Re: Triggered spark gap resistors

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>



> I tried using a series _ballast_ resistor since it has been common
> practice in the automotive application for many years, either in the
> form of resistance wire in the harness or a firewall mounted ceramic
> power resistor. I cannot say whether GM has used or is currently using
> ballast resistance with the HEI coil. Any wrench twisters who can
> comment here?

Modern solid state HEI's use the pass transistor to limit the current
(which is what the ballast resistor was for).

> 
> Since probing this trigger design with a scope and Pearson CT, I've seen
> that the triac as a trigger driver leaves a _lot_ to be desired. Jitter
> is quite broad, I've measured upwards of 2 mSec. If this is used with a
> resonant matched cap and improperly set or no safety gaps, there exists
> a real potential for _missed_ firings and we all know what that means.

I'd think that the jitter is from the pretty crude circuit driving the
triac trigger, rather than the triac itself.  The basic circuit is an RC
feeding a DIAC (or, in older designs, a neon bulb) into the gate of the
TRIAC or SCR. In a two wire device, all that timing stuff is essentially in
parallel with the triac and (here's where the problem is) in series with
the load.  Ideally, one would put the timing circuit across the line.
Then, the problem is that on one polarity, the trigger pulse (from gate to
MT1) is across the triac only, then on the other polarity, the trigger
pulse (from G to MT2) is across the triac and the load)

http://home.earthlink-dot-net/~jimlux/hv/dimmer.htm has some drawings and
explanation.


> 
> Wealze, if my memory is correct, I believe you reported that you
> modified a dimmer with the _third_ lead brought out and found the jitter
> greatly improved? I'd like to compare notes with you as I have not been
> able to confirm this. Feel free to contact me off list if you wish.
> 
> Hope this helps explain the _why_.
> --
> Jon G. Tebbs
> <jgtebbs-at-eos-dot-net>