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Many years ago all dimmers contains same solution with UJT trigger circuit.SCR comutate on all zerocross, with trigger impulse delay You can control the AC voltage effective value. In static application (as switch) apply DC voltage between cathode and gate. It will comutate on zerocross, but fired always immediately when the voltage rising about 2.5V after zerocross.
Heres a simple circuit with a switch on the SCR gate,
i imagine sebi would put forward a similar one.
The values are only guessed, i would start with them
and see how it goes.
I would be aiming for about 10 volts or a bit more on
the 5 mfd. It doesnt have to hold much, or for long, it
doesnt want to be so big as to cause a delay when switching
off. If it does make a small delay when switching off,
just reduce the value a bit till its ok.
Sebi, if you feel these values are within reason or
maybe too far out, please say so.
They are only guessed.
I imagine 3k would work the gate ok, they are pretty high
impedance, but some are lower than others.
30k at 120 volts sounds like about half a watt to me,
so that shouldnt get too hot.
In response to john1's original schematic, I tried the capacitor thing, it didn't work. I also tried connected the gate to the anode so that when I want the light switched on, the anode wil never fall to zero volts. This killed the pin that was connected to the gate. Is there any way to use an SCR to turn on a 120 VAC light that actually works for real?
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
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.