Electronic – the purpose of this transistor in this circuit

capacitorcircuit analysistransistors

What is the purpose of the Q1 transistor in this circuit?

I understand that I'm going to have approximately 15V for the Vi positive part, and the Q2 transistor works as a current source.

Also, I know that when the voltage on the capacitor is Vc > Vp the PUT will allow the current go through the 50ohm resistor.

But, how does Q1 affect the charging capacitor?

circuit

Best Answer

Q1 simply turns on and discharges C during the negative half cycle, to reset the delay for the next positive half cycle.

Spehro makes a good point; it is not immediately clear if the risetime on C is fast enough to limit the reverse Vbe on Q1 to less than the 5V typically permitted : you might want to check this in simulation, to prevent reverse breakdown of this transistor.