Sharp R930AK Service Manual - Page 27
Absolute Humidity Sensor Circuit - p parts
UPC - 074000606036
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R-930AK R-930AW Pin No. Signal 48 P10 49-53 54-56 57-59 P07-P03 P02-P00 P37-P35 60-64 P34-P30 I/O OUT OUT OUT OUT OUT Description Segment data signal. Signal similar to P23. Key strobe signal. Signal applied to touch-key section. A pulse signal is input to P24-P27 terminal while one of G-5 line keys on key matrix is touched. Segment data signal. Signal similar to P23. Digit selection signal. Signal similar to P55. Digit selection signal. Signal similar to P55. Used for initial balancing of the bridge circuit (absolute humidity sensor). ABSOLUTE HUMIDITY SENSOR CIRCUIT (1) Structure of Absolute Humidity Sensor The absolute humidity sensor includes two thermistors as shown in the illustration. One thermistor is housed in the closed vessel filled with dry air while another in the open vessel. Each sensor is provided with the protective cover made of metal mesh to be protected from the external airflow. Sensing part Sensing part (Open vessel) (Closed vessel) Ventilation openings Sensing part (Open vessel) Thermistor element Sensing part (Closed vessel) Thermistor element Sensor case View of sensor case removed Cross section view (2) Operational Principle of Absolute Humidity Sensor The figure below shows the basic structure of an absolute humidity sensor. A bridge circuit is formed by two thermistors and two resistors (R1 and R2). The output of the bridge circuit is to be amplified by the operational amplifier. Each thermistor is supplied with a current to keep it heated at about 150˚C (302˚F), the resultant heat is dissipated in the air and if the two thermistors are placed in different humidity conditions they show different degrees of heat conductivity leading to a potential difference between them causing an output voltage from the bridge circuit, the intensity of which is increased as the absolute humidity of the air increases. Since the output is very minute, it is amplified by the operational amplifier. C S R3 Operational R1 amplifier Output + voltage - R2 S : Thermistor open vessel C : Thermistor closed vessel Absolute humidity vs, output voltage characterist Absolute humidity (g/m 2) (3) Detector Circuit of Absolute Humidity Sensor Circuit This detector circuit is used to detect the output voltage of the absolute humidity circuit to allow the LSI to control sensor cooking of the unit. When the unit is set in the sensor cooking mode, 16 seconds clearing cycle occurs Output voltage R51 0.1 µF 0.01µF 0.015µF 0.01µF than the detector circuit starts to function and the LSI observes the initial voltage available at its AN6 terminal. With this voltage given, the switches SW1 to SW5 in the LSI are turned on in such a way as to change the resistance values in parallel with R50-1. Changing the resistance values results in that there is the same potential at both F-3 terminal of the absolute humidity sensor and AN7 terminal of the LSI. The voltage of AN6 terminal will indicate about -2.5V. This initial balancing is set up about 16 seconds after the unit is put in the Sensor Cooking mode. As the sensor cooking proceeds, the food is heated to generate moisture by which the resistance balance of the bridge circuit is deviated to increase the voltage available at AN6 terminal of the LSI. Then the LSI observes that voltage at AN6 terminal and compares it with its initial value, and when the comparison rate reaches the preset value (fixed for each menu to be cooked), the LSI causes the unit to stop sensor cooking; thereafter, the unit goes in the next operation automatically. When the LSI starts to detect the initial voltage at AN6 terminal 16 seconds after the unit has been put in the Sensor Cooking mode, if it is not possible to balance, of the bridge circuit due to disconnection of the absolute humidity sensor, ERROR will appear on the display and the cooking is stopped. 1) Absolute humidity sensor circuit IC2(IZA495DR) C. Thermistor in closed vesssl S. Thermistor in open vessel 620k 300k 150k 75k 37.4k 47k 4 64 SW1 P30 5 63 SW2 P31 6 62 SW3 P32 7 61 SW4 P33 8 60 SW5 P34 35 AN7 F-1 11 C LSI (IC1) F-3 3.57k S F-2 3.32k 1.8k 10 + 10k 2 - 47k 360k 6 AN6 9 12 1 VA : -15V VA : -15V VC : -5V 25