Parameter Generation and Control 9354-4250, Installation And Operation Manual, Page: 23 / 90

PGC, Inc 500-1000 CFM Air Handler May 2006
Appendix A A-7
D
ESCRIPTION OF
C
IRCUITRY
CAUTION
Although this conditioner utilizes 24 volts DC for the control circuits, some
components require 120 or 460VAC; these voltages can be present even when the
conditioner is not operating.
CAUTION
Power to the unit should be removed at the wall disconnect prior to opening any of
the access panels.
Line voltage is applied to the 24 VDC power supply. The 24VDC power supply provides power
to the Programmable Logic Controller (PLC), the HygroClip T/Rh transmitter, the air bypass
damper actuator, safety devices, and the line contactor.
The PLC has an on-board DC-DC converter that will p5VDC for the board-level logic
components. A second DC-DC converter pr15VDC for use by the SmartPad user
interface, stepper motor, and the RTD amplifiers.
When power is applied to the conditioner, and control circuit breaker(s) are closed, the SmartPad
user interface will be energized. If all of the safety devices are satisfied (pressure switches,
thermostats, float switches, etc.), then the conditioner can be energized by pressing the Standby
key on the SmartPad™ user interface. This will instruct the PLC to energize the line contactor
(1CON).
When 1CON is closed, power is applied to the motor starter protectors (MSPs) and the heater
circuit breaker. If the MSPs are closed, then the compressor, blower, and pump will be energized.
If the heater circuit breakers are closed, then the air and water heaters will be enabled. The
operation of the air heaters and water heater is controlled by the PLC; the PLC determines the
duty cycle (On time verses Off time) for each of the heaters, based on the requirements of each
temperature control loop. The PLC will open and close digital outputs that will control the input
signals to solid-state relays, which will apply power to the respective heater based on the
respective control loop. An LED on each relay will indicate the On or Off status of that particular
heater control loop.
The UV filter will also be enabled by the control circuit breaker(s). Any time power is applied to
the conditioner, the control circuit breakers are closed, and the unit is operational (i.e. 1CON is
closed), the UV filter should be energized.
On initial power up, the PLC will determine the hot gas bypass valve position by driving the
stepper motor to fully close (0% output) the hot gas bypass valve in order to “zero” the valve.
This involves driving the motor past the full closed position; the valve will be seated against the
full closed stop but is not damaged by this over driving maneuver. Bumping the valve against the
full-closed stop can correct for any “lost steps” or accumulated errors that could occur over time
due to the constant reversal of the stepper motor.
During normal operation, the position of the hot gas valve is based on the output percentage of
the water temperature control loop. This output percentage is translated into a percentage of
“open” value for the stepper motor. The PLC converts this percentage open value into a step
count that will position the valve accordingly. When the valve is adjusted to the desired position,
the PLC will cease to step the motor, and the motor will maintain its current position.