# Simple way to measure inductance?

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I have some inductors with unknown inductance and I'm looking for a very simple but reliable method to measure its value(doesn't have to be precise or accurate but just get me in the neighborhood).

I was thinking that I could simply using a constant current source to drive the inductor and measure the voltage drop across it then use the formula

L = V/I/(2*Pi*f)

to compute L.

This works in theory but is it pratical? I was thinking of the mains to generate the frequency and about 1mA of current or so... the current acts as a scaling factor that can be used to for small or large inductors... and I suppose one could throw in a resistor to "fine tunning" but I'm mainly want something simple.

Would this work? or is there an easier method(this seems like the easiest but who knows ;)?

Thanks, Jon

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60 cycles is NOT Very Practical, Unless there Very Large Inductors.
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Not sure why this should matter? Does it have to do with the non-ideal characteristic of the inductor at those frequencies?

Jon

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I can't find anything on those pages that talks about measuring real inductors.

Jon

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In an equarion, "L" is inductance. When the voltage across the resistor peaks, note the frequency and solve for L.

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um... and how is that any different than what I did? Also where does it talk about real world issues that are involved?

Jon

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You were talking about using line frequency As Gary said, that will have limited utility.

You also mentioned (completely out of left field) a constant current source (a DC construct). . .

There is an assumption that the "Q" (quality factor) of the coils isn't complete crap. Your initial request WAS for a ballpark number. If you want to spend some \$\$ and quantify the parts more completely:

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On Sat, 4 Feb 2006 23:03:09 -0600 in sci.electronics.basics, "Abstract Dissonance" wrote,

What is the inductive reactance of the inductor you are likely to be measuring at 60Hz? At 60kHz?

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If your inductor is 1uF, then the impedance is

1uF*2*PI*60 = 377u ohms.

So, if you put a 1mA RMS current through it, you'll get 377nV across the inductor.

A better way is to measure the resonant frequency that your mystery inductor produces when resonated with a known capacitor. You pluck it somehow, then measure the frequency of the resulting oscillations. You generally need a spectrum analyzer or an oscilloscope to do this.

You could also buy one of these:

It uses a microcontroller to measure the frequency. I have one, and it works pretty well.

```--
Regards,
Bob Monsen```
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I have no idea. I have some inductors I ripped off some old junk stuff a long time ago and they don't have any markings on them and have some "sleeve" on them. I guess it has about 50 wraps of about 18awg type of wire in a toroidal form with, I guess, a ceramic(or whatever) or iron core... I also have some other smaller inductors about th size of a nickel or so.

Jon

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what I ment was a constant current AC source(in that it delievers a sin wave with constant rms value(and not just some arbitrary current wave)).

heh, but the sites you gave don't talk about measuring an inductor but just the mathematics behind it. Theoretically I could use any frequency and the simple configuration I gave using the math I gave(which I guess you overlooked)...

This doesn't mean that it will even get me in an order of a magnituide in the real world and thats why I asked if it was good enough cause I can't look at an inductor and say its about x henries so I need something that can give me some idea and is going to work over a large range. Maybe a wein bridge type of method is good but I don't have any precision components to make it with and I really don't need something with more than 2-3 significant digits(atleast for now).

It as already been pointed out that using a low frequency can throw off the result but I need to know more about this and why.

Jon

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uH? isn't that kinda small?

Yeah... I noticed that in simulating some results I can get very wide swings of voltage depending on the inductance and frequency... I can use anywhere from 1mA to 1A though but it doesn't help much(but a factor of a 1000 can kill my circuit if my inductance happens to be larger than what I expect(since with 1A I can get upwards of 1000V or something(don't remember the specifics now but had 1000V show up on the volt meter);/)

naw... I don't need to spend that kinda money on something I probably won't use much and that I might be able to make on my own(to a good enough approximation to satisfy my needs). I'd rather spend the money on the components and try to make my own and screw up but learn something.

I have seen some schematics on some pwm type of inductor meters that seems relatively easy to do. If I recall correctly it uses a uC or some other method to generate a square wave at some frequency and passes it through the inductor(or some circuit with it in there) and then determines its inductance from that circuit(I think it uses the resonance method you spoke of).

Thanks, Jon

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The simplest and most reliable method kinda depends on what test equipment you have avaiable. Scope? Signal Gem? DMM?

Mike

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[snip...snip...]

I put together one of those LC meters also and agree with Bob that they're a pretty good value for a home shop.

For one-off measurements, if you have a signal generator, frequency counter, and o-scope you can use a simple bandpass layout to get the value of the inductor. Vary the frequency to peak the output; at the peak Zc = Zi.

___ ___ Input| \\_------|___|-----------. measure here here |___/ | | .---o---. | | | | --- C| --- C| | C| | | '---o---' | | === GND (created by AACircuit v1.28 beta 10/06/04

```--
Rich Webb   Norfolk, VA```
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You could build a colpitts oscillator and measure the output frequency.

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```--
Using a series-resonant circuit, you\'ll get a better peak if you do
it like this:```
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If you use 60 Hz to measure a small coil, the inductive reactance will be so small that the resistance in the wire of the coil will swamp it. Been there done that. You can build an inductance meter