Here is a circuit of an emergency light. As long as the power supply is present, transistor Q1 conducts. Since the base of the transistor Q2 is connected to the collector of Q1, transistor Q2 and Q3 do not conduct and hence the lamp remains off. LED glows as long as the supply is present.
When the power supply fails, the base drive to Q1 disappears. Thus Q1 stops conducting and its collector voltage jumps to battery voltage and starts conducting, switching on the lamp instantly. The load current is supplied by the battery. Whenever the power supply is restored, Q1 starts conducting turning Q2 & Q3 off and the lamp is switched off. Transistor Q2 conducts and provides sufficient base drive to transistor Q3.
The circuit above is too complex. The first diode is not needed and the rest of the circuit can be re-arranged. The 2R2 will overcharge the battery and dry it out in a few months.
It can be simplified to this:
The 100R gives 40mA charging with a 12v battery and 12v DC plug pack.
When the power supply fails, the base drive to Q1 disappears. Thus Q1 stops conducting and its collector voltage jumps to battery voltage and starts conducting, switching on the lamp instantly. The load current is supplied by the battery. Whenever the power supply is restored, Q1 starts conducting turning Q2 & Q3 off and the lamp is switched off. Transistor Q2 conducts and provides sufficient base drive to transistor Q3.
The circuit above is too complex. The first diode is not needed and the rest of the circuit can be re-arranged. The 2R2 will overcharge the battery and dry it out in a few months.
It can be simplified to this:
The 100R gives 40mA charging with a 12v battery and 12v DC plug pack.
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