Re: TESLA COIL REVISED

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

Original poster: dhmccauley-at-spacecatlighting-dot-com 

Nicely said, Paul.

Dan


 > Hi Jaro, All,
 >
 > Further to the wealth of correct advice already given in this
 > thread:
 >
 > The reason that secondary coil Q factor is not too important
 > in TCs is that
 >
 > a) Other losses tend to dominate: mainly primary drive (gap or FET
 >     resistance), ground circuit resistance, and load resistance;
 >
 > b) The TC is usually fairly heavily loaded so that the loaded Q is
 >     much less than the unloaded Q and efficiency is high as a result.
 >
 > It is important to distinguish between two quite different modes of
 > operation of the TC:
 >
 > 1) The cap discharge type, in which the primary cap acts as a power
 > compression circuit;
 >
 > 2) The CW drive, in which there is no power compression and energy
 > is stored/accumulated in the resonant secondary.
 >
 > (In both cases a kind of 'resonant rise' is involved, but the term
 > is too vague to be of much use in discussion).
 >
 > In case (1) the output voltage is at most sqrt(Lp/Ls) or sqrt(Cp/Cs)
 > times the input voltage, and is reduced only a little by poor Q
 > factor due to (a) and (b) above.  Transfer of the compressed power
 > through the TC is very quick - only taking a few or several RF
 > cycles, and as a result there is time only for a few percent of the
 > input energy to dissipate in the secondary coil loss.  Increasing
 > secondary Q from say 50 to 500 would only make a few percent
 > increase in output voltage.  As the Q factor approaches infinity,
 > the output tends, not to infinity, but to Vin * sqrt(Cp/Cs), etc,
 > because only a finite quantity of energy is injected into the coil
 > with each cap discharge.
 >
 > In case (2) the output voltage is at most Vin * Q, where Vin is
 > the equivalent CW voltage applied to the secondary base.  The Q in
 > this formula involves all the circuit loss resistances, not just
 > those of the secondary.  Unlike type (1), the energy accumulation in
 > the secondary takes typically many tens to a few hundred RF cycles,
 > and as a result secondary Q factor becomes more important, but is
 > still not the dominant loss.
 >
 > For example, my CW TC secondary is about 90mH, with an AC
 > resistance of around 43 ohms (accounting for proximity and skin
 > losses - the DC res is only 7 ohms).  At the operating Fres of
 > 91.5kHz the Q factor of the coil alone might in theory be
 > 2 * pi * 91500 * 0.09/43 which is around 1200.  In practice the
 > measured values are nearer 200, suggesting a total loss resistance
 > of 2*pi*91500*0.09/200, which comes out to around 250 ohms. The
 > coil itself is only accounting for around 20% of the total losses.
 >
 > In the CW case if the resonator Q tended to infinity, so would the
 > output voltage, because with CW drive, an infinite amount of energy
 > is available (in principle you can wait as long as you like while
 > more and more input power steadily accumulates in the resonator).
 > In practice, stored energy rises until it is leaking into the losses
 > at the same rate at which input power is supplied.  When this steady
 > state is achieved (after maybe a few hundred RF cycles) the output
 > is steady at a peak voltage of sqrt(Q*Pin/(pi*F*Cs)).
 >
 > If the coil is well matched to the intended load, the load
 > resistance itself will dominate the operating Q, so that coil
 > resistance and other loss resistances are small in comparison. The
 > efficiency is then high and further reduction of coil resistance
 > only makes a marginal improvement in efficiency and output voltage.
 >
 > For example, roughly, if I was to double the inductance of my coil,
 > or reduce its resistance by a half, I would only gain another 10%
 > on the output voltage.
 >
 > So, resonator Q factor is only important in the CW case, because
 > the energy storage quality of the resonator is what counts
 > in that mode of operation.  But you must consider total loss
 > (including the output load) in determining the Q, and not just go
 > by the coil resistance.
 >
 > Yes, you can make coils with few turns resonate with very high Q
 > at very high frequencies.  Q values of several hundred are possible
 > with silver plated coils inside silver plated cavities.  They give
 > hot and bright but short discharges, due to the high frequency
 > involved, not the small coil size.
 >
 > With cap discharge TCs, the dominant losses are those of the
 > primary spark-gap and primary proximity loss.  Ground circuit
 > resistance comes a close third, secondary coil resistance trails in
 > at fourth place.  Despite these losses a well designed TC can
 > deliver over 60% of its bang energy to the output discharge. (Figure
 > comes from measurements on Terry's OLTC).
 >
 > Naive reasoning (eg focusing too much on secondary inductance and
 > resistance) may lead the experimenter to try high frequencies
 > with few secondary turns.  But the other losses increase dramatically
 > as operating frequency is increased, so unless you have an
 > application which requires some particular high frequency, better
 > results overall are obtained with lower frequencies (35-300kHz) and
 > fairly high secondary turn counts (500-3000).  The low frequency
 > of modern TCs encourages long but not so bright discharges, which
 > seems to be the preferred fashion these days.
 >
 > The point is, the operating frequency is a matter of taste, not
 > physics.  Go for a HF coil if you like, but don't expect long
 > streamers or high efficiency.  Examples are available in the ham
 > radio literature, eg base loading coils for short vertical antennas.
 >
 > Jaro wrote:
 >  > you don't have to take my word for it ... here's what Tesla
 >  > had to say...
 >
 > I'm afraid that quotes from Tesla don't carry much weight. A century
 > later we know a lot more about Tesla coils than Tesla ever did.  You
 > should study modern research, and beware the pseudo-scientific
 > drivel that lurks around the topic.  You've come to the right place
 > here on this list for reliable and up to date info.
 > --
 > Paul Nicholson
 > --
 >
 >
 >
 >
 >