Jenn-Air JMD2124WS Technical Education - The Absolute Humidity

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2. The coil of shut-off relay (RY-1) is en-   

  ergized, but the power transformer is not  

  turned on.

3. After about 16 seconds, the cook  

  relay(RY-2) is energized. The power    

  transformer is turned on, microwave en- 

  ergy is produced and first stage is started.

   The 16 seconds is the cooling time re-  

  quired to remove any vapor from the oven  

  cavity and sensor.

NOTE: During this first stage, do not open 

the drawer or touch STOP/CLEAR pad.

4. When the sensor detects the vapor emit- 

  ted from the food, the display switches  

  over to the remaining cooking time and the  

  timer counts down to zero.

  At this time, the drawer may be opened to  

  stir, turn or season food.

5. When the timer reaches zero, an audible  

  signal sounds. The shut-off relay and cook  

  relay are de-energized and the power   

  transformer, oven lamp, etc. are turned off.

6. Opening the drawer or touching the    

  STOP/CLEAR pad, the time of the day will  

  reappear on the display and the oven will  

  revert to an OFF condition. When the tim- 

  er reaches zero, an audible signal sounds.

Cooking Sequence

1. Touch one of the SENSOR pads.

NOTE: The oven should not be operated on 

sensor immediately after plugging in the unit. 

Wait two minutes before cooking on SENSOR.

THE ABSOLUTE HUMIDITY 

(AH) SENSOR CIRCUIT

(1) Structure of Humidity Sensor

  The humidity sensor includes two therm- 

  istors 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.

(2) Operational Principle of Humidity Sensor

  The figure below shows the basic struc- 

  ture of an absolute humidity sensor. 

  A bridge circuit is formed by two thermis- 

  tors 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.

Ventilation Opening for Sensing

Sensing part

(Open vessel)

Sensing part

(Closed vessel)

Thermistors

Continued next page.