[sdiy] Speaking of filters with coils...

[aankrom]

so L= z/(2 * pi * f), right? So if I am assuming a range of 0.4H - 2H, 
I should use a relatively low frequency - I mean probably lower than 
1kHz. This is what the math tells me if I have it right... If the 
frequency I need is much higher than the coils probably have too small 
of an inductance to use.

Anthony

On Wed, 4 Jan 2012 17:26:26 +1100, Paul Perry wrote:
> 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