Inductors
Here are several emails about measuring the inductance of inductors.
Date: 19990528
From: Declan Moriarty <genius@esatclear.ie>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: Inductance testing
I very occasionally want to find the value of an inductor.
This does not justify the purchase of dedicated space consuming
equipment. I'd like to make a simple circuit to allow for an educated
guess to the nearest value, maybe even using a 'scope or frequency
meter. Any suggestions or urls?
Date: 19990528
From: Clint Hamilton <orpheusaudiovideo@home.com>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: Re: Inductance testing
Inductance meters are very cheap. I have one I got from Parts Express that
is a capacitance meter, and checks ohms, hfe on xistors, diodes, and
inductance. Costs about 85 bucks US. There are many out there a little
cheaper and for a whole lot more too. www.partsexpress.com
Date: 19990528
From: John E Freeman <edfree@concentric.net>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: Re: Inductance testing
Declan Moriarty wrote:
I very occasionally want to find the value of an inductor.
This does not justify the purchase of dedicated space consuming
equipment. I'd like to make a simple circuit to allow for an educated
guess to the nearest value, maybe even using a 'scope or frequency
meter. Any suggestions or urls?
This may work, don't know for shure though.
Lets say that the inductor is fairly large so we will use 60(50) Hz line
frequency as a standard. Using a filament type transformer with a safe
secondary voltage of 6-24 RMS volts, place in series to the secondary a
resistor and the inductor in question (the resistor should be great
enough to limit the current to a safe value) Measure the voltage drop
across the resistor, then calculate, using Ohms law the RMS current,
then measure the voltage across the inductor and calculate, using Ohms
law and the RMS current, the effective impedence, now with the frequency
and impedence known you should be able to determine the inductance. for
smaller inductors a higher frequency (must be a known frequency) will be
needed.
Let me know if this works.
Date: 19990528
From: Bob Gordon <kb2zgn@mindspring.com>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: Re: Inductance testing
If you have a function generator, you have all you need to determine the
value of any inductor when used in conjunction with the o'scope and
frequency counter/meter. The added benefit is that you can also get the Q of
the inductor as well with that set-up.
Date: 19990528
From: Bob Gordon <kb2zgn@mindspring.com>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: Re: Inductance testing
This may work, don't know for shure though.
Lets say that the inductor is fairly large so we will use 60(50) Hz line
frequency as a standard. Using a filament type transformer with a safe
secondary voltage of 6-24 RMS volts, place in series to the secondary a
resistor and the inductor in question (the resistor should be great
enough to limit the current to a safe value) Measure the voltage drop
across the resistor, then calculate, using Ohms law the RMS current,
then measure the voltage across the inductor and calculate, using Ohms
law and the RMS current, the effective impedence, now with the frequency
and impedence known you should be able to determine the inductance. for
smaller inductors a higher frequency (must be a known frequency) will be
needed.
Let me know if this works.
I'm afraid that this method will not work. The impedance of an inductor
is not directly proportional to the frequency of the voltage applied,
otherwise this would be a valid test. An inductors impedence is at it's
lowest at a frequency (resonant) determined by several factors including
wire size, coil diameter, and core material and is not linear over a wide
frequency range. The "Q" or bandwidth is determined by finding the points on
either side of the peak frequency where the loss reaches the 3db point and
varies widely depending on the construction of the coil. This can be easily
plotted using a function generator or signal generator, o'scope, and to make
frequency measurement simpler, a frequency counter. Frequency can be
determined from the scope display, but it is not as precise and assumes that
the scope is calibrated. With the advent of cheap inductance meters
available, this procedure is time consuming and the only benefit is in
finding the natural "Q" of the inductor. Save yourself a load of work and
get an inexpensive inductance meter. They have the generator, counter and
can calculate the inductance instantly. The time alone that it saves will
pay for itself.
