30-41-00
 ALL  ú ú 01 Page 8 ú Jul 15/96
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 10. Operation_________
 A.  Each of the No. 1 and 2 window heat systems are operated by the actuation of the respective control switch to the ON position (Fig. 2). With either switch for either side in the ON position, 115-volt ac control power will be directed from the load control center to the window heat control unit for the respective window, and 115-volt ac heating power will be directed to windows No. 4 and 5 by the side switch of the respective side. The No. 1 and 2 windows are maintained at 107 |7°F by the window heat control unit and the No. 4 and 5 windows are regulated between 80 to 120°F by the window thermal switch on the No. 5 window.
 B.  With control power supplied to the heat control unit in the absence of an overheat condition, the control unit overheat circuit will direct 115-volt ac heating power from the load control center to the control unit temperature control circuit for modulation to the control unit heat transformer. If the window is below 100°F, the window heat sensor will signal the temperature control circuit for heating power and the temperature control circuit will gradually increase power to the control unit transformer circuit. The transformer circuit in turn will apply the preselected voltage to the terminal strip on the E3-3 electrical shelf then directly to the window. The window heating power will continue to increase until the temperature control circuit output reaches full operating power within 3 minutes or until the window sensor detects the window temperature approaching operating temperature. As operating temperature is approached, the temperature control circuit will begin to decrease power until power output and heat dissipation from the window are held in equilibrium at the required operating temperature. When the temperature control circuit supplies power to the control unit transformer, it also supplies a ground for the power on lamp driver circuit, turning on the ON light and providing visual indication that power is actually being supplied to the window. The window heat control unit may be confidence tested for power output through the use of the system test switch. Placing the switch in the POWER position completes a circuit to ground from the temperature control circuit which forces the temperature control circuit to full power output, regardless of window temperature.
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 30-41-00
 ALL  ú ú 03 Page 9 ú Jul 15/96
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 C.  If the window heat sensor detects a window temperature above 145°F while power is being applied to the window, the temperature control circuit will de-energize the control unit overheat circuit. Having the overheat detection section of the temperature control circuit operate only while power is actually being applied to the window allows a lower overheat trip setting, without nuisance system trips and lockouts under hot summer day conditions. With the control unit overheat circuit de-energized, the 115-volt ac heating power will be cut off from the temperature control circuit, removing window heat and turning off the power ON indicating light. Also, whenever the control unit overheat circuit is de-energized with the control switch in the ON position or energized with the control switch in the OFF position, the overheat indication circuit through the control switch will be completed to go through the control unit overheat circuit. This will turn on the overheat indication light and, after a 1-second delay, activate the master caution indicating circuit, to turn on the master caution and anti-ice indicating lights. The master caution and anti-ice lights will remain on until the master caution indicating circuit is reset by pressing the master caution light cover. The overheat indication lights will remain on until the system is reset by momentarily turning the control switch to the OFF position. While the system is in operation, the control unit overheat detection system may be confidence tested through the use of the system test switch. Placing the switch in the OVERHEAT position completes a circuit to ground from the temperature control circuit which forces the temperature control circuit to apply power to the window and then simulates the overheat condition, de-energizing the control unit overheat circuit and turning on the master caution, anti-ice and system overheat indicating lights.
　
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