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 D.  Each cabin temperature control bridge utilizes two legs which contain variable resistances outside of the controller. One resistance is located in the temperature selector and is manually set to establish the desired temperature reference. The other is a temperature sensor located in the cabin whose resistance varies according to cabin temperature. The bridge is balanced when cabin temperature causes the temperature sensor resistance to equal the selector resistance.
 E.  The temperature control damping bridge also has the variable resistors of two of its legs remotely located. The resistors are contained in a single duct anticipator sensor in the main distribution manifold. One resistor is thermally impeded by insulation so that it reacts slowly to temperature change. The other is not insulated, and reacts repidly to temperature changes. As a result of a sudden temperature change the resistors will not vary at the same rate and the bridge will become unbalanced. The damping bridge slows down system response to prevent delivery of excessively hot or cold air to the cabin. It also assists to prevent overshooting and hunting of the temperature control system when a new temperature is selected.
 F.  The duct temperature limit bridge has only one of its resistances remotely located. The duct limit temperature sensor is located such that it senses the temperature of the conditioned air. As the temperature of the air rises the resistance changes and the bridge becomes unbalanced. If the duct temperature approaches an unsafe level, the temperature limit bridge signal will cancel out heat demand signals of the control bridge. A polarizing diode between the duct temperature limit bridge and the amplifier prevents the temperature limit bridge having any effect on the control bridge signals during a cooling demand.
 G.  Each of the three bridge circuits is connected to an 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 temperatue 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.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 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 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. 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.
　
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