Date: 19990528
From: Andrew Ingraham <Andrew.Ingraham@compaq.com>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: RE: Inductance testing
Of the three basic passive electrical components (resistors, capacitors,
inductors), inductors are the least ideal.
They tend to have sizable parasitic components (series resistance, shunt
capacitance). Their electrical characteristics may depend more on those
parasitics than on the inductance, over wide frequency ranges. That makes
measuring their inductance more difficult; you need to find the right
frequency range (for each type of inductor) where you are actually able to
see and measure the inductance. Too low a frequency, and DC resistance
swamps it. Too high, and you're above resonance where capacitance
dominates; or you may be into a region of second-order effects, like lead
inductance which could be a lot smaller than the low frequency inductance.
The "right" frequency range depends greatly on the inductor model ... or on
what frequency range is of interest to you.
Many inductors also are very nonlinear in the sense that inductance varies
greatly (orders of magnitude) versus DC current. Also they are affected by
placement, nearby iron, nearby planes, wires, etc.
So take any inductance measurement with a few grains of salt.
Date: 19990528
From: Kerry Berland <kerry@siliconengines-ltd.com>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: Re: Inductance testing
I'm sure you'll get some excellent suggestions
on a roll-your-own approach. But we use a Wavetek
DM27XT multimeter. In addition to the usual--
volts, ohms, amps--it also measures capacitance
and inductance. Takes very little space, and
not much more money than a standard multimeter.
Date: 19990528
From: Logger <wireline@mindspring.com>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: Re: Inductance testing
We use the same meter and like it alot.
Date: 19990531
From: Herne Renaud <Renaud.Herne@UCB-Group.com>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: RE: Inductance testing
What about building an LC oscillator?
This circuit has an unique resonance frequency.
So it will oscillate with a frequency related to L and C.
C is known or can be measured easily
(by measuring this way a known inductance ie: calibrate).
The problem is resumed to a frequency measurement.
The correct frequency is automatically selected provided.
the feedback amplifier of the oscillator has sufficient bandwidth.
The advantage of the frequency measurment is that
it has a very wide dynamic response (1Hz-10MHz is common)
I think that this method of measurement should work
... but I never tryed.
Any comments?
Date: 19990531
From: Declan Moriarty <genius@esatclear.ie>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: RE: Inductance testing
On Fri, 28 May 1999, you informed me:
Of the three basic passive electrical components (resistors, capacitors,
inductors), inductors are the least ideal.
They tend to have sizable parasitic components (series resistance, shunt
capacitance). Their electrical characteristics may depend more on those
parasitics than on the inductance, over wide frequency ranges. That makes
measuring their inductance more difficult; you need to find the right
frequency range (for each type of inductor) where you are actually able to
see and measure the inductance. Too low a frequency, and DC resistance
swamps it. Too high, and you're above resonance where capacitance
dominates; or you may be into a region of second-order effects, like lead
inductance which could be a lot smaller than the low frequency inductance.
The "right" frequency range depends greatly on the inductor model ... or on
what frequency range is of interest to you.
Many inductors also are very nonlinear in the sense that inductance varies
greatly (orders of magnitude) versus DC current. Also they are affected by
placement, nearby iron, nearby planes, wires, etc.
So take any inductance measurement with a few grains of salt.
Can I refine my query? I am amazed by the huge response, and
thanks to you all. I do not have a function generator - I do have an
RF generator (valve operated Heathkit ;-).
The main inductors I come across are small ones from switched
mode power supplies, 20-400 uH. These may have a lead broken. Apart
from actual value in Henries or fractions thereof, nothing matters. I
will not be able to specify all design details; But I can replace with
one from the correct range and hope that most things turn out equal.
It's called making money, not ethical, or perfect. I had thoughts
about a few things (part of a D.C. current transformer) to do this,
a square edge into a capacitor with the inductor inline. Give me a few
days, and I'll draw up and submit the idea on a .gif.
