Viking CVMOS200, DMOS200 Absolute Humidity Sensor Circuit, Structure of Absolute Humidity Sensor

CDMOS200

DMOS200

VMOS200

CVMOS200

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.

Thermistors	ventilation opening for sensing
Sensing part	Sensing part
(Open vessel)	(Closed vessel)

(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 150oC (302oF), the resultant heat is dissipated in the air and if the two thermistors are placed in 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 inceases. Since the output is very minute, it is amplified by the operational amplifier.

				Absolute humidity vs,
C		Operational		output voltage characteristic
	R1
		amplifier		e
		+	Output	agt
			voltage	volOutput
S	R2		S : Thermistor
R3			open vessel
			C : Thermistor
			closed vessel
				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 then 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 R107 ~ R111 of IC2 . Changing the resistance value results in that there is the same potential at both F-3 terminal of the absolute humidity sensor and AN0 terminal of the LSI. The voltage of AN1 terminal will indicat about 16 seconds about -2.50

V. 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 AN1 terminal of the LSI.

Then the LSI observes that the 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 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

			R98		620k			26	SW1
									P20
			R99		300k			25	SW2
									P21
			R100		150k			24	SW3
									P22
			R101		75k			23	SW4
									P23
C. Thermistor in			R102		37.4k			22	SW5
									P24
closed vessel
S. Thermistor in
open vessel			R97					50	AIN4
			47k		0.01uF	0.015uF
							0.01uF
F-1				C93					LSI
C		R96			C91		C92		(IC1)
F-3	3.57k
						R94
							R95	49
		90	0.1 uF	8 7 6 5					AIN5
S		C		1 2 3 4		10k	47k
		R91	R92
F-2	3.32k			IC2	R93
			1.8k		360k		D91
	R90			D90
VA : -15V				VA : -15V		VC : -5V

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