The circuit of emergency light presented here is unique in the sense that it is automatic, com-pact, reliable, low-cost, and easy to assemble for anyone. The circuit consists of four sections, namely, battery charging section, inverter section, changeover section, and low battery voltage indication section. In the battery charging section, 230V AC mains is converted to 9V AC using step-down transformer X1. The diodes D1 and D2 from a full-wave rectifier, and capacitor C1 filters the rectified voltage.
The output of filter is about 12V DC, which is connected to the collector of transistor T1 provides a fixed bias of 8.2V. Thus, transistor T1 works as a regulator and provides a constant voltage for charging the lead-acid battery. LED1 indicates the charging of battery. The inverter section comprises transformer X2, transistor T2, capacitor C2 and resistor R3. Transformer X2 is ferrite core type. Its winding details are shown in Fig. 2.
While core details are shown in Fig. 3. Resistor R3 pro-vides DC bias to the base of transistor T2, while capacitor C2 couples the positive AC feed-back from winding L1 to the base of transistor T2 to sustain the oscillations. The AC power developed across primary winding L2 is transferred to secondary winding L3, which ultimately lights up the fluorescent tubes.
The changeover section uses diodes D3 and D4 as an automatic switch. In the presence of AC mains supply, diode D3 keeps transistor T2 in its cut-off state, while diode D4 provides DC path for charging of the battery. But, in the absence of AC mains supply, diode D4 is reverse biased and acts as an ‘off’ switch, inhibiting the conduction of diode D3, which allows normal functioning of transistor T2. The inverter can be switched ‘off’,when not required, by using ‘on/off’ switch S1.
Low battery voltage indicator circuit comprises transistor T3, senser diode D6, LED 2, variable resistor VR1, and resistors R4 through R6. The low battery indication can be adjusted from 4.7V to 5V by using variable resistor VR1. When the battery voltage is above 4.7V, zener diode D6 comes out of conduction, keeping transistor T3 at cut-off level. At the same time,LED2 gives the indication of low battery voltage.
The whole circuit can be assembled in a cabinet of emergency light suitably.
Author : Rajesh Kamboj - Copyright: EFY