[AMRadio] AM Audio

Geoff w5omr at satx.rr.com
Sat Aug 20 11:23:02 EDT 2005

Bob Bruhns wrote:

> <>Yes, that will work. But remember, you want equal
> voltages across the resistor and the inductor.
> Surprisingly, this will not be 50% of the applied
> voltage, but it will be about 71% of the applied
> voltage. This adds up to about 1.414 times the applied
> voltage! You should confirm this by measuring both
> voltages directly.
> This happens because the impedance of the inductor is
> reactive, and the current is equal, so the phases of
> the voltages differ by 90 degrees. This phase shift
> causes a partial cancellation when the voltages are
> added, so 71 + 71 = 100 instead of 142. It's
> interesting to observe.

Actually, Bacon, John's entire message was thus... you two agree.  and I 
present it here, so that others may learn from this experience, as well.


If all you need is a measurement of inductance the easiest thing, is to series up a variable resistor with the choke and apply a low voltage from a filament XFMR (6 - 20 volts.  Adjust the resistor so that the voltage measured across the choke is equal to the voltage across the resistor.  If for instance, a 10 volt source is used you will measure about 7 volts across the resistor as well as across the choke.  

You are probably asking why it is not 5 Volts across each and the answer to that is because of the phase difference of 45 degrees  when XL = R.  The sine of 45 degrees is .707 and .707 X 10 Volts is about 7 volts.  But the main thing is what ever the voltage is, by equalizing the voltage across the resistor and choke results in the creation of a resistance that is equal to XL of the choke at 60 CPS.

Measure the resistance of the variable resistor after equalizing the voltage
drops and you have the XL of the choke at 60 cps. 

L = XL / 2 * pi * F

L = XL / 6.28 * 60

L = XL / 376.8

Divide the resistance by 376.8 and you have the inductance.


Thanks for the response.  From all the active posters in here, I admire 
and respect the intellect for the electronic abilities and talents 
mostly of  WA3WDR, K4KYV, WA3VJB, KD5OEI and of course, WA5BXO, and not 
necessarily in that order.

73 = Best Regards,

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