This White LED Emergency light uses a Relay for the automatic working when power fails. When the Mains power is available, the relay allows the battery to charge and when the power fails, the relay switches on the emergency light. Too simple working ,and no complicated circuits. This Circuit is designed on request from Thomas PD.
A 12-0-12 volt 500 mA transformer is used to step-down the 230 V AC to low volt Ac which is rectified by the diode D1 and D2 and made ripple free by C1. Green LED indicates the availability of mains power and the charging status. The battery is connected to the Common contact of the relay. Charging current passes through the NO( Normally Open) contact of the relay while the Battery power passes to the White LED through the Common and NC( Normally Connected) contacts of the relay.
Six 1 watt white LEDs are used which can give sufficient light similar to 11 watts CFL. The high watt white LED requires 3 volts and around 100 mA current for sufficient brightness. So to get sufficient backup time, 12 volt 7.5 Ah UPS battery or 12 Volt Car battery is necessary.
When the mains power is available, Relay energizes and battery charges through D3, NO and Common contacts of the relay. At this time, Lamp will be off since the NC contact is detached from the common contact. When power fails, relay de energizes and the NC contact again makes connection with the common contact and the lamp turns on. A switch can be connected between the NC contact and the LEDs to switch off the lamp when not in use.