

110khz knee frequency? Interesting, how will it sound for audio?

[XEAGLEKEEPER]

probably just about any general purpose op-amp will do.Instead of using a freq counter you can use a scope F = 1/T and T=1/F

110khz knee frequency? Interesting, how will it sound for audio?

maybe a bat could here it. LOL

[unclejed613]

you could build a wheatstone bridge circuit, and find where the inductor nulls a potentiometer at a known freq. you then measure the resistance of the potentiometer, and look up in a chart (or use the calculation) to find out what value of inductance has a reactance equal to the value on the potentiometer. so for instance if the inductor nulls a potentiometer at 47 ohms, you use the formula L= XL/2*pi*f so at 100khz it would be 47/628000, which would be 75uH. so you will need 1Mhz to bring XL up into the 10-100 ohm range for coils around 1-10 uH. it's best to use a 10 turn pot for your nulling element, as the nulls will probably be quite sharp.

[Elvee]

When I have no suitable inductancemeter available, I use the resonance method, based on the Thomson formula:
http://en.wikipedia.org/wiki/Resonant_circuit
You need a good reference capacitor, say 100nF 0.5% or 1%, polystyrene or polypropylene, you make a tank circuit with the unknown coil and you feed the circuit via a resistor of some Kohm.
You connect a scope across the circuit and you sweep your generator until you find a signal peak; you then calculate the inductance using the reverse Thomson formula: 1/C*(6.28*F)².
With 100nF and 1µH, the frequency is around 500KHz.
The signal generator has to be preferably sinusoidal, otherwise you will get parasitic peaks at harmonic frequencies. Even if it is the case you can still get the correct value by selecting the highest and strongest peak.
The method is very accurate and only depends on the accuracy of the reference cap. In addition you can select a cap value giving a resonance frequency near the intended operating frequency of the inductance, giving an even better accuracy.
LV