Date: 19990601
From: Andrew Ingraham <Andrew.Ingraham@compaq.com>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: RE: Inductance testing
What about building an LC oscillator?
This circuit has an unique resonance frequency.
A possible problem is that the real circuit DOESN'T have an unique resonance
frequency, due to all the parasitic effects I've mentioned. If your
amplifier has enough gain at high frequencies, it is possible to excite one
of those modes rather than the primary parallel resonance. I don't know how
often this happens, but it is a potential pitfall.
Also you should account for the amplifier's capacitance, which might be
significant compared to the inductor's own.
As a sanity check, connect a known capacitor in parallel, repeat the
measurement, and see if the oscillation frequency changes the right way by
the right amount. If not, maybe you caught one of those parasitic modes.
Anyone remember a Grid Dip Meter? In the old days, no radio engineer would
be without one. One use for those things was to "measure" inductance, by
finding the lowest resonant frequency, with a known capacitor in parallel.
The idea is similar to making an oscillator, but a whole lot less automatic.
I imagine it was tedious, because you had to sweep the meter over several
ranges, and you needed to make sure you found the lowest frequency where
there was a dip ... not one of those higher modes.
Some coils (ferrite beads, but also some leaded components) are made to
appear resistive rather than inductive (much less than 90 degrees phase
shift between voltage and current) over a wide frequency range. You might
not be able to get them to oscillate readily. But with such a device, maybe
you don't care so much about the inductance anyway.
Date: 19990606
From: Declan Moriarty <genius@esatclear.ie>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: Inductance Testing Notion
I have submitted a file called induct1.gif. Those interested
can collect it. It contains my notion on Inductance Testing which I
threatened you all with last week. This is part circuit, part block
diagram, part omitted. Here is a decscription: (Fairly boring for
everyone else :-)
Transformer 1 is a ferrite pot core; It has one feedback winding, and
one centretapped main winding: the centretap goes to +Vcc*; One end
of the main winding goes to T1, and the other end to T2, via a n/c
switch across which the device to be tested is connected. The n/c
switch is simply to facilitate zeroing, if needed, and to keep
tempratures equal.
T1 and T2 have 10 ohm resistors R1 & R2 on the emitters, and are driven
such that when either T1 or T2 are on, the other is off; they switch
over when the voltage across the emitter resistor reaches a threshold
of 1V, so the current rises from 0 to 100 mA. With me so far? Good.
With the test/zero button closed, the inductance of the pot core
governs frequency, and, all things being equal, the dutycycle of the
waveform across R1, and R2 should be 50% 0V, followed by a sawtooth
with a peak of 1V, as ahown.
If another inductor is added to the equation by opening this test/zero
switch , duty cycle should change, and T2 will have a longer sawtooth
rise than T1 when the test button is open circuit. That facilitates
measurement. The math would involve L1(the pot core) * L2 / L1 + L2
type sums, something even I can manage. Another and perhaps better
approach would be to measure duty cycle on T1 & T2 - the longer the
duty cycle, the bigger the device.
I like the idea because
- Because it's mine :) - funny how that happens always, isn't it?
- Because it measures inductance only. It would have to have fairlylarge parasitic capacitance to affect things at the currents used.
Date: 19990607
From: Radek Soltys <rspublic@friko6.onet.pl>
To: Multiple recipients of list CHIPDIR-L <CHIPDIR-L@fatcity.com>
Subject: Re: Inductance testing
I very occasionally want to find the value of an inductor.
This does not justify the purchase of dedicated space consuming
equipment. I'd like to make a simple circuit to allow for an educated
guess to the nearest value, maybe even using a 'scope or frequency
meter. Any suggestions or urls?
Here's a simple circuit that should allow you
inductance approximate recognition.
I haven't tested it yet, but it is very simple,
thus worth trying.
If you have any questions about its operation
or assembly - contact me personally and I'll
refer to the Polish article where I found the
schematic.
