Electronic Solid State Relay Circuit


Joined Apr 7, 2010
The LED current passes through R1. When the LED current is about 15ma or so, there will be 0.6 volts across the B-E junction of the transistor (E = IR = 0.015 * 43) which turns it on. This causes its C-E junction to begin stealing current from the LED. As the current through the LED drops, less voltage will be developed across the BE junction, turning the transistor off which allows the LED to receive more current. This repeats itself continuously. Long story short, its a 15ma current regulator.