Thank you Ron, Thank you sebi,
Sebi, i have never had an SCR conduct with an open circuit gate,
but i suppose if it had lengths of wire over to a switch, then
yes i can see that a spurious spike could easily trigger the SCR,
so i have to agree, a resistor between cathode and gate would be
a wise inclusion to make sure it stays off when its supposed to.
Without the series resistor, you would effectively be discharging
the capacitor using the gate, which may be alright, but i am a
bit wary of damage, i have always thought the gate to be a bit
sensitive.
Ron, yes sometimes its not easy to know what is required, and
sometimes its easy to just get things wrong.
Batman, if you're using cmos stuff on a home made assembly, i
think you should make sure that its on a stable footing, and not
flapping around at supply voltages, because cmos stuff is well
known to be extra easy to damage from external discharges that
normally would go un-noticed. This type of circuit has the
potential to damage cmos stuff very easily, because its running
at supply voltages from ground.
I would suggest you consider TTL equivalents for the cmos you
have in mind,
Or more sensibly consider using a triac instead of an SCR,
which would put the circuitry on a much more traditional footing,
that is one side could be neutral, which makes for a more
easily understood circuit anyway.
Best of luck with it,
John