MMD24S
MMD24B
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
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Sensing part | Sensing part |
(Open vessel) | (Closed vessel) |
(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.
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| Absolute humidity vs, |
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| Operational | output voltage characteristic | |
R | amplifier | voltageutput | ||
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C | R2 |
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| open vessel |
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| C : Thermistor |
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| closed vessel |
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| Absolute humidity (g/m 2) |
the resistance values results in that there is the same potential at both
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.
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| R40 | 430 |
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| closed vessel |
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| 0.1µF | 1 | 63 | AN7 | |||
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| R50 10K | R52 47K |
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F1 | R4 |
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| R43 1.8K |
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| R44 360K |
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| R51 47K |
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| R45 620K |
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| 52 | SW1 |
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| R46 300K |
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| 51 | SW2 |
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| R47 150K |
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| 50 | SW3 |
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| R48 75K |
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| 49 | SW4 |
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| R49 37.4K |
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| 48 | SW5 |
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(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
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