Re: 60 vs. 30 ma

Subject:  Re: 60 vs. 30 ma
  Date:   Wed, 11 Jun 1997 19:19:31 -0700 (PDT)
  From:  "Edward V. Phillips" <ed-at-alumni.caltech.edu>
    To:  tesla-at-pupman-dot-com

"Are you certain? In my Tesla Coil the primary capacitor is
directly across the high voltage leads of the xformer. For
most cases, in order to draw the rated current (ie 60mA) you
need a capacitance value so that the capacitive reactance (Xc)
is equal to your voltage (ie 12000v) divided by max current
output (ie 60mA). Or Xc=12000/.06"
        No way, if I understand what you mean.....  The transformer
will provide rated current into a short circuit, and less than rated
current into a resistive or inductive load.  HOWEVER, if you connect
a capacitive load, particularly the "matched" case you present,
The transformer will be series resonant with the capacitor, and
the voltage will rise ABOVE rated voltage to a value limited by
one of these three conditions:
1. The current is limited by the equivalent series resistance
of the transformer (Rsecondary + (12000/100)^2 Rprimary,
2. The transformer core saturates, changing the effective inductance
and losses,
3. The poor secondary winding shorts out due to excess voltage,
a condition which is responsible for many, if not all, neon
transformer failures.
        The total equivalent resistance of a typical 12 kV, 60 ma
transformer of the type I use here is about 10,000 ohms.  If this
make that the secondary is resonant, this would result in a current
of about 1.2 amps.  The resultant voltage would then equal to 1.2
amps flowing through the reactance of the capacitor, which in this
case is 200,000 ohms.  Resultant voltage 240 kV !!!!!!!  No way the
poor, cheap secondary winding insulation is going to withstand that.
        By the way, in my opinion is is poor practice to place
the capacitor across the transformer, with the gap in series with
it and the primary, as your note suggests.  This will mean that
the full RF voltage appears across the winding, which may or may
not take it.  Putting the gap across the transfomer and the
capacitor and primary in series will result in much lower voltage
on the transformer, since the gap keeps the transformer shorted
(it can take it fine, and the peak current is the same as with
your connection) until the oscillations decay.