LF coil inductance?

Discussion in 'Electronic Design' started by Robert Sterns, Sep 5, 2004.

1. Robert SternsGuest

How can I determine the inductance of a coil at frequencies below
10Hz?

It seems most hobbiest meters apply a higher frequency.

Robert

2. Pooh BearGuest

10 Hz is impracticably low for most puroposes.

Why do you think the inductance will change significantly with frequency
?

Graham

3. Tim WescottGuest

10Hz, or 10kHz? 10Hz is _very_ low unless you're working on high-end audio.

In any case, if you can safely assume that the coil's impedance will be
largely a result of it's inductance do the following: connect the coil
in series with a resistor and apply a sine wave at your chosen
frequency. Measure the AC voltage across the resistor, and across the
inductor. The impedance of the coil at that frequency will be:

|Zl| = R * |Vl|/|Vr|.

Assuming that the coil is purely inductive then Xl = |Zl|, and the
inductance of the coil is Xl/(2*pi*f).

If you _can't_ assume that the coil is purely inductive, but you _do_
have an oscilloscope, then see if you can measure Vl and Vr
simultaneously to get both amplitude and phase information. In that
case you can express them as vector quantities and get a vector
impedance. The real part of this impedance will then be the series
equivalent resistance of the coil, and the inductance will be the series
equivalent inductance, at your frequency of interest.

4. Tim WescottGuest

Inductance wouldn't change with frequency, but effective inductance
would. I could see measuring inductance at 10Hz either because I'm
working on high-end tube audio and I'm concerned about performance down
there, or because I'm working at some higher frequency and I want to
eliminate the effects of some suspected parasitic that shows as a
parallel impedance.

5. Jim ThompsonGuest

Isn't "high-end tube audio" an oxymoron ?

And be sure you wire up your speakers using parallel-plate
transmission lines ;-)

...Jim Thompson

6. Michael A. CovingtonGuest

The inductance would have to be very large in order for the reactance to be
appreciable at 10 Hz. What are you measuring the inductance of?

7. Michael A. CovingtonGuest

I think we can assume that anything that has appreciable inductive reactance
at 10 Hz will also have appreciable resistance.

8. Tim WescottGuest

them. I was speaking of high \$\$ = high end.

9. John PopelishGuest

Depends on whether you are talking performance or price.

10. Robert BaerGuest

The lower the frequency, the less effect any parasitics like stray
The only "parasitic" element is the DC resistance, which has an effect
at all reasonable frequencies.
You are getting into an area that is untenable.

11. James MeyerGuest

You failed to warn readers to use **exactly** the same length of
transmission line for every connection in order to preserve proper phase
relationships.

Jim (the other one) Meyer

12. Dave VanHornGuest

How can I determine the inductance of a coil at frequencies below
Assuming you know it's resistance, then you should be able to build a
divider against a known resistor, feed it with a known amplitude at the test
frequency, and measure amplitude and phase shift.

13. Tim WescottGuest

Right. So using a lower frequency will reduce these effects, just like
I said.
Right. So you can't go too low in frequency below the design range of
the inductor or you'll just be measuring the resistance. Hopefully the
OP knows this.
Oh? How?

14. Paul BurridgeGuest

ASCII's not the best medium for reproducing formulas in. What exactly
are the quantities

"Zl" and "Vl" supposed to refer to?

Thanks,

p.

15. John WoodgateGuest

I read in sci.electronics.design that Paul Burridge

No, but with care, it is possible to write quite complex expressions
that can still be deciphered.
Now this is a much more prevalent problem. People don't define their
symbols. Sometimes, you even find five or six defined and then see in
the equations a symbol k or x that isn't defined, so the whole thing is
useless.

16. James MeyerGuest

Perhaps not too useless...

The symbol k is usually taken as a scalar serving as a proportionality
constant. It's normally derived by experimental methods and not really
important to the understanding of the mechanism that the equation is trying to
describe.

And as for x...

The symbol x is usually reserved for an unknown quantity. If it could
be defined, then it wouldn't be unknown, would it?

Jim

17. Tim WescottGuest

Zl would either be the first complex impedance, or the complex impedance
of the coil. Vl refers to voltage, same thing. In this case one would
assume that it's an 'L' and they're referring to the coil.

When I was taking computer programming oh so long ago at age 15 one of
the girls in the class was confused by the model 33 teletype's lack of
lower-case letters, because she had been taught in typing class to
_always_ ignore the '1' key, and use a lower case 'L' instead. She
tried typing numbers with an upper case 'L', but that didn't work.

It was a measure of my ignorance of girls that rather than commiserating
with her (she was very good-looking) I laughed instead.