Electrolux EI30MH55GB Service Manual - Page 29

HUMIDITY SENSOR CIRCUIT EI30MH55GSA EI30MH55GBA EI30MH55GWA (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. Thermistors ventilation opening for sensing Sensing part (Open vessel) Sensing part (Closed vessel) (2) Operational Principle of Humidity Sensor The figure below shows the basic structure of a 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 150oC (302oF), 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. Operational C R amplifier Output + voltage Absolute humidity vs, output voltage characteristic S R2 R3 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 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 AN1 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 R98 ~ R102. Changing the resistance values results in that there is the same potential at both F-3 terminal of the humidity sensor and AN0 terminal of the LSI. The voltage of AN1 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. Output voltage R90 C90 0. uF C93 C9 0.0uF 0.uF C92 0.0uF 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 AN1 terminal of the LSI. Then the LSI observes that voltage at AN1 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 AN1 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 humidity sensor, ERROR will appear on the display and the cooking is stopped. 1) Humidity sensor circuit C. Thermistor in closed vessel S. Thermistor in open vessel R98 R99 R00 R0 R02 620k 300k 50k 75k 37.4k R97 47k F- C R96 F-3 3.57k S F-2 R9 3.32k R92 .8k 8765 R94 234 5k IC2 R93 360k R95 47k D90 8 SW P50 7 SW2 P5 6 SW3 P52 5 SW4 P53 4 SW5 P54 0 AN0 LSI (IC) 9 AN VA : -5V VA : -5V VC : -5V 29