Sharp r-8580 manual Absolute Humidity, Sensor Circuit, JJj\i

R-8580

ABSOLUTE HUMIDITY

(1) Structure of Absolute Humidity Sensor

The absolute humidity sensor includes two

thermistors as shown in the illustration. One thermister 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.

Internal Structure of Sensor

(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 (Rl and R2).

The output of the bridge circuit is to be amplified by the operational amplifier.

Each thermister is supplied with a current to keep it heated at about 150 “C(302 “F)and the resultant heat is dissipated in the air and if the two thermistors are placed in different humidity condi-

tions they show different degrees of heat

conductivity leading to potentiaJ difference be- tween 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 will be amplified by the operational amplifier.

			t	Absolute humidity	vs.
		Operational		output voltage characteristic
GIPC: Thermistor
C	RI	amplifier	H
T		output	=
		vottaga	e
-	.		5
sit	: RI	-	0.
			2
		S: Thhenms
R3			\JJj\i
		closed vessl
			1	-Absolute humidity	b
					(g/m’ 1

IDetector Circuite 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 later the detector circuit starts to func- tion and the LSI observes the initial voltage availabel at its AN2 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 R50.

-1. Changing the resistance values results in that there is the same potential at both F-3 terminal of the absolute humidity sensor and AN3 terminal of the LSI. The voltage of AN2 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.

SENSOR CIRCUIT

As sensor cooking proceeds, the food is heated to generate moisture by which the resistance balance of the bridge circuit to deviate increasing the volt- age available at AN2 terminal of the LSl. Then the LSI observes that voltage at AN2 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 into the next necessary operation automatically. When the LSI starts to detect the initial voltage at AN2 terminal 16 seconds after the unit has been put in the Sensor Cooking mode, if it is im possible to take a balance of the bridge circuit due to dis- connection of the absolute humidity sensor, ER- ROR will reappear on the display and the cooking is stopped.

(RD72001 U)

1 )Absolute			humidity sensor		circuit
					Q!3l	swt
			RSO	R.watOr Block
				-.,mJ
			!	*.*
			I,	RsO-4)6	24	“’
					E23d
				RSO-III7	23	SW301
			0
					E22^I	SW47
				RSO&	22
			0
				”	EZl/	WC-J
c	Thcrmlrtor	m		RSO-7&	2l
	closed vessel		,C	c.	_ ma--I	7
S	Thcrmlrtor	In		I

open vessel

2)lnternal equivalent circuit of 12

Top View

28