# Simple way to measure inductance?

Discussion in 'Electronic Basics' started by Abstract Dissonance, Feb 5, 2006.

1. ### Abstract DissonanceGuest

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

2. ### Guest

60 cycles is NOT Very Practical, Unless there Very Large Inductors.

4. ### Abstract DissonanceGuest

Not sure why this should matter? Does it have to do with the non-ideal
characteristic of the inductor at those frequencies?

Jon

5. ### Abstract DissonanceGuest

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

Jon

7. ### Abstract DissonanceGuest

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

Jon

8. ### JeffMGuest

[Resonance technique]
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:

9. ### David HarmonGuest

On Sat, 4 Feb 2006 23:03:09 -0600 in sci.electronics.basics,
What is the inductive reactance of the inductor you are likely to be
measuring at 60Hz? At 60kHz?

10. ### Bob MonsenGuest

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

appears to me (whether rightly or wrongly) that direct arguments
against christianity & theism produce hardly any effect on the public;
& freedom of thought is best promoted by the gradual illumination of
therefore, been always my object to avoid writing on religion, & I
have confined myself to science. I may, however, have been unduly
biassed by the pain which it would give some members of my family, if
I aided in any way direct attacks on religion"
-- Charles Darwin

11. ### Abstract DissonanceGuest

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

12. ### Abstract DissonanceGuest

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

Jon

13. ### Abstract DissonanceGuest

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

14. ### MikeGuest

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

Mike

15. ### Rich WebbGuest

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 www.tech-chat.de)

16. ### Guest

You could build a colpitts oscillator and measure the output frequency.

17. ### John FieldsGuest

---
Using a series-resonant circuit, you'll get a better peak if you do
it like this:

INPUT----+
|
[C]
|
+---->>---+
| |
[L] [DET]
| |
GND------+---->>---+

Where the impedance of DET at resonance is >> than Xl at the
frequency of interest,

Or a suck-out if you do it like this:

INPUT--[R]--+---->>---+
| |
[C] |
| |
| [DET]
| |
[L] |
| |
GND---------+---->>---+

Where the impedance of the detector becomes less critical.

18. ### kellGuest

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
http://earthground.8m.com/indcap.htm

19. ### kellGuest

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
http://earthground.8m.com/indcap.htm