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G.  Each of the three bridge circuits is connected to a magnetic amplifier. The amplifier interprets the signals received, and then signals the silicon-controlled rectifier actuator control to shut off current to the mix valve, complete a circuit to move the valve toward cold, or to complete a circuit to move it toward hot.
6.  Operation
A.  Cabin temperature may be adjusted either by a manual or automatic control system. Both systems utilize 115 volt ac current to adjust the mix valve so that air of the desired temperature is directed into the airplane distribution system. Circuit breakers are provided for temperature control system circuit protection. The PACK VALVE circuit breaker and the OVERHEAT circuit breaker provide protection during both manual and automatic control operation, the MANUAL TEMP CONT circuit breaker protects during manual operation, and the LEFT and RIGHT AUTO TEMP CONT circuit breakers protect during automatic control operation (Fig. 4).
B.  When air conditioning switches are turned ON the pack valves open and air from the pneumatic system is ducted through the mix valves to the air cycle system and the mixing chamber. The mix valves adjust to allow the proper proportion of cold air from the air cycle system and hot air from the pneumatic system to enter the distribution system for a selected cabin temperature.
C.  Manual control requires monitoring of the passenger cabin and supply duct temperature indicator while adjusting the mix valve position to obtain and hold the desired cabin temperature. With the air conditioning switches ON, 115 volt ac current is provided to three switches in the cabin temperature selector. If the selector knob is in the MANUAL OFF position all three switches are open. Moving the knob to COOL closes one of the switches and the circuit is completed to move the mix valve such that more air is passed through from the air cycle system and less from the pneumatic system. Moving the knob to WARM closes a different switch moving the valve in the opposite direction. Only one of the switches in the selector can be closed at a particular time. A 190°F (88°C) duct overheat thermal switch gives system protection to prevent.
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May 01/75  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  21-61-0 Page 9 

adjustment of the mix valve such that air entering the cabin becomes too hot. At approximately 190°F (88°C) the thermal switch closes, energizing the cabin duct overheat relay. The energized relay completes a circuit to move the mix valve to the full cold position. The thermal switch, when closed, also completes a circuit to illuminate the DUCT OVERHEAT light. After correcting the overheat condition the system may be returned to normal by pushing the reset button. Another thermal switch protects against duct overheat should control power be lost. At approximately 250°F (121°C) this switch closes to energize the pack overheat relay and complete a circuit to close the pack valve and illuminate the PACK TRIP OFF light. Return to normal after a trip off requires pushing the PACK RESET switch after the condition has been corrected.
D.  When the selector knob is moved to AUTO the third switch closes and a circuit is completed to the temperature regulator. Setting the knob pointer for a particular cabin temperature adjusts a potentiometer fixed to the knob shaft. This potentiometer serves as a reference resistance in the regulator temperature control bridge. The cabin temperature sensor provides the resistance in the other leg of the bridge. If cabin temperature is already the same as that asked for by the selector, the silicon-controlled rectifier actuator control will prevent any current passing on to the mix valve. At a cabin temperature other than that selected the temperature sensor will provide a resistance either higher or lower in the other leg of the control bridge. As a result the regulator actuator control will complete a circuit to move the mix valve either toward hot or cold, as required, to bring cabin temperature to agree with selected temperature. The anticipator bridge and the duct temperature limit bridge at the same time sense conditioned air temperature to slow down changes requested by the control bridge and prevent duct overheat. The three bridge circuits feed into a magnetic amplifier which takes the resultant signal to the actuator control. The actuator control then moves the mix valve so that cabin temperature changes without sudden blasts of cold or hot air and without raising duct temperature above limits. The same system overheat protection described under manual control is in effect during automatic control.
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21-61-0 Page 10  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 01/75 

5C8  Temperature Control Schematic - Right Side Shown (Left Control Similar) 
Jun 20/84  Figure 4  21-61-0 
　
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