556 STSG, simple zero-crossing

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

Original poster: "Peter Lawrence by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>


I've been building a STSG using a 556 (dual-555) timer chip so that I can
have one adjustment for phase and a separate one for dwell.

I found a "555 circuits" at RadioShack, and in it there is something called
a "cascade timer", that is what I'm basing my design on.

I use a 12v transformer through a bridge rectifier to get 120 Hz pulsed DC
and take that signal (with a 10k pull-down resistor) to the input of the
first timer as a "zero detect". This is much simpler that what someone else
recently posted using multiple transistors.

With suitably chosen R1-potentiometer and C1 I can get 1 to 7 milliseconds 
phase delay.

With suitably chosen R2-potentiometer and C2 I can get 0.5 to 1.5 milliseconds
dwell time.

the timer chip needs filtered DC, but the zero detect needs unfiltered, so
put an additional diode between the bridge output and the filter cap for the
chip and take the zero detect signal ahead of that diode.

This timer circuit works very well. I'm still working on the power-output
part, using the 556 output to drive a 2n3055 to drive a "horizontal output
driver transistor" to connect to the ign-coil.

Output of the ign-coil seems to depend a lot on the phase timing, but should
be independent of phase and only dependent on dwell. I think I need a large
cap to supply the the ign-coil, but so far experimenting with 2uF and 8uF
have not seen much change. Another thing to change is try a 10v-3a xfrmr,
since I've just got a 12v-1a now (12v rectified and filtered ends up 16v,
which is a bit much for the 555/556, and 1a may be a bit low for the ign-coil).



Pete Lawrence.