Re: Pythagorean Idea for Inductance Meter?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Couldn't get a huge number of opamps, lay out a suitable PC board, choose
appropriate discrete components, and get it suitably linear (or nonlinear,
as the case may be), and have it work over time and temperature...  Each
component costs money, in layout, soldering, etc.  And, while junky opamps
in the surplus bin might be cheap, good ones aren't, nor are good quality
sockets, nor are precision resistors, etc. It all adds up.

I'll take a modern microcontroller based LCR meter against any 40 year old
design analog bridge.. especially if you need to make measurements at more
than one frequency so that you can get the frequency dependent variations
and figure out the parasitic C around the L.

Not that it can't be done entirely analog, and that analog designs have
their definite place (say, 1-5% accuracy, real low power, limited
measurement range needed...).  If you're going to read the result on an
analog meter with a pointer (5% at best), then, yes, a microcontroller might
be overkill.

I don't know that laziness has anything to do with it. Sure, I could make
microwave measurements with a slotted line and a diode detector, but, gosh,
the 8510 does a much better job, and is a lot more accurate to boot. If I
had to do all the cal measurements and back out the various factors by hand
it would take weeks to reduce just one set of measurements.  I don't know
that one could even, with infinite amounts of hand calculation, get the same
quality results from hand measurements that you could from an automated
analyzer.. stability over time between making the cal measurements and the
unknown is probably a limiting factor.

I'd rather spend my time working with the inductor than making a measurement
tool to measure it.

That said, there is a satisfaction in making precision measurements or
measurement tools the old fashioned way, but it's for some visceral
satisfaction, or as a hobby, not for productivity or quality.  Heck, very
few people build tesla coils because they're useful.. We build them because
the building and operating of them is fun.  You could probably count the
number of people int he world who actually earn a living making tesla coils
on one hand.


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, November 10, 2002 8:33 PM
Subject: Re: Pythagorean Idea for Inductance Meter?


 > Original poster: "Crow Leader by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<tesla-at-lists.symmetric-dot-net>
 >
 > You can get a huge number of op amps for $50 as well.
 >
 > I like the idea of analog math. Everybody is lazy these days and just
throws
 > microcontrollers at everything. I doubt anybody could even build a
 > mechanical adding machine anymore.
 >
 >
 > KEN
 > ----- Original Message -----
 > From: "Tesla list" <tesla-at-pupman-dot-com>
 > To: <tesla-at-pupman-dot-com>
 > Sent: Sunday, November 10, 2002 6:22 PM
 > Subject: Re: Pythagorean Idea for Inductance Meter?
 >
 >
 >  > Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
 > <jimlux-at-earthlink-dot-net>
 >  >
 >  > This is kind of how some of the older impedance bridges work.  These
days,
 >  > they digitize everything and use a microprocessor to do the math.  You
can
 >  > get a more than powerful enough micro and display to do this sort of
thing
 >  > for around $50...(it doesn't need to be a ball of fire, speed wise..)
 >  > ----- Original Message -----
 >  > From: "Tesla list" <tesla-at-pupman-dot-com>
 >  > To: <tesla-at-pupman-dot-com>
 >  > Sent: Sunday, November 10, 2002 11:58 AM
 >  > Subject: Pythagorean Idea for Inductance Meter?
 >  >
 >  >
 >  >  > Original poster: "Jolyon Vater Cox by way of Terry Fritz
 >  > <twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
 >  >  >
 >  >  >
 >  >  > For the purpose of measuring the inductance of the windings used in
TCs
 > ,
 >  >  > is it possible (and economical) to design a meter for true
inductance
 >  > using
 >  >  > Pythagorean principles -based on fact that the AC voltage across a
coil
 >  >  > with no resistance is proportional to the AC current alone?
 >  >  >
 >  >  > I envisage using two opamps (wired as squarers) to register voltages
 >  > across
 >  >  > L the test inductor and a reference resistor R connected in series
 > across
 >  >  > an AC supply
 >  >  >
 >  >  > the squares of the two voltages would be subtracted in a difference
amp
 >  >  >
 >  >  > then square-rooted (by an opamp with a squarer in the feedback loop)
to
 >  >  > give the voltage component due to inductance alone; the latter would
be
 >  >  > displayed on a meter as a measure of the inductance.
 >  >  >
 >  >  > Is this method as simple as it sounds or more complicated in
practice?
 >  > Does
 >  >  > anyone have a schematic for such a circuit?
 >  >  >
 >  >  >
 >  >
 >  >
 >  >
 >
 >