Hello.
I understand that one of the method to turn-off an scr is by
connecting a transistor parallel to it in order to short it.
I would like to turn on the transistor(BC547B)by touching the wires
with my finger(see photo) and make the scr turn-off.
Through the base the current may be about 8 microamper,so the max hfe will be
1.6mA(X200)and that current will be,only if i will connect the collector of the transistor before the R2(second position).
My question are:
Does it enough current(1.6mA)to make the SCR turn-off?
What is the minimum current that i need to pass through the transistor in
order to turn-off the scr?
Should i connect the collector wire of the transistor as the 1 position
or the second position?(before R2 or after the R2,LED?).
Thanks.
Best Answer
The transistor must conduct essentially all of the load current in order to reduce the SCR current below its specified holding value. In this case, the LED is the load, so the transistor must handle whatever current you're putting through it.
And position 1 is the correct connection. You wouldn't want the transistor to put a short circuit directly across your power supply.