Measuring small inductances

P

Paul Burridge

Guest
Hi all,

I've got quite a huge stash of inductors in my parts bin. The colour
codes don't always seem to relate to the values I've been able to
measure, with my multi-function DVM, however, and I can't accurately
measure any inductor < about 10uH. Is there a circuit anywhere that
would enable me to get a reasonably accurate idea of the values I've
got down to say 100nH or thereabouts?

Thanks,

p.
--

The BBC: Licensed at public expense to spread lies.
 
"Paul Burridge" <pb@osiris1.notthisbit.co.uk> wrote in message
news:d52b50l8hqa05qrj05gchhng7ft0lh4tq4@4ax.com...
Hi all,

I've got quite a huge stash of inductors in my parts bin. The colour
codes don't always seem to relate to the values I've been able to
measure, with my multi-function DVM, however, and I can't accurately
measure any inductor < about 10uH. Is there a circuit anywhere that
would enable me to get a reasonably accurate idea of the values I've
got down to say 100nH or thereabouts?
Click to expand...
www.google.com
 
"Paul Burridge" <pb@osiris1.notthisbit.co.uk> wrote in message
news:d52b50l8hqa05qrj05gchhng7ft0lh4tq4@4ax.com...
Hi all,

I've got quite a huge stash of inductors in my parts bin. The colour
codes don't always seem to relate to the values I've been able to
measure, with my multi-function DVM, however, and I can't accurately
measure any inductor < about 10uH. Is there a circuit anywhere that
would enable me to get a reasonably accurate idea of the values I've
got down to say 100nH or thereabouts?
Click to expand...
If you have a signal generator you can connect them to a suitable capacitor
and measure the resonant frequency.

Leon
 
On Mon, 15 Mar 2004 12:48:43 -0000, Leon Heller wrote:

"Paul Burridge" <pb@osiris1.notthisbit.co.uk> wrote in message
news:d52b50l8hqa05qrj05gchhng7ft0lh4tq4@4ax.com...
Hi all,

I've got quite a huge stash of inductors in my parts bin. The colour
codes don't always seem to relate to the values I've been able to
measure, with my multi-function DVM, however, and I can't accurately
measure any inductor < about 10uH. Is there a circuit anywhere that
would enable me to get a reasonably accurate idea of the values I've
got down to say 100nH or thereabouts?

If you have a signal generator you can connect them to a suitable capacitor
and measure the resonant frequency.

Leon
Click to expand...
Nyuk, nyuk, nyuk. I bet if he had a signal generator, he wouldn't be
looking for inductors to shotgun that multiplier :) In fact, he
wouln't even be building a multiplier.
--
Best Regards,
Mike
 
On Mon, 15 Mar 2004 12:48:43 -0000, "Leon Heller"
<leon_heller@hotmail.com> wrote:

"Paul Burridge" <pb@osiris1.notthisbit.co.uk> wrote in message
news:d52b50l8hqa05qrj05gchhng7ft0lh4tq4@4ax.com...
Hi all,

I've got quite a huge stash of inductors in my parts bin. The colour
codes don't always seem to relate to the values I've been able to
measure, with my multi-function DVM, however, and I can't accurately
measure any inductor < about 10uH. Is there a circuit anywhere that
would enable me to get a reasonably accurate idea of the values I've
got down to say 100nH or thereabouts?

If you have a signal generator you can connect them to a suitable capacitor
and measure the resonant frequency.
Click to expand...
Hi Leon,

That's what I have been doing, in fact. It's just I'd prefer to have a
more convenient, portable method to get instant read-outs of coil
values...
--

The BBC: Licensed at public expense to spread lies.
 
On Mon, 15 Mar 2004 10:46:53 +0000, Paul Burridge
<pb@osiris1.notthisbit.co.uk> wrote:

I've got quite a huge stash of inductors in my parts bin. The colour
codes don't always seem to relate to the values I've been able to
measure, with my multi-function DVM, however, and I can't accurately
measure any inductor < about 10uH. Is there a circuit anywhere that
would enable me to get a reasonably accurate idea of the values I've
got down to say 100nH or thereabouts?
Click to expand...
_________________________________________________________

I'd recommend a grid dip meter and a known capacitor, but be sure to
make the test at a frequency well below the self resonant frequency of
the inductor alone.

Another thing to be wary of: Some inductors are wound on a resistor to
reduce the Q for a specific purpose. Be suspicious of any which have an
unusually broad dip.

--
Bill, W6WRT
QSLs via LoTW
 
"Bill Turner" <noway@nohow.com> wrote in message
news:0qhb50515j45ikdfcaefsqbu1bev6ole56@4ax.com...
On Mon, 15 Mar 2004 10:46:53 +0000, Paul Burridge
pb@osiris1.notthisbit.co.uk> wrote:

I've got quite a huge stash of inductors in my parts bin. The colour
codes don't always seem to relate to the values I've been able to
measure, with my multi-function DVM, however, and I can't accurately
measure any inductor < about 10uH. Is there a circuit anywhere that
would enable me to get a reasonably accurate idea of the values I've
got down to say 100nH or thereabouts?

