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With neat circuit diagram explain working of emergency light system. 
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Emergency lighting system: 

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In the residential, commercial and industrial areas, the illumination system work with available AC supply. In the event of AC supply failure, partial illumination can be obtained using emergency lighting system that works on DC supply. A very simple single source emergency lighting system which is most suitable for household application is shown in the figure. The input 230v AC supply is stepped down to 6-0-6V AC supply by centre-tapped transformer. The diodes D1 and D2 form full wave rectifier and comnvert 6-0-6V AC supply into 6V DC supply for 6V lamp. When AC supply is available, 6V DC supply appears across lamp & it glows. The pulsating current also flows through D3, R1 to trickle charge the battery. Thus battery charging is carried out when AC supply is availabl;e. The capacitor C get charged with upper plate positive to some voltage less than 6V. due to capacitor voltage, gate-cathode junction of thyristor T get reverse biased. The anode is at battery voltage and cathode is at rectifier output voltage, which is slightly higher, hence thyristor is reverse biased and can not conduct. The lamp glows due to rectifier output DC voltage. When AC supply fails, rectifier output DC voltage is reduced to zero. The capacitor C then discharges through lamp and R2. After discharging, due to battery, it charges through R3 and lamp with lower plate positive. Due to capacitor voltage, gate-cathode junction get forward biased and gate current flows. Since the anode is now at higher potential than cathode, thyristor T is turned-on. The lamp get connected across battery through thyristor and therefore, it glows. In rthis way, on failure of AC supply, light is obtained from DC supply.

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