Service manual

30
Theory of Operation
DCB, DCFCB and DCFCTB Models
Theory of Operation
Control Board Theory of Operation
Description of DCFCTB Block Diagram
Power Supply Section
You can power the unit by either 110 VAC or 220 VAC (on JP22). The unit is
protected by MOVs V5, V6 and V7, resistors R147 and R148 and by fuse F2. These
absorb spikes and limit over-voltages. The power is stepped down by the transformer,
rectified by D59 and filtered by C99. The voltage at this point is about 24 Volts DC.
Transorb D61 blows fuse F1, if the voltage exceeds 30 VDC. This voltage is
regulated to 12 VDC by switching regulator U27. This voltage turns on Q14 thru
R104. The collector of Q14 sends a low to U22 pin 2 indicating the presence of AC
Power.
U27 can also be powered by up to 75 Volts DC thru JP5 pin 9 from an external DC
source. The 12VDC from U27 turns on Q15 thru R116. This pulls the gate of switch
Q20 low allowing it to pass voltage from the 12 Volt battery thru Q20 to the rest of
the board if needed due to a power failure. The passed battery voltage turns on Q16
thru R118 and Zener diode D57. Q16 also pulls the gate of Q20 low turning it on. If
AC power fails, Q15 releases its low to Q20. Then if the battery voltage drops below
about 9.6 VDC, Q16 releases its low to Q20. This turns off the power to the board;
this stays off until AC power is restored.
U28 regulates the voltage further to 8 VDC and U24 regulates the voltage further to 5
VDC. An LED (D34) is also tied to this point and indicates power. U18 is an isolated
DC to DC converter which converts the 12 Volt supply to an isolated 5 Volt supply.
This is used for sensor and remote activation inputs. LED (D36) is also tied to this
point and indicates isolated 5 Volt power. U29 is an isolated DC to DC converter
which converts the 12 Volt supply to an isolated 12 Volt supply. This is used for
relay outputs. LED (D53) is also tied to this point and indicates isolated 12 Volt
power.
U14 monitors the voltage of the 5 Volt power. If the power supply voltage falls
below 3.3 Volts, U14 resets the microprocessor.
CPU Section
The microprocessor section is comprised of the microprocessor (U15). This processor
contains a watchdog timer, a day/date clock, non-volatile EEPROM and FLASH
memory, I/O pins and A to D convertors. The EEPROM is the non-volatile memory
where all customer specific information held while the FLASH is the non-volatile
memory where the main program is held. One (1) allows the microprocessor to
communicate over serial port JP8 for programming.