[sdiy] Speaking of filters with coils...

[Paul Perry]

Want to determine inductance with a frequency generator and voltmeter? Found 
this:
"Get a sine wave oscillator, and put the inductor and a resistor in series 
across the output. I generally start with about 100 ohms. Adjust the 
oscillator frequency until the voltage across the inductor and the resistor 
is equal. Since they are 90 degrees out of phase, each will be 0.707 times 
the oscillator voltage. At this frequency, the inductor has an impedance of 
100 ohms, and the inductance can be calculated from Z = 2 * pi * f * L. "

This would assume that (at the test frequency) the resistance of the wire in 
the coil is negligible compared to the impedance of the inductor, but that 
is a pretty safe bet.

Centre tapped audio coils were often used in single transistor oscillators 
back in The Day - by grounding the centre tap, the two ends are 180 deg out 
of phase & it is simple to get poisitve feedback around a common emitter 
amplifier stage (not that I can say for sure that was happening in the organ 
in question.)

paul perry Melbourne Australia

----- Original Message ----- 
From: "aankrom" > They have three terminals. One is a tap. They have a 
ferrite barrel
> over the outside and a slug inside for adjustment. A local electronics
> store used to have a DMM with inductance mode that they'd let me use,
> but someone bought it. Sometimes I guess at a coil's inductance by
> seeing what a 1kHz square wave looks like when I pass it through it on a
> scope (and compare it with a coil of known inductance and similar
> construction), but that's not a very good indicator. They look like they
> could be anywhere from 400mH to 2H. The organ had a couple of 2H
> inductors on it as well that were used in filters (and they're not very
> big in size). I suppose I could make a filter or oscillator out of one
> of those and then swap in the inductor I want to use and see the
> frequency change. I have an Optoelectronics frequency counter that's
> accurate and precise down to 0.5 Hz. I also have a Wavetek 182A function
> generator and an Radio Shack Optimus EQ with a spectrum analyzer. Not
> the best equipment, but it may help. The frequency counter should be a
> big help at any rate...
>
> The fact that the coil is tapped shouldn't hurt anything, should it?
> I've hear of people using Radio Shack audio transformers as 500mH coils
> in Crybaby clones.
>
> Anthony