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'[EE]: measuring inductance'
2001\05\30@182914 by Andy N1YEW

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how do i measure inductance of an inductor?

andy

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2001\05\30@185155 by David VanHorn

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At 05:39 PM 5/30/01 -0400, Andy N1YEW wrote:
>how do i measure inductance of an inductor?

Couple ways.

Hang a known cap on it, and measure resonant freq, or hang a known voltage,
and measure dI/dT.

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2001\05\30@190627 by Mike Hardwick

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>how do i measure inductance of an inductor?

Assuming you don't have an impedance bridge or inductance meter, what
equipment *do* you have, and what does the inductor look like?

Mike Hardwick

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2001\05\30@221541 by Gabriel Caffese

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With a Q-meter. You also can get th Rd and Cd asocaited with it. (for a
specific working frequency).

{Original Message removed}

2001\05\30@232909 by James R Albers

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Hello Andy,

I connect the unknown inductor across a known capacitor.  Then I take my
dip meter and dip the parallel LC circuit that I have made.  Then I read
the frequency off of my dip meter, and plug this number into the standard
formula for resonance.  This only works well up to about 150 MHz.

Of course, if you're an engineer, you'll want to use a computer model.

73,

Jim Albers N9CYL

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2001\05\31@011022 by hard Prosser

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In my experiance it depends very much on the type of inductor & its losses.
For air cored RF types, a known Cap and a dip meter or some other form of
resonance test seemd to work best.
For large iron cored types at low frequency a voltage/current phase
measurement has been more successful - particularly if the Q is low.

An LCR meter can give you a good quick (and sometimes rough) indication
also!

You also may need to make sure that it is measured as close to the actual
operating frequency & drive level (including any DC) as possible to
minimise errors due to skin effect, core loss, saturation etc. This can
increase the measurement difficulties significantly.


Richard P





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Hello Andy,

I connect the unknown inductor across a known capacitor.  Then I take my
dip meter and dip the parallel LC circuit that I have made.  Then I read
the frequency off of my dip meter, and plug this number into the standard
formula for resonance.  This only works well up to about 150 MHz.

Of course, if you're an engineer, you'll want to use a computer model.

73,

Jim Albers N9CYL

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2001\05\31@044350 by David VanHorn

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At 10:06 PM 5/30/01 -0500, James R Albers wrote:
>Hello Andy,
>
>I connect the unknown inductor across a known capacitor.  Then I take my
>dip meter and dip the parallel LC circuit that I have made.  Then I read
>the frequency off of my dip meter, and plug this number into the standard
>formula for resonance.  This only works well up to about 150 MHz.
>
>Of course, if you're an engineer, you'll want to use a computer model.

I don't know about that, I have and use, a heathkit dip meter. :)
Models are fine, as long as you don't expect detailed accuracy in the real
world.
Too often, little details aren't in the model..


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2001\05\31@075658 by Olin Lathrop

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> how do i measure inductance of an inductor?

I assume you want an answer other than "use an inductance meter, of course".

The first thing I would do with an unknown inductor is measure the DC
resistance.  You will probably need to know this so you can account for it
when trying to measure the pure inductance.  In other words, you can think
of real inductors as a series combination of a resistor and an inductor.

The simplest way to measure the inductance is to find the time constant with
a known resistance.  Connect a known resistor to a regulated +5V supply and
the other end to the inductor.  Look at the resistor/inductor junction
voltage with a scope.  It should show 5V for now because the other end of
the inductor is unconnected.  When the other end of the inductor is
connected to ground, you should see an exponential decay towards ground
(almost, depends on inductor series resistance).  Adjust the resistor value
so that time constant is reasonable.  You can now compute inductance from
(time constant) = RL.


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Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, spamBeGoneolinspamBeGonespamembedinc.com, http://www.embedinc.com

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2001\05\31@080757 by Olin Lathrop

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> In my experiance it depends very much on the type of inductor & its
losses.
> For air cored RF types, a known Cap and a dip meter or some other form of
> resonance test seemd to work best.

This works fine, but note that the accuracy is only as good as the cap.
Accurate resistors are much more obtainable than accurate caps.  If accuracy
is an issue, then use an LR method instead of an LC method.


