Re: Ignition coil (anti-parallel pairs) array and other questions

To: tesla-at-pupman-dot-com
Subject: Re: Ignition coil (anti-parallel pairs) array and other questions
From: Ed Phillips <evp-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Sun, 18 Jul 1999 10:18:07 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

"1)Anyone ever tried using an array of pairs of ignition coils connected
in 
anti-parallel as a substitute for a NST? I'm just looking for a cheaper 
alternative to a NST."

	Yes.  I have a system here consisting of two GM "high energy" coils fed
as you suggest, and driven by a TRIAC driver.  Produces sparks of up to
5", limited by voltage breakdown of the secondaries (whole setup is
immersed in oil).  However, the power output is so low (<50 watts) that
it is really of no use in driving all but the smallest TC.  I do have
another very small coil using a conventional ignition coil to charge the
primary capacitor, It is driven from 12 volts, draws about an amp, and
the coil will put out 4" streamers.  The driver is an HV silicon bipolar
transistor with dual 555 timers to generate the drive waveform, and a
2N2219 buffer amplifier.

	A note about driving ignition coil primaries.  The maximum output
voltage will occur when the core is just driven to saturation; this is
determined by the operating voltage and the duration of the drive
voltage.  Any excess width of the drive pulse just results in heat loss
in the drive transistor.  I adjust the drive waveform by setting the
pulse width to the point where the voltage across the drive transistor
just reaches zero at the end of the pulse, then set the pulse repetition
frequency to vary the effective power input.  If things are adjusted
right there is very little power dissipation in the switch transistor. 
Think the cases where people report overheating of the switch are those
in which the drive waveform results in core saturation and high drive
currents.

Ed