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.