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Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, RemoveMEolinspamTakeThisOuTembedinc.com, http://www.embedinc.com

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2001\05\31@103135 by Andy N1YEW

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ok, so how would i do it with a pic?  i assume i would have to measure the
decay time and know the resistor value and then compute it and display it.

andy
{Original Message removed}

2001\05\31@122423 by rottosen

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Andy N1YEW wrote:
>
> ok, so how would i do it with a pic?  i assume i would have to measure the
> decay time and know the resistor value and then compute it and display it.
>


You could make an LR oscillator and measure its period with the PIC  :-)


-- Rich




> andy
> {Original Message removed}

2001\05\31@192516 by Robert A. LaBudde

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At 10:18 AM 5/31/01 -0600, Rich wrote:
>Andy N1YEW wrote:
> >
> > ok, so how would i do it with a pic?  i assume i would have to measure the
> > decay time and know the resistor value and then compute it and display it.
> >
>
>
>You could make an LR oscillator and measure its period with the PIC  :-)

Actually, any method to measure capacitance could be used to measure
inductance.

Just replace the R-C voltage divider with an L-R voltage divider in the
circuit used.

This would include the single- or dual-slope PIC-pin methods, a monostable
or astable multivibrator or simple impedance voltage divider.

So just find a circuit for a capacitance meter application, and swap L for
C and flip with R.

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Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
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2001\05\31@210518 by Olin Lathrop

face picon face
> ok, so how would i do it with a pic?  i assume i would have to measure the
> decay time and know the resistor value and then compute it and display it.

Yeah, that's a bit more tricky than doing it manually.  I would use a
comparator to indicate when the decay got to a particular level and time how
long that takes.  You may have to switch in different resistors for
different ranges of inductances.  You will also have to measure the steady
state level to account for and measure the inductor's series resistance.


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Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, RemoveMEolinspam_OUTspamKILLspamembedinc.com, http://www.embedinc.com

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'[EE]: measuring inductance'
2001\06\01@080527 by Andy N1YEW
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hi
----- Original Message -----
From: "Olin Lathrop" <EraseMEolin_piclistspamspamspamBeGoneEMBEDINC.COM>
To: <RemoveMEPICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Thursday, May 31, 2001 6:14 PM
Subject: Re: [EE]: measuring inductance


> > ok, so how would i do it with a pic?  i assume i would have to measure
the
> > decay time and know the resistor value and then compute it and display
it.
>
> Yeah, that's a bit more tricky than doing it manually.  I would use a
> comparator to indicate when the decay got to a particular level and time
how
> long that takes.  You may have to switch in different resistors for
> different ranges of inductances.  You will also have to measure the steady
> state level to account for and measure the inductor's series resistance.

thats a bit harder than i thouht it would be :(

andy
{Quote hidden}

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2001\06\01@172954 by Peter L. Peres

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> ok, so how would i do it with a pic?  i assume i would have to measure
> the decay time and know the resistor value and then compute it and
> display it.

If I was to do this I'd try to build a 'gate oscillator' using a 74HC00 or
such and implement a counter using the PIC. By using a set of good quality
capacitors and maybe a transistor or two to switch them into the circuit
it could cover a pretty wide range. Certainly better than 1:10000 with 3
ranges. (I predict that the next thread will be about square root
extraction using PIC).

Peter

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2001\06\01@174610 by Andy N1YEW

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hello
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To: <EraseMEPICLISTspamEraseMEMITVMA.MIT.EDU>
Sent: Friday, June 01, 2001 5:12 AM
Subject: Re: [EE]: measuring inductance


{Quote hidden}

why a 74hc00 instead of a 74hc04?

i have both....
why would i need a sqr root?(the formula maybe?)

andy

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2001\06\02@055049 by Peter L. Peres

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> why 74HC00

Because it helps you later with the counter gate. You can also use a
resistor instead as gate together with a tristateable PIC pin (my
favorite).

> square root

Because of the formula (Thomson).

You could use a 4066 switch to switch the capacitors in. For three scales
you need only two switches (one cap stays in the circuit all the time).

Peter

PS: L-meters are sometimes built using an analog RL circuit with a DVM
used as readout. You may want to look into this. Mostly you output a
square wave from the PIC, it goes through a R and the L to be measured, it
is rectified by a opamp 'perfect rectifier' and read by an A/D. Since you
can change the frequency as you please. Maybe this is better than the
oscillator for you.

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