Re: Resonant measure

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

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Mddeming-at-aol-dot-com>

In a message dated 1/23/01 10:13:17 PM Eastern Standard Time, 
tesla-at-pupman-dot-com writes: 


>
> Original poster: "Phillip Heslin by way of Terry Fritz <twftesla-at-uswest-dot-net> 
> " <pheslin-at-home-dot-com> 
>
> Hi Matt, 
>
>  I like your method, it sounds rather elegant. I was wondering if you 
> could leave the signal generator set at that frequency and hook your 
> setup to the primary circuit to tap the primary until the LEDs are 
> brightest (just to get you close before applying any power). 
>
>        Thanks, 
>
>        Phil Heslin



Hi Phil, 

Thanks for the kind words. I don't think the LED technique works too well on 
the primary, because at resonance, the impedance seen by the generator is 
just the resistance of the primary coil (i.e., almost a dead short). I now 
use the AADE L/C meter described in the list about a month ago to measure the 
tank capacitance and then back-calculate the required inductance as 
L=1/(C*(2pi*f)^2) to match the secondary. Then I use the L/C meter to find 
that tap point on the primary. This is all done after disconnecting the tank 
capacitor and the power transformer from the coil. :-) 
         A second way which I used, before I had the LC meter and before 
smoking my non-digital HP 'scope, was as follows: 
1. With tank cap reattached, short the spark gap. 
2. Connect generator to the coil thru a 1K-ohm resistor and set generator to 
resonant frequency. 
3. Monitor the voltage on the coil with your 'scope. Peak voltage should 
appear at resonance. 
A dip meter can also be used but I don't have one. This method does not give 
the exact point because the resistance of the spark gap under power is not 
zero as it is when shorted for this test, but it should get you fairly close. 
The formula for frequency is not exactly f=1/(2pi*sqrt(LC)). It is actually 
f=sqrt(1/LC - (R/2L)^2) /2pi. 
This means the exact resonance point is harder to find with L very small. 
        At this point it is very important to remove the short from the gap, 
the signal generator,  and BOTH scope leads before applying full power to the 
circuit!! :-))) There are probably far more accurate and sophisticated ways 
to pinpoint the resonance tap if you have optical couplers,  antenna arrays, 
and/or a digital service monitor, but most po' boys & girls can live with a 
few percent error. 

Matt D.