Re: Balancing L/C Sizes

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

Original poster: "Scott Fulks by way of Terry Fritz <twftesla-at-uswest-dot-net>" <darkthing-at-earthlink-dot-net>

 Matt D.  wrote:
> Is High Q really the defintion of operating efficiency in a Tesla coil?

Not the definition of efficiency, but it does help.  I'll give you my
interpretation of the value of Q in a TC, and let the real experts correct
me for the edification of both of us.

A tesla coil has to boost voltage from the transformer voltage to a very
high value (say around a million volts) to get a large impressive spark
length.  There are two ways this happens.  One is from a simple turns ratio
transformation, from the coupling of a few turns of primary to many turns of
secondary.  The other is through "resonant rise".    The Q is important in
the resonant rise only.

"Resonant rise" occurs when the second and following cycles from the primary
add to the current already stored in the secondary.  This addition will
continue until either of two things happen. The primary will eventually
"ring down" so it doesn't have any more energy to impart to the secondary.
This is a function of the Q of the primary circuit, since that gives the
number of useful cycles before ringdown.  The second case is that the losses
in the secondary will start eating more current than is being delivered by
the primary.  This is a function of the Q of the secondary.

So, if I have this right, a tesla coil is limited by the Q in either the
primary or the secondary.  It doesn't help to have a huge Q in the secondary
if the Q of the primary is low, or vice versa.  However, a low Q in either
circuit could limit an otherwise excellent coil.

Cap-discharge coils are limited more by the Q of the primary circuit than
the secondary, because the inductance of the primary is so low.  However, a
tube coil is generally more limited by the Q of the secondary, since the
primary circuit is fed continuously and doesn't ring down.

Regards,
Scott Fulks (darkthing-at-earthlink-dot-net)