J1 12V input, J2 CAMERA IR-CUT switching signal feedback output; CSD1 photoresistor, my question is what is the role of Q3 transistor?
Trevon ポストする August 6, 2020
Q2.Q3 is enough for a simple constant current circuit, I is about 0.7/R7
Aiyana ポストする August 6, 2020
I did not see that it is a constant current, I see that Q3 is an overcurrent protection function. When the resistance under Q2 reaches about 0.7V, Q3 turns on, so Q2 turns off, and the LED does not light up.
Kaiya ポストする August 6, 2020
Q2 is off, can Q3 still be turned on?
The situation is: the current rises -> Q3 Ib increases -> Q2 base current decreases -> R7 voltage drop decreases -> Q3 Ib decreases.
This is a typical negative feedback process.
