RE: Tesla Coil Blunders

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

Original poster: "John H. Couture by way of Terry Fritz <twftesla-at-uswest-dot-net>" <couturejh-at-worldnet.att-dot-net>


Matt -

The resistance R can be very large. For example the default coil shown at

    http://home.att-dot-net/~couturejh/

The resonant frequency is 169.8 Khz (click calculate) using the
Wheeler/Medhurst equations. Assume a frequency reduction of 11.5% (150.34
Khz) the R would be 35000 ohms. Test data I have collected indicates 11.5%
is realistic. I haven't figured out how to find this resistance at the
design stage so have "blundered" along like other coilers with the calcs.
You might want to test your coils for resonant frequency at low voltage with
a generator and meter and also at operating high voltage with a scope and
antenna probe to determine the reduction in resonant frequency.

John Couture

------------------------------


-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Wednesday, March 28, 2001 8:50 PM
To: tesla-at-pupman-dot-com
Subject: Re: Tesla Coil Blunders


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

In a message dated 3/28/01 8:44:41 AM Eastern Standard Time,
tesla-at-pupman-dot-com
writes:



>
> > Original poster: "John H. Couture by way of Terry Fritz
> > <twftesla-at-uswest-dot-net>" <couturejh-at-worldnet.att-dot-net>
> >
> >
> > Tesla Coilers may be making the worst of TC blunders when doing
> > calculations for determining the operating resonant frequency of their
> > coils. It is a well known fact that the calculated frequencies do not
> > agree with the actual operating test frequencies. Many important
> > design decisions are based on this unique frequency so it is important
> > that it be correct. To circumvent this problem a fudge factor is used
> > for all TC computer programs but this is only a bandaid for TC design.
> >
> > The equation that coilers use for finding the TC resonant frequency is
> >
> >       F = 1/(6.283 x sqrt(LC)
> >
> > The TC secondary circuit is an RCL circuit but the "R" is omitted by
> > coilers in the above equation which may be the worst of TC blunders.
> > In the Tesla Coil Construction Guide page 5-1 the complete resonant
> > frequency equation is shown including the "R" parameter. It is also
> > mentioned that the "R" reduces the resonant frequency and if the "R"
> > is large enough there will be no resonant frequency. In other words
> > the "R" could be the reason the operating frequency does not agree
> > with the calculated frequency when the "R" is omitted.
> >
> > To my knowledge no one has ever determined how to find this very
> > important "R" for an operating Tesla coil either by calculation or by
> > tests. TC programs are hurting because of this lack of information.If
> > anyone has any comments I would be interested in them.
> >
> > John Couture



Hi John & All,
        I'm not sure that a known simplification is necessarily a "blunder".
The full original formula is:
f=1/2pi x sqrt(1/(LC - (R/2L)^2))
taking the derivative of f w/resp to R gives us:
df/dR=R/(f*(4*pi*L)^2)
For a coil ~4"  -at-270 KHz, R would have to be>500 ohms to change the
frequency
even 1%. Since DC resistance is on the order of 20 ohms, the AC resistance
would have to be 25 times higher to be responsible for even a 1% change.
IMO,
this is not the culprit.

Matt D.