Sharp KB-3300JS Service Manual - Page 34

KB-3300JS KB-3300JK KB-3300JW HUMIDITY SENSOR CIRCUIT (1) Structure of Humidity Sensor The 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. ventilation opening for sensing Thermistors Sensing part (Open vessel) Sensing part (Closed vessel) potential at both F-3 terminal of the absolute humidity sensor and AN6 terminal of the LSI. The voltage of AN7 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 AN7 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 AN7 terminal 16 seconds after the unit has been put in the Sensor Cooking mode, if it is not possible to balance the bridge circuit due to disconnection of the absolute humidity sensor, ERROR will appear on the display and the cooking is stopped. 1) Humidity sensor circuit (2) Operational Principle of 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 150οC (302οF), 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 varied every minute, it is amplified by the operational amplifier. R3 Operational S R1 amplifier Output + voltage Absolute humidity vs, output voltage characteristic Output voltage C R2 S : Thermistor open vessel C : Thermistor closed vessel Absolute humidity (g/m 2) (3) Detector Circuit of 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 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 R45 ~ R49. Changing the resistance values results in that there is the same 32 VA : +15V VC : +5V R40 430 C S F2 F3 F1 R42 3.57K R41 3.32K C. Thermistor in closed vessel S. Thermistor in open vessel D40 D41 86 C42 0.1 F 38 + 2 - IC3 4 1 R50 10K R43 1.8K R44 360K R52 47K 86 AN7 LSI (IC1) R51 47K R45 620K R46 300K R47 150K R48 75K R49 37.4K 85 AN6 95 SW1 P00 96 SW2 P01 97 P02 98 P03 SW3 SW4 99 SW5 P04