Efficient, Automatic, White LED Emergency Light Circuit

 Description:

Referring the figure, we see that the circuit is actually very easy to understand, let’s evaluate it with the following points:

The transformer, bridge and the capacitor forms a standard Power supply for the circuit. The circuit is basically made up of a single PNP transistor, which is used as a switch here.

We know that PNP devices are referenced to positive potentials and it acts like ground to them. So connecting a positive supply to the base of a PNP device would mean grounding of its base. Here, as long as mains power is ON, the positive from the supply reaches the base of the transistor, keeping it switched off. Therefore the voltage from the battery is not able to reach the LED bank, keeping it switched off.

In the meantime the battery is charged by the power supply voltage and it’s charged through the system of trickle charging.

However, as soon as the mains power disrupts, the positive at the base of the transistor disappears and it gets forward biased through the 10K resistor.

The transistor switches ON, instantly illuminating the LEDs.

Initially all the diodes are included in the voltage path, and are gradually bypassed one by one as the LED gets dimmer.

Parts List

R1 = 10K,

C1 = 100uF/25V,

D1, D2 = 1N4007,

D3---D6 = 1N5408,
T1 = BD140

Tr1 = 0-9V, 500mA,

LEDs = white, hi-efficiency, 5mm,

S1 = switch with three changeover contacts.