Honeywell Chadwick-Helmuth 8500C - Figure 4-4. Heater;El Control Board

4-8 Chadwick-Helmuth A Business Unit of Honeywell International, Inc.
Chapter 4 - System Theory of Operation
4.2.1.1.3 Heater/EL Control Board (Figure 4-4)
The heater/EL control board provides power and control of LCD screen heater and EL lamps used
behind the keypanel and LCD. The board contains a power supply filter, a display heater controller,
and an EL controller.
Figure 4-4. Heater/EL Control Board
a. Power Supply Filter. The 28 VDC input power is applied to the power supply filter from
the external power connector. The input power passes through a 2.5 Ampere fuse, a diode
bridge, and a filter. Voltages greater than 60 to 85 Volts are shorted to ground by a surge
protector which causes the fuse to open. The diode bridge supplies +27 VDC with the
correct polarity if the input +28 VDC polarity is reversed. The thermal cutoff switch shuts
off power to the display heater and EL lamps when the internal temperature is above
+65°C. This protects the EL lamps whose life span is decreased when operated at high
temperature. It also prevents the display heater from operating at high temperatures, in the
event of a controller failure.
2.5A
FUSE
+28 VDC
DIODE
BRIDGE
FILTER
THERMAL
CUTOFF
SWITCH
+27 VDC
POWER SUPPLY FILTER
DISPLAY HEATER
+27 VDC
VOLTAGE
REGULATOR
+12V
HEATER
CONTROLLER
HEATER
EL CONTROLLER
+27 VDC
VARIABLE
VOLTAGE
REGULATOR
EL
INVERTER
10K
BRIGHTNESS
ADJUSTMENT
EL LAMPS
(KEYPANEL)
EL LAMPS
(LCD DISPLAY)
FILE: FIG4-4.DRW