Figure 8-16. - Control unit with annunciation.

found where some form of automatic fire detection or automatic fire extinguishing is connected to the alarm system. However, recent conversion by most alarm system manufacturers to solid-state electronic design, which is essentially a low-voltage direct-current (dc) technology, means that the most recent installations are of the low-voltage type.

The power supply of the system refers to the circuitry and components used to convert the ac line voltage to low-voltage ac or dc for operating the alarm system and for charging standby batteries. If the system is an older one with a dry cell, nonrechargeable standby battery (no longer permitted by NFPA standards), the lower supply probably contains a switching arrangement for connecting the battery to the system when ac power fails. Figure 8-17 is a simplified diagram of a typical dc power supply for powering a low-voltage dc alarm system and for charging a rechargeable standby battery.

Transformer T drops the line voltage from 120 volts ac to a voltage in the range of 12 to 48 volts ac. The low ac voltage is rectified by diode bridge D, and the resulting dc voltage powers the alarm system through relay contacts S1 and charges battery B through the current limiting resistor R. When normal ac power is available energizing relay coil S, contacts S1 are closed. If ac power fails, S1 opens and S2 closes, connecting the battery to the alarm system. Fuse F1 protects against a defect in the power supply or the alarm system during normal ac operation. Fuse F2 protects against alarm circuits defects that would cause a battery overload during dc-powered operation.

Figure 8-17. - Typical dc power supply and battery charger.