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Trying to switch on my SCR

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Member level 4
Sep 23, 2008
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I'm not aware of any transistor output optocoupler that can withstand >600V as required by a 400V AC switch. EL817 e.g. has 35 V rating. You would use an standard opto triac, possible with a series rectifier diode to cut negative halfwaves.

yes it breaks down at the higher voltage.....but not catastrophically....

only solution put two in series with 1Meg-ohm voltage balancing resistors and a and a gate-cathode R on the SCR
yes it breaks down at the higher voltage.....but not catastrophically...

Obviously the transistor optocoupler breaks down first. Either it triggers the triac early, protecting the optocoupler. Or the optocoupler breaks down at a higher current, which happens only once.

FVM, once it breaks down the SCR turns on dropping the volts and taking current away from the device - hence not catastrophic, actually repeatable n times....
Did you notice the low voltage transistor optocoupler in the linked schematic?

It will survive breakdown only by chance and if the power dissipation before triggering the SCR isn't too high. E.g. without gate-cathode resistor.

But apart from this guesses, the OP wants a 400 VAC capable switch, and that's not possible with a transistor optocoupler (at least without an auxilary power supply).

FVM, did you notice the 47k ohm resistor mentioned?, pretty hard to destroy the opto with this in series...
Maximum power dissipation of the optocoupler isn't less than 1.5 W with the 47 k resistor. But it's not guranteed at all that the device will survive a voltage above maximum ratings at a lower power.

But I think we are discussing a circuit that's useless because it can't serve the intended purpose, trigger the SCR when operated with high voltage.

I suggested a working alternative in post #2, what's your suggestion?

OK. I am not sure if putting a 1Mohm in series to the gate will give me enough current to switch on the SCR's gate (40mA according to datasheet and tetsing) I have also tried lower values. But it does stop the SCR from turning on once the Line is active.

I will try MOC3020M I think this with a 200ohm resistor in series to the gate should do the trick.

Samuelr; a 3020 unfortunately will not work for 370Vac (it might, just, but not guaranteed) as it is a 400Vpk device and the peak volts for 370Vac is 523V. An opto SCR or opto triac as suggested by FVM is a better way to go, or a simple pulse transformer (small) to an 800V rated SCR.

[ 2 x 3020's with their outputs in series and say a meg ohm across each for voltage sharing would work, the leds being driven in series too, needs a small gate to cathode resistor on the SCR to stop un-intentional firing] <- this is what I was suggesting in post 3#

@ FVM 8mA rms in the opto xtor is unlikely to kill it in this circuit, as, as soon as 70V is exceeded the SCR is on and the applied mains volts go away, the xtor is 7V in reverse which is 28mW, so the opto xtor should survive indefinitely...

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