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How to measure inductance indirectly?

D

DaveC

Jan 1, 1970
0
I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking for
a peak in V (accross the inductor) and a drop in I?

Thanks,
 
M

Mike

Jan 1, 1970
0
I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking for
a peak in V (accross the inductor) and a drop in I?

Thanks,

It can be, but your results might be better if you add a series resistance
before the tank circuit (and possibly a terminating resistance to the
transmission line from the signal generator).

If your tank circuit is placed in parallel with a low resistance (like 50
ohms from your signal generator, for example), the effective Q of the
circuit will be reduced, and it will be more difficult to distinguish the
amplitude peak when you sweep the frequency. On the other hand, increasing
the series resistance before the tank will reduce the peak amplitude that
you see at resonance, but the frequency peak will be sharper. Infinite
resistance will give the sharpest peak, but also zero voltage - somewhere
between infinite resistance and zero resistance is the 'best' measurement
point.

-- Mike --
 
D

DaveC

Jan 1, 1970
0
Just feed any frequency across the inductor.
Read the voltage across it and the current through it (like across a 1 ohm
resistor or a .1 ohm resistor)

RMS voltage? RMS current?
The voltage divided by the current is the reactance of the inductor at that
frequency and then just use the formula for reactance.
Xl= 2piFL

XL = V inductor / I inductor

so

L = X / (2*pi*F) right?
And viola with just your signal gen and a resistor and a voltmeter you can
measure the L.

Try it at a couple different freqs and it should come out pretty close.

You have to use a high enough freq to swamp out the resistance though. But a
freq with in the range of your voltmeter.

Can you suggest a few good frequencies to measure at? I've got an old Fluke
handheld meter (don't recall the model #...)

The inductance value stays (relatively) constant across a range of
frequencies? If I determine L at 1 Khz, for example, will that inductor have
the same value at 20 Khz?

Thanks,
 
T

Tony Williams

Jan 1, 1970
0
DaveC said:
I've got a scope and signal generator. Is it as simple as hooking
a known value of C in series with the L and sweeping the
frequency range looking for a peak in V (accross the inductor)
and a drop in I?
/|\
peak in the feed-I for a series-tuned LC.

More or less, yes. Although I go for a parallel L-C
and high value resistive connection to the generator.
Using different values of tuning-C then allows an easy
guesstimate of the inductor's own stray-C.
 
W

Winfield Hill

Jan 1, 1970
0
Fred Bloggs wrote...
If the inductors are to be used for RF or some other linear
application where Q might be of interest then a simple technique
is to use the Hay bridge at the application frequency:

Hay Bridge

+-----------+--------------------------------+
| | |
| \ _ \
| R1 / /| R3 /
| \ \
| // /
| | _ |
| | /| |
| C1 === +---------------------------+
--- /| | +-----+ |
/ \ | )|( | |
| ~ | sig | )|( | |
\ / gen | )|( === |
--- | | | | |
| +----+ | sd | +------+
| | +-|>|-+-/\/\-+-> VOM | |
| \ | | ) \
| R2 / | === Lx ) Rx /
| \ | detector | ) \
| / | | ) /
| | | | | |
+-----------+------+------------+------------+------+
|
---
///


Adjust R1 C1 for null at operating w = 2pi f, where:

(R1-j/(wC1)) (Rx*jwLx) = R2R3 (Rx+jwLx)

Lx
=> Rx* -- = R2R3Rx or Lx = C1*R2*R3
C1

and
R2*R3
=> R1RxwLx = R2R3wLx or Rx = -----
R1

Rx
Q = ---
wLx


Very nice. Tell us about the requirements on the transformer.

Thanks,
- Win

whill_at_picovolt-dot-com
 
J

Jerry G.

Jan 1, 1970
0
The simplest way, is to get a meter that is designed to read inductance. A
good inductance meter is fairly expensive. Leader has a very good laboratory
type at a reasonable cost, compared to many of the others.

The more complex way is to make a resonant circuit with the inductor, and
use your generator and scope, with some math to figure it out, as per the
suggestions you received.

I've used a lot of Leader equipment, and found it to be very good.


Site for inductance meters
http://www.tequipment.net/LeaderLCR-745G.html

http://www.testequipmentconnection.com/manu-search.asp?smanufacturer=leader



--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking for
a peak in V (accross the inductor) and a drop in I?

Thanks,
 
D

DaveC

Jan 1, 1970
0
The simplest way, is to get a meter that is designed to read inductance. A
good inductance meter is fairly expensive. Leader has a very good laboratory
type at a reasonable cost, compared to many of the others.

I looked around and found Wavetek's Meterman series LCR55 which retails for
about US$180. Cheapest solution for a meter.

