Figure 4-4. Heater/El Control Board - Honeywell Chadwick-Helmuth 8500C Maintenance Manual

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.
2.5A
FUSE
+28 VDC
+27 VDC
+27 VDC
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.
4-8 Chadwick-Helmuth A Business Unit of Honeywell International, Inc.
POWER SUPPLY FILTER
DIODE
FILTER
BRIDGE
DISPLAY HEATER
VOLTAGE
+12V
CONTROLLER
REGULATOR
EL CONTROLLER
VARIABLE
VOLTAGE
REGULATOR
10K
BRIGHTNESS
ADJUSTMENT

Figure 4-4. Heater/EL Control Board

THERMAL
CUTOFF
SWITCH
HEATER
HEATER
EL
INVERTER
+27 VDC
EL LAMPS
(KEYPANEL)
EL LAMPS
(LCD DISPLAY)
FILE: FIG4-4.DRW