My project requires a backup power supply, and I want to include an LED to determine when the backup power supply becomes active. My thought was a voltage divider to limit current to the LED, and allow the excess to bypass the LED and go straight through to the load.Would connecting a BJT as an amplified Zener where the Zener is in fact the LED be relevent?
I am told the correct way to do this would be to utilize a transistor, however I cannot think of a way to connect a transistor where I am not connecting it or an LED to ground(else there will always be current regardless of whether the backup power supply is active), or running the full load current through the gate of a transistor.Would a BJT be better here?
What is the correct way to approach this? I suspect using a transistor would be correct, I have just not been able to create a working circuit with a transistor myself!
EDIT: More info
When the 12v supply is droped it is disconnected and creates an open circuit.