_________________________________________________________

I'd recommend a grid dip meter and a known capacitor, but be sure to
make the test at a frequency well below the self resonant frequency of
the inductor alone.

Another thing to be wary of: Some inductors are wound on a resistor to
reduce the Q for a specific purpose. Be suspicious of any which have an
unusually broad dip.

--
Bill, W6WRT
QSLs via LoTW
Click to expand...
This is what I do; it works well even on my questionable dip meter.

Google for "LCR" meter; see what you come up with (mostly high $
instruments, probably).
 
On Mon, 15 Mar 2004 07:40:36 -0800, "Tim Wescott"
<tim@wescottnospamdesign.com> wrote:

Google for "LCR" meter; see what you come up with (mostly high $
instruments, probably).
Click to expand...
This one is great:

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


John
 
Hi,

http://www.aade.com/lcmeter.htm
Click to expand...
Or, as you have a counter, you could save yourself the money by
copying his oscillator and use a calculator to find the value of L
or C from the change in frequency.


Cheers - Joe
 
On Mon, 15 Mar 2004 07:12:42 -0800, Bill Turner <noway@nohow.com>
wrote:

I'd recommend a grid dip meter and a known capacitor, but be sure to
make the test at a frequency well below the self resonant frequency of
the inductor alone.
Click to expand...
I'm afraid my experience of using GDMs is *very* unfavourable. They
were fine in the days of big old HF kit, but nowadays with
PCB/surface-mounted stuff, they're *ucking useless.

Another thing to be wary of: Some inductors are wound on a resistor to
reduce the Q for a specific purpose. Be suspicious of any which have an
unusually broad dip.
Click to expand...
ISTR some valve anode chokes used this arrangement in HF rigs a couple
of decades ago...

--

The BBC: Licensed at public expense to spread lies.
 
On Mon, 15 Mar 2004 08:10:02 -0800, John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

On Mon, 15 Mar 2004 07:40:36 -0800, "Tim Wescott"
tim@wescottnospamdesign.com> wrote:


Google for "LCR" meter; see what you come up with (mostly high $
instruments, probably).


This one is great:

http://www.aade.com/lcmeter.htm
Click to expand...
Crumbs! Thanks, John!
--

The BBC: Licensed at public expense to spread lies.
 
On Mon, 15 Mar 2004 15:12:24 +0000, Paul Burridge wrote:

On Mon, 15 Mar 2004 12:48:43 -0000, "Leon Heller"
leon_heller@hotmail.com> wrote:


"Paul Burridge" <pb@osiris1.notthisbit.co.uk> wrote in message
news:d52b50l8hqa05qrj05gchhng7ft0lh4tq4@4ax.com...
Hi all,

I've got quite a huge stash of inductors in my parts bin. The colour
codes don't always seem to relate to the values I've been able to
measure, with my multi-function DVM, however, and I can't accurately
measure any inductor < about 10uH. Is there a circuit anywhere that
would enable me to get a reasonably accurate idea of the values I've
got down to say 100nH or thereabouts?

If you have a signal generator you can connect them to a suitable capacitor
and measure the resonant frequency.

Hi Leon,

That's what I have been doing, in fact. It's just I'd prefer to have a
more convenient, portable method to get instant read-outs of coil
values...
Click to expand...
So why beat yourself to death building a sweep gen to test filters
when you got that there sig gen?
--
Best Regards,
Mike
 
On Mon, 15 Mar 2004 18:40:07 +0000 (UTC), Joe McElvenney
<ximac@btinternet.com> wrote:

Hi,

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

Or, as you have a counter, you could save yourself the money by
copying his oscillator and use a calculator to find the value of L
or C from the change in frequency.


Cheers - Joe
Click to expand...
To save $100, I'm not going to copy *anybody's* circuits.

John
 
"Paul Burridge" <pb@osiris1.notthisbit.co.uk> wrote in message
news:d52b50l8hqa05qrj05gchhng7ft0lh4tq4@4ax.com...
Hi all,

I've got quite a huge stash of inductors in my parts bin. The colour
codes don't always seem to relate to the values I've been able to
measure, with my multi-function DVM, however, and I can't accurately
measure any inductor < about 10uH. Is there a circuit anywhere that
would enable me to get a reasonably accurate idea of the values I've
got down to say 100nH or thereabouts?
Click to expand...
"splat" test

V=L*dI/dt

charge up a large cap to some voltage V

short the cap thru the inductor, and measure the current with a scope
(digital is great, as long as the single-shot sample rate is high enough - I
did this once with an HP 56000 series scope, and got 1 data point - very
easy to fit a line to, very hard to fit the right line to!)

knowing dI/dt and V, calculate L

then, you can see saturation from where dI/dt increases rapidly (invaluable
for power electronics)

also, ensure that 0.5CV^2 >> 0.5LI^2 at the current of interest - this
ensures that V is approximately constant

I have a couple of 33mF 35V caps in parallel, with 10 x 1R MRS25 resistors
paralleled as a current sensor. It takes about 45s to set up for a
measurement. I often use the shaft of a screwdriver to make the "splat" - a
nice hard material is good.

