Re: 555 50% duty cycle

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "rheidlebaugh by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>
> 
> A consideration often overlooked by car-coil experimenters, is the fact that
> the coil ,like the TC, is a resonant device requiring a .22u capacitor to
> develop max voltage. Brute force can over come this, but at a high power
> loss. Even the lamp dimmer circuits use a .2u cap in series for best results
>     Robert  H

	With a mechanical interruptor (i.e. contact points in the distributor)
the capacitor is necessary to avoid arcing, but use of too large a
capacitor results in problems because it discharges through the contacts
when the close again.  Values of the order of 0.25 ufd to 1 ufd were
used quite often; seems to me the flat capacitor in a model T spark coil
is around 1 ufd.  When you are using a solid-state driver things get a
little more critical.  When driving from an SCR larger capacitor values
seems better, up to the point where the peak currents blooie the SCR. 
I've used up to 10 ufd here in a "light-dimmer" circuit where the peaks
of current flow through the power line, causing great glitches in line
voltage and attendent interference.  When operating with a 12V supply
and switch transistor there is an optimum value of capacitance which can
be determined by experiment, using a scope to monitor the voltage across
the transistor.  I have found that values around 1.5 ufd seem about
optimum for an Exel coil.  Lower values result in collector breakdown
with the 600V transistors I used, and higher values eventually reduce
the output. 

	That's kinda all mixed up.  Bottom line is that experimenting with the
capacitor value will help determine the optimum value, and use of a
scope sure makes it easier.

Ed