Explain with circuit diagram the operation of emergency lighting system.

Ans:

Working:-

• Fig. above shows a simple emergency lighting circuit. The 230v ac supply is applied as input. Supply is stepped down using a Center tapped transformer. The full wave rectifier converts ac to dc voltage. 

•  When supply is ON, voltage appears across it and the lamp glows. Pulsating current also flows through D3 & R1 to charge the battery. Thus battery charging is carried out. 

•  The capacitor C gets charged with upper plate positive to some voltage less than secondary voltage of transformer. Due to capacitor voltage, gate cathode junction of SCR1 gets reverse biased. The anode is at battery voltage & cathode is at rectifier output voltage, which is slightly higher, hence SCR1 is reverse biased & cannot conduct. The lamp glows due to rectifier output dc voltage. 

•  If power fails, the capacitor C discharges through D3, R1 & R3 until the cathode of SCR, is less positive than anode. At the same time the junction of R2 & R3 becomes positive & establishes a sufficient gate to cathode voltage to trigger the thyristor. Once the thyristor turns ON, the battery discharges through it, & turns the lamp ON. When power is restored, the thyristor is connected & commutated & capacitor C is recharged.