(2)  
The passenger cabin temperature sensor is installed in the same duct which encloses the passenger cabin temperature bulb (Ref par. 3.C.[1]).

4.  Temperature Selectors
A.  The control and passenger cabin temperature selectors are identical units mounted on the forward overhead panel. The face dial is divided into an automatic and MANUAL range. Three switches inside the selector are provided to direct 115 volts ac to the mix valve. Two cams on a cam plate, fixed to the selector knob, close each switch separately as the knob is turned.
B.  In MANUAL, turning the knob clockwise to COOL causes one of the cams to close a switch connected to the mix valve actuator motor, and operate the valve to increase the proportion of cold air passing through the valve. Turning the knob counterclockwise to WARM causes the cam to close a switch connected to the mix valve actuator and operate the valve to increase the proportion of warm air passing through the valve. When the knob is returned to OFF, both switches are open and the mix valve remains in the position to which it was last driven.
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C.  When the temperature selector knob is turned past the MANUAL range into AUTO, the second cam on the knob shaft closes a switch which directs power to the automatic temperature control circuit. The cam keeps the switch closed throughout the automatic range. In the AUTO range, the knob also drives a potentiometer that forms one leg of a cabin temperature control bridge circuit.
5.  Temperature Regulator
A.  Control and passenger cabin automatic temperature regulation is obtained from a single unit located in the electronic compartment. This unit contains all parts of each regulation system which are not required to be mounted remotely. Separate identical networks are enclosed for each cabin (Fig. 3).
(1)  On some regulators (temperature controllers) a built-in test circuit has been added to provide a quick electrical check of temperature control system components. On these controllers a rotary test switch, two sets of GO, NO GO lights and a test instruction decal are provided on the face of the controller. When temperature control system is not being tested the switch must be returned to START position.
B.  Each network contains a separate power supply, the fixed resistance legs of three different bridge circuits, a control amplifier, a silicon-controlled rectifier actuator control, and a pulse-forming network.
C.  The three bridge circuits in use for automatic control are cabin temperature control bridge, temperature control damping bridge, and duct temperature limit bridge.
D.  Each cabin temperature control bridge utilizes two legs that contain variable resistances outside of the regulator. 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 rapidly 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.
500 
May 15/78  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  21-61-0 Page 7 

Temperature Regulator Circuit  500 
21-61-0  Figure 3  Feb 15/67 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

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 magnetic amplifier prevents the temperature limit bridge having any effect on the control bridge signals during a cooling demand.
　
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