The scenario
An emergency light that runs on mains and automatically switches to battery when power fails. A PNP transistor senses DC presence to keep LED off, while a diode and resistor trickle-charge the battery.
What is in this drawing
Read the decisions behind it.
01
Using a PNP transistor for automatic switchover instead of a relay
02
Isolating the battery charging path with a diode to prevent backflow
03
Transistor biasing ensures rapid switch when mains voltage collapses
Suitable for low-power emergency lighting where relay coil power consumption is not justified, and a simple transistor switch suffices.
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