Thanks,
 
P

Paul Burridge

Jan 1, 1970
0
The simplest way, is to get a meter that is designed to read inductance. A
good inductance meter is fairly expensive. Leader has a very good laboratory
type at a reasonable cost, compared to many of the others.

The more complex way is to make a resonant circuit with the inductor, and
use your generator and scope, with some math to figure it out, as per the
suggestions you received.

I've used a lot of Leader equipment, and found it to be very good.


Site for inductance meters
http://www.tequipment.net/LeaderLCR-745G.html

http://www.testequipmentconnection.com/manu-search.asp?smanufacturer=leader
Alternatively, get a grid dip meter! Made for the job...
 
R

Robert Monsen

Jan 1, 1970
0
DaveC said:
I looked around and found Wavetek's Meterman series LCR55 which retails for
about US$180. Cheapest solution for a meter.

Various folks have mentioned this one as a cheap meter that does a fairly
good job:

http://www.aade.com/lcm2binst/LC2Binst.htm

Its $100 in kit form, or $130 assembled and tested. That's USD.
 
D

daestrom

Jan 1, 1970
0
DaveC said:
RMS voltage? RMS current?


XL = V inductor / I inductor

so

L = X / (2*pi*F) right?


Can you suggest a few good frequencies to measure at? I've got an old Fluke
handheld meter (don't recall the model #...)

The *inductance* will stay pretty constant over a range of frequencies,
provided you're not driving the core (if it has one) into saturation. But
measure the DC resistance also, and compare with the reactance. If they are
the same order of magnitude, then you may need to do some more math to
figure out the true inductance.

If you put a resistor in series (say, 100 ohms), then 'tune' the frequency
so the voltage across the inductor is about the same (albeit, not in phase
with) as the resistor, they you should have a pretty good voltage/current
reading to do the calculations.

You may also need to apply some 'gut' feeling. After all, if its a big
multi-turn iron core, or little air-core inductor, your technique may have
to be adjusted.

daestrom
 
D

DaveC

Jan 1, 1970
0
Just feed any frequency across the inductor.
Read the voltage across it and the current through it (like across a 1 ohm
resistor or a .1 ohm resistor)

My question is how to calculate reactance:
The voltage divided by the current is the reactance of the inductor at that
frequency (XL = VL / IL)
Or:

and then just use the formula for reactance.
XL= 2piFL
And viola with just your signal gen and a resistor and a voltmeter you can
measure the L.

Your two formulae confused me. Which one should I use for calculating XL?
Does it matter? Are they equivalent?

I hooked a choke in series with a 1-ohm 10 watt resistor. I took some
measurements (VL, VR).

I used XL = 2*pi*F*L and reformed the formula to:
L = X / (2*pi*f) -- is this right?

None of the known L values I measured worked out in calculation.

What am I doing wrong?

Thanks,
 
T

Terry Given

Jan 1, 1970
0
DaveC said:
I have several inductors I'd like to know the values of. I have no way to
measure L directly, ie, with a meter.

I've got a scope and signal generator. Is it as simple as hooking a known
value of C in series with the L and sweeping the frequency range looking for
a peak in V (accross the inductor) and a drop in I?

Thanks,

If you are mesuring smps type inductors (or, for that matter, any form of
power inductor/transformer) then V=LdI/dt is the simplest way. All you need
is a scope and a couple of bits. Charge up a large cap to some voltage V,
then short your inductor across the cap while measuring current (a hall
effect DCCT is useful here, but so is a resistor. beware peak pulse power
and resistor ESL). If the energy stored in the cap 0.5CV^2 >> energy in L at
current of interest 0.5LI^2, then V remains pretty much constant, and from
the slope of the current waveform, L=V/(dI/dt). If you dont have a
storage/digital scope, repetitively pulse the choke using a big FET. I
typically charge the caps with a current limited PSU & a series R. This
approach costs very little, takes a few seconds and gives a lot of
information - saturation current, and inductance-vs-current - iron powder &
step-gap chokes have very non-linear inductance, and the inductance at very
low current can be order(s) of magnitude higher than that at rated current,
leading to rubbish results from most inductance meters. Its easy to test a
choke at 5,000A this way too.

Terry
 
K

Klaus Vestergaard Kragelund

Jan 1, 1970
0
Fred Bloggs said:
Not really- it depends on what the application is. If for example the
inductors are for a switching power supply application, then you would
want to measure the current response to a step voltage at various levels
of DC bias- inductance is inferred from rise time L/R. If the inductors
are to be used for RF or some other linear application where Q might be
of interest then a simple technique is to use the Hay bridge at the
application frequency:

Also for SMPS or RFI filters one major thing to look out for is the
saturation current (core saturation) versus the core temperature. At
elevated temperatures, 100 degrees or so, the saturation current may be
significantly reduced.....

Cheers

Klaus
 
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