This unit charges a nickel-cadmium battery from the mains to provide a standby power supply for emergency lighting in the event of a mains fail- ure. When the mains supply drops out, the light- ing is switched on automatically.
The circuit of the unit is extremely simple. Trl, Dl and CI provide a halfwave rectified and smoothed DC supply of approx. 6 V, which is used to continuously charge the Ni-Cad battery atabout100mAviaRlandD2.A2AhNi-Cad can safely be charged at this rate.
The voltage drop across D2 reverse-biases the base-emitter junction of Tl, so that this transis- tor is turned off and the lamps are not lit. When the mains supply fails, however, Tl is supplied with base current via R2; the transistor therefore turns on and the lamps are lit. As soon as the mains supply is restored, Tl will turn off, the lamps are extinguished and the battery is once I more charged via Rl and D2.
The unit can be mounted wherever emergency lighting will be required in the event of a power failure. An obvious example is in that infamous
dark cupboard under the stairs, so that, should a fuse blow, a replacement can be easily found and fitted.
A transformer with a slightly higher secondary voltage can be used, provided that Rl is uprated to limit the current through this resistor to 100 mA.