Re: HV power source.

To: tesla-at-pupman-dot-com
Subject: Re: HV power source.
From: Jim Lux <James.P.Lux-at-jpl.nasa.gov> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Fri, 04 Sep 1998 14:53:41 -0600
Approved: terryf-at-verinet-dot-com
Tesla List wrote:
> 
> Original Poster: Hollmike-at-aol-dot-com
> 
> >Like a standard automotive "condensor"... They have low internal
> >inductance, relatively low loss, and they are designed for the
> >application.
>    Actually, the condenser that comes with the car coil is just for noise
> suppression.  
Au contraire...
Take it out and the engine won't run, because the energy is dissipated
in the spark across the points. The capacitor absorbs the start of the
pulse as the points open, delaying the voltage rise sufficiently until
the points are far enough apart to hold off the few hundred volts. If
you take the cap out, the moment you start to open the points (don't
forget there is about 5 amps flowing in the circuit), you draw a low
voltage arc, which dissipates all of the energy stored in the inductance
of the coil, and, therefore, no spark at the plugs. Also, the points
will quickly be eroded.

I discovered this empirically when the newly replaced cap that came with
the points failed about a day after I did the replacement. Turns out it
used to be a fairly common occurance.  These days, with electronic
ignitions, the cap makes no difference, because the points are only used
to provide a sync signal. (they still do run a fair amount of current
through them, so that the system is dirt tolerant, though).

You will want to find the right capacitance that will eliminate
> the spark in the relay(or what ever type of break controller you use).  It
> wouldn't hurt to try it since it comes with it, but you will probably need
> more capacitance than it will provide.   I tried many different values and
> never found the "perfect" one.  I wound up replacing the relay several times
> before going with a neon tranny.  You might look into a solid-state ignition
> circuit and try that out, since the relay would then see much less current.

You can fire a solid state ignition directly with a 555. For instance,
the very common MSD-6A (about $100) has a low level trigger input that
can be run from almost any source providing a suitable edge.  And, of
course, most solid state ignitions (including the MSD) also provide a
magnetic pickup input which only requires a few tens of mV to switch. 
You'll have no problem firing a MSD at 500 Hz. 8000 RPM in a 8 cylinder
engine 533 Hz.  The MSD puts out about 100 mJ per pulse at -400Volts,
relative to the chassis ground.