Power Supply and Battery Backup for Pyroelectric Fire Alarm System. Power supply comprises a 0-12V, 1A step-down transformer having a standard full-wave rectifier formed by D1 through D4 and filter capacitor C1. A battery backup is supplied when the mains supply is cut-off because of short-circuit and fire. A 12V, 4.5Ah rechargeable battery is utilized for backup to provide enough current for the solenoid pump. When mains electrical power is available, diode D5 forward biases. It provides power towards the circuit and also charges the battery via resistor R2, and it limits the charging current to 120 mA. When power fails, diode D5 reverse biases and diode D6 forward biases, providing immediate backup for the circuit. LED1 signifies the availability of mains electrical power.