As the name implies,emergency light is a source of light available during an emergency.It is an automatic system in which a rechargeable battery operated light source turns on as soon as the ac mains fail.When the grid supply is restored,the lamp is turned off.
An emergency light essentially consists of a source of electricity – a rechargeable battery pack and an automatic sensing circuit to check the presence(or absence) of mains supply.The sensing circuit connects a lamp (or a compact fluorescent tube inverter) circuit in the event of mains failure and disconnects it when the mains supply is restored.When ac mains supply is available,a charger circuit charges the battery pack to keep it in top condition(fig 2).
Over the years, emergency lights have become better designed and more reliable. However, they retain a few weak facilities. Usually 6 volt sealed maintenance free (SMF) batteries are used in portable emergency lights. Unfortunately, many low cost lights give no indication when the battery has gone flat, so that there is a risk of the battery failure. Only high-end systems have built in electronic deep discharge protection circuitry to disconnect the output load in case the battery voltage happens to fall below a safe value.
The miniature add-on circuit-SMF(6V) BATTERY GUARD- presented here(fig 1) can be connected to almost all types of 6 v battery powered emergency lights. As can seen from the circuit diagram,the ‘hardware’ is rather simple. There are only two transistors,one zener diode,a couple of passive components , and, for the ‘user interface’, a push to off reset button(S1) and one LED indicator(D1). The finished circuit can be fitted (internally) to one side of the emergency light cabinet by a set of screws & spacers. Next,connect the relay contacts(N/C & C) in series with the existing on/off switch of the emergency light(fig 3). The user interface(S1&D1) are then mounted (externally) on the same side of the cabinet using suitable washers/bushes.
Working of the circuit is very simple.At the moment of initial power up,or after a reset by S1, capacitor C1 holds transistor T2 cut off. The switch off voltage level can be set with preset pot VR1.When the potential across C1 has risen to a value high enough(0.8V) to for relay driver T2 to be switched on,the relay is activated and its contacts disconnects the output load from the battery. Now ,in this active mode , the current drain is determined almost entirely by the relay coil (here around 13mA , including the LED current) only.