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>

The approach proposed by Jolyon is sometimes used to measure antenna
impedances (look for references to the "three meter" technique)..  It is a
way to measure both R and X of an unknown by measuring three voltages in a
series circuit consisting of source (any impedance), a (mostly) reactive
component, a (mostly) resistive component, and the unknown.  As he points
out, simple algebra can be used to solve for R+jX of the unknown.  I've
never seen the calculations done with analog computers (usually, some form
of voltage detector is used, and solved either graphically, or with a
computer), although I'll bet some of the "antenna analyzers" around do it
with opamps, etc. (and, read out on a little tiny analog meter.. which is
fine, because all you're usually looking for is the zero reactance
condition)

Some provisos:
1) The reactive and resistive components should have reactance/resistance
comparable to the unknown (so the voltages are all roughly of the same
magnitude).
2) You need some calibration references to calibrate the system. Shorts,
opens, and some intermediate values.
3) If your detectors/meters are nonlinear (most are), then you need to back
out the calibration characteristic of the meter.
4) Small parasitic reactances can make this kind of measurement tricky (you
get inadvertent resonances near the measurement frequency.) The parasitics
add more degrees of freedom to the system.

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, November 11, 2002 6:13 AM
Subject: RE: Pythagorean Idea for Inductance Meter?


 > Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<tobias.hofer-at-bluewin.ch>
 >
 >
 >  >-- Original-Nachricht --
 >  >Date: Sun, 10 Nov 2002 12:58:30 -0700
 >  >From: "Tesla list" <tesla-at-pupman-dot-com>
 >  >To: tesla-at-pupman-dot-com
 >  >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?
 >  >
 >  >
 > hello
 >
 > this method is possible, in my opinion it's to complicated. a conventional
 > inductance meter works in an other way.
 > the voltage on a inductor depends on the change of current and time.
 > it's given by the equation UL = L*dI/dt. it means the current follows the
 > voltage.the inductance is also the oposite to a capacitor.
 > i you put a dc voltage to an inductor the current rise in 'e-function'.
 > in practice a known voltage is applied to the L an the the current via.
 > time
 > is meassured. so you can  calcultage easily the inductance.
 >
 >