Simple way to measure inductance?

[Abstract Dissonance]

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

 

[[email protected]]

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

 

[JeffM]

I have some inductors with unknown inductance

and I'm looking for a...method to measure its value
Jon (Abstract Dissonance)

http://www.google.com/images?q=signal-generator+resonance+auckland
http://www.google.com/images?q=series-resonance+L+C+ibiblio

 

[Abstract Dissonance]

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

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

Jon

 

[Abstract Dissonance]

http://www.google.com/images?q=signal-generator+resonance+auckland
http://www.google.com/images?q=series-resonance+L+C+ibiblio

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

Jon

 

[JeffM]

http://www.google.com/images?q=signal-generator+resonance+auckland

I can't find anything on those pages
that talks about measuring real inductors.
Jon ( Abstract Dissonance)

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

 

[Abstract Dissonance]

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

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

Jon

 

[JeffM]

[Resonance technique]

um... and how is that any different than what I did?
Jon (Abstract Dissonance)

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).
..
..

Also where does it talk about real world issues
that are involved?

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:
http://froogle.google.com/froogle?scoring=p&price1=1&price=between&q=impedance-bridge

 

[David Harmon]

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

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

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

 

[Bob Monsen]

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 ?

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:

http://www.aade.com/lcmeter.htm

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

--
Regards,
Bob Monsen

"I am a strong advocate for free thought on all subjects, yet it
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
men's minds, which follow from the advance of science. It has,
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

 

[Abstract Dissonance]

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?

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

 

[Abstract Dissonance]

[Resonance technique]
When the voltage across the resistor peaks,
note the frequency and solve for L.
JeffM

um... and how is that any different than what I did?
Jon (Abstract Dissonance)

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).

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)).

.
.
There is an assumption
that the "Q" (quality factor) of the coils isn't complete crap.
Your initial request WAS for a ballpark number.

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.

If you want to spend some $$
and quantify the parts more completely:
http://froogle.google.com/froogle?scoring=p&price1=1&price=between&q=impedance-bridge

Jon

 

[Abstract Dissonance]

If your inductor is 1uF, then the impedance is

uH? isn't that kinda small?

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

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

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);/)

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:

http://www.aade.com/lcmeter.htm

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).

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

--
Regards,
Bob Monsen

"I am a strong advocate for free thought on all subjects, yet it
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
men's minds, which follow from the advance of science. It has,
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

Thanks,
Jon

 

[Mike]

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

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

Mike

 

[Rich Webb]

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 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)

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).

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

--
Regards,
Bob Monsen

"I am a strong advocate for free thought on all subjects, yet it
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
men's minds, which follow from the advance of science. It has,
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

Thanks,
Jon

 

[[email protected]]

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

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

 

[John Fields]

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

---
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.

 

[kell]

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

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

 

[kell]

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

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