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9 – Defrost Control CMC1
The CMC1 defrost control (figure 6) is a solid state control
manufactured by Hamilton Standard. The control
provides automatic switching from normal heating
operation to defrost mode and back. The control
provides 14 minute defrost periods at 30, 60 or 90 minute
field changeable intervals. The control monitors
thermostat demand and “holds” the timer in place
between thermostat demand. A set of diagnostic pins are
also provided for troubleshooting the unit.
The control contains a solid state timer which switches
an external defrost relay through 1/4” male spades
mounted on the control’s circuit board. When the
defrost thermostat closes (call for defrost), the defrost
timer initiates a 30, 60 or 90 minute (depending on how
the control is preset) timing sequence. If the defrost
thermostat remains closed when the timing sequence
ends, the defrost relay is energized and defrost begins.
SOLID STATE DEFROST CONTROL CMC1
Timing
Jumper
Timing Pins
Troubleshooting Pins
Control Terminals
30
60 90
FIGURE 6
A defrost period can last up to 14 minutes and can be
terminated two ways. If the defrost thermostat does
not open within 14 minutes after defrost begins, the
timer will de–energize the defrost relay and the unit
will resume normal operation. If the defrost
thermostat opens during the 14 minute defrost period,
the defrost relay is de–energized and the unit resumes
normal operation. Refer to figure 7.
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CLOSED, ON
OPEN, OFF
THERMOSTAT DEMAND
DEFROST THERMOSTAT
DEFROST RELAY
DEFROST THERMOSTAT
DEFROST RELAY
THERMOSTAT DEMAND
DEFROST THERMOSTAT
DEFROST RELAY
THERMOSTAT DEMAND
DEFROST THERMOSTAT
DEFROST RELAY
HP23 SERIES UNITS TYPICAL DEFROST TIMINGS
NORMAL HEATING OPERATION: DEFROST TERMINATED BY DEFROST THERMOSTAT
NORMAL HEATING OPERATION: DEFROST TERMINATED BY TIME
NORMAL HEATING OPERATION INTERRUPTED BY THERMOSTAT DEMAND: “HOLD” FUNCTION
DEFROST PERIOD INTERRUPTED BY THERMOSTAT DEMAND: “HOLD” FUNCTION
30/60/90 MINUTES
“HOLD” TIME
“HOLD” TIME
14 MIN. PLUS “HOLD” TIME
30/60/90 MINUTES
DEFROST THERMOSTATOPEN WITHIN 14 MINUTES
30/60/90 MINUTES
14 MIN.
30/60/90 MINUTES
30/60/90 MINUTES PLUS “HOLD” TIME
Note – Control begins timing at 0 when defrost thermostat closes. Defrost is terminated when defrost
relay is de–energized. Anytime defrost thermostat opens, defrost relay is immediately de–energized,
defrost timer resets and “HOLD” function stops.
DEFROST THERMOSTAT
MUST REMAIN CLOSED
FOR TIMER TO REMAIN
IN “HOLD”
DEFROST THERMOSTAT
MUST REMAIN CLOSED
FOR TIMER TO REMAIN
IN “HOLD”
FIGURE 7
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THERMOSTAT DEMAND
