Who might be able to measure inductance?

[fazman]

I have some coils which I have no idea what the milihenries are.
Without going and buying a meter to measure them what type of businesses might have this capability that I can go and ask them for a favour? Would an electric motor company be able to do this ya think?
Thanks for any suggestions

Dave.

[owdi]

If you can handle the steep learning curve of Speaker Workshop, you can use it to measure the coils yourself. All you need is a computer and about $10 in parts.

Dan

[BobEllis]

Fora about the same cost (an op amp and a couple resistors on a prototype board) you can make the bridge shown here and using the program you have likely more accurate measurement without the learning curve.

[critofur]

Bob, thanks for those links ~ is there a web page about them on that site too?

[BobEllis]

Dr. Steber published the linked article in the ARRL magazine QEX. You might find other interesting articles at www.arrl.org.

[anatech]

Hi Dave,
If you run up against a brick wall, I can measure them for you. That's unless you have a ton of the darn things. I use an Agilent 4263A LCR meter.

Last resort.

-Chris

[mashaffer]

As a quick and dirty you can always use one of those freeware signal generator programs for the pc and a simple voltage divider with the coil in parallel with a resistor. Adjust frequency until voltage across each is equal. At that freqency the coil has the same imped. as the resistor. Then use Z=2*pi*L=R. So L=R/(2*3.1415)=R/6.283.

You might have to try a couple of different sized resistors to get a reasonable frequency. This is not as accurate as a bridge but will get you in the ballpark without having to build a bunch of stuff or figure out the complexity of a bridge.

mike

[rpapps]

Originally posted by mashaffer
with the coil in parallel with a resistor.
mike

I think he meant to say series. In parallel, the voltages will always be equal.

So, when the voltages are equal, Z=R.
Now Z is the vector sum of Xl and Rl but for an air core inductor of the type usually found in crossover networks, Rl may be small enough to ignore so Xl=R. But, if you want to be absolutely accurate, measure the DC resistance and apply the formula Xl=SqRoot(Z^2/R^2)
Also, you have to factor the test frequency into the math.
Xl=2*Pi*F*L.
Transposing gives L=Xl/2*Pi*F.

You could try electronics repair shops in your area to get them measured or find a local ham radio operator who would be willing to help.
Cheers
Rob

[mashaffer]

My bad. I shouldn't try to post when I am sick. I meant series and I left out the frequency term. I was lying down and it hit me so I returned to correct it.

L=R/(2*pi*f) as Rob pointed out.

mike