I have used this technique successfully to measure inductors ranging from
the tiny (< 500nH) to huge (3mH 2,000A chokes). Actually, big chokes tend
not to read true on LCR meters, as the magnetic material permeability is
often much higher at very low currents - especially true of iron powder.

if you dont have a digital scope, make a tiny, LF oscillator with a 555.
Drive a grunty FET, with a small (I use 0.1R - 10R) source resistor. Then,
you'll get a repetitive waveform, which works nicely on any analogue scope.
If Rsense is nice and low, use a 50R BNC cable to connect to scope, with a
50R terminator at the scope - lovely clean waveforms.

I built a little tester like this to test air gaps in planar cores for a 55W
smps that lives inside an LED video screen (actually, thousands of them live
inside). works a treat, and cost < $5.

One of my techs once built a splat tester for big chokes - he had several
1200V 600A IGBT's in parallel running as a series pass regulator, with about
100mF of capacitance (charged to 700Vdc thru several lightbulbs) to control
V, even at currents on the order of 5,000A. A honking great LEM DCCT
measured the current, and the "switch" was purely mechanical - he used the
pole faces from an HV contactor, as the material is astonishingly arc
resistant, so no pitting/welding
 
On Mon, 15 Mar 2004 19:01:01 -0500, Active8 <reply2group@ndbbm.net>
wrote:

So why beat yourself to death building a sweep gen to test filters
when you got that there sig gen?
Click to expand...
Because it has to be small and light enough to be mounted on our robot
for field checks. Try doing that with a 120lb Marconi. :)

--

The BBC: Licensed at public expense to spread lies.
 
On Tue, 16 Mar 2004 13:02:44 +0000, Paul Burridge wrote:

On Mon, 15 Mar 2004 19:01:01 -0500, Active8 <reply2group@ndbbm.net
wrote:

So why beat yourself to death building a sweep gen to test filters
when you got that there sig gen?

Because it has to be small and light enough to be mounted on our robot
for field checks. Try doing that with a 120lb Marconi. :)
Click to expand...
That's more macaroni than even *this* eyetalian can eat.

You're confusing me again. If you have a filter on a bot in the
field, it's the same filter that was on the bot before you got there
and should do the same thing.

If you change a filter in the field, you'd want to sweep it before
it's installed in der bot.

--
Best Regards,
Mike
 
On Tue, 16 Mar 2004 10:40:27 -0500, Active8 <reply2group@ndbbm.net>
wrote:


If you change a filter in the field, you'd want to sweep it before
it's installed in der bot.
Click to expand...
Sigh.. Yes, I know this has been raised (and answered at considerable
length before). Raised so often, in fact, that I wrote a full
explanation and stuck it on our website. Sadly, that's not up and
running at the moment so I can't refer you to it. Take it from me,
however, that there *is* method in my madness. :)

--

The BBC: Licensed at public expense to spread lies.
 
On Tue, 16 Mar 2004 20:26:17 +0000, Paul Burridge wrote:

On Tue, 16 Mar 2004 10:40:27 -0500, Active8 <reply2group@ndbbm.net
wrote:


If you change a filter in the field, you'd want to sweep it before
it's installed in der bot.

Sigh.. Yes, I know this has been raised (and answered at considerable
length before). Raised so often, in fact, that I wrote a full
explanation and stuck it on our website. Sadly, that's not up and
running at the moment so I can't refer you to it. Take it from me,
however, that there *is* method in my madness. :)
Click to expand...
I'll make a mental note to visit yer site once in a while. same
domain as the gifs you've been posting?
--
Best Regards,
Mike
 
On Tue, 16 Mar 2004 16:18:30 -0500, Active8 <reply2group@ndbbm.net>
wrote:

I'll make a mental note to visit yer site once in a while. same
domain as the gifs you've been posting?
Click to expand...
(Answered by e-mail)

--

The BBC: Licensed at public expense to spread lies.
 
On Mon, 15 Mar 2004 10:46:53 +0000, Paul Burridge
<pb@osiris1.notthisbit.co.uk> wrote:

Hi all,

I've got quite a huge stash of inductors in my parts bin. The colour
codes don't always seem to relate to the values I've been able to
measure, with my multi-function DVM, however, and I can't accurately
measure any inductor < about 10uH. Is there a circuit anywhere that
would enable me to get a reasonably accurate idea of the values I've
got down to say 100nH or thereabouts?

Thanks,

p.
Another idea which not seems to be mentioned, but won't let you
Click to expand...
measure down to nH, is to put an accurate 10ľH coil in series with the
meter, and you should at least measure with reasonable accuracy down
to 1ľH or lower. You may buy ready made coils down to below 100nH
which can be used to check the calibration.

My over 50 years old Radiometer multimeter uses this technique and
measures well down to 20nH. I made my own meter with 18MHz oscillator
to meassure 0.01-1uH

LA8AK

----
Jan-Martin, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/
 
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