TM-9-1200-215-34-P
AUTOMATIC FIRE CONTROL SYSTEM (AFCS)
TECHNICAL MANUAL; DIRECT SUPPORT AND GENERAL SUPPORT MAINTENANCE MANUAL
FEBRUARY 1999
TM-9-1200-215-34-P - Page 161 of 266
TM 9–1200–215–34&P
6–7
When the vehicle input voltage is greater than 24 volts the PWM is enabled which pulses the MOSFET gate
drives allowing current to flow through the inductor (L), resistor (R), and diode (CR2). The output voltage (V
out
)
and current (l
out
) are continuously monitored. The output current will never exceed 11 amperes. As the battery
recharges the output current will taper off to a small trickle. The applied output voltage will not be constant but will
vary as a function of the temperature of the back–up batteries.
The charger output voltage adjusts with the temperature to compensate for the charging characteristics of
lead–acid batteries. For low temperatures the applied charging voltage must be increased to fully charge the
battery while at high temperatures the voltage needed to fully charge the back–up batteries must be deceased,
otherwise the life of the back–up batteries will be reduced.
A thermistor mounted in the battery assembly is monitored by the PCU. The PCU automatically adjusts the
charging voltage as the temperature of the back–up battery assemblies change by comparing the voltage that is
compensated by the thermistor to a reference voltage and using this to adjust the pulse drive to the MOSFETs.
When the MOSFETs are pulsed (i.e., conducting, then not conducting current) the inductor will smooth the abrupt
change in current by forcing diode CR1 to conduct current when the MOSFET is off. The result is a triangular
shaped current waveform that is filtered by an output capacitor and then applied to the back–up
batteries for
recharging.
(8)
Status monitor circuit. The PCU monitors the vehicle battery and the back–up battery voltages and
generates the vehicle battery status and back–up battery status signals which are connected to power busses A
and B. Figure 6–5 shows the status monitor circuit and nuclear event detector interface diagram.
Figure 6–5. Status Monitor Circuit and Nuclear Event Detector Interface Diagram
J5
J1
J2
RADIO OUTPUT
BACKUP BATTERY VOLTAGE
LEU OUTPUT
IR/NS OUTPUT
DIAGNOSTIC
PORT
POWER
BUS A
POWER
BUS B
ON SIGNAL
PCU TEST
SHUTDOWN
SWITCH VOLTAGE
VEHICLE BATT. STAT.
BACKUP BATT. STAT.
MONITORS
AND
CONTROL
AND
NUCLEAR
EVENT
DETECTOR
(9)
Nuclear event detector. A Silicon Controlled Rectifier (SCR) that will conduct current when it is exposed
to gamma radiation is used as the nuclear event detector. The Nuclear Event Detector (NED) will latch NED to
the “on” condition after exposure.
When the SCR conducts current it immediately opens both the input power relay and the back–up power relay,
along with interrupting the 28 volts to the external ‘Power On’ switch. NED will continue to disable the PCU
outputs after a nuclear event unless the vehicle power circuit breaker on the PCU is manually turned “off” and
then back “on” or until vehicle input power is removed from and reapplied to the PCU.
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