R420LW
9 – 1
R420LW
Service Manual
CHAPTER 9.
TOUCH CONTROL PANEL ASSEMBLY
[1] OUTLINE OF TOUCH CONTROL PANEL
The touch control section consists of the following units.
(1) Key Unit
(2) Control Unit (The Control Unit consists of Power Unit and LSI Unit).
The principal functions of these units and the signals communicated
among them are explained below.
1. Key Unit
The key unit is composed of a matrix, signals generated in the LSI are
sent to the key unit through P20-P27. When a key pad is touched, a
signal is completed through the key unit and passed back to the LSI
through P70, P71,P41 and P45 to perform the function that was
requested.
2. Control Unit
Control unit consists of LSI, reset circuit, indicator circuit, power
source circuit, relay circuit, buzzer circuit, synchronizing signal circuit,
absolute humidity sensor circuit and back light circuit.
1) Reset Circuit
This circuit generates a signal which resets the LSI to the initial
state when power is supplied.
2) Indicator Circuit
This circuit consists of 25 segments and 4 common electrodes
using a Liquid Crystal Display.
3) Power Source Circuit
This circuit generates voltages necessary in the control unit from
the AC line voltage.In addition, the synchronizing signal is available
in order to compose a basic standard time in the clock circuit.
4) Relay Circuit
A circuit to drive the magnetron, fan motor, turntable motor and light
the oven lamp.
5) Buzzer Circuit
The buzzer is responsive to signals from the LSI to emit audible
sounds (key touch sound and completion sound).
6) Synchronizing Signal Circuit
The power source synchronizing signal is available in order to com-
pose a basic standard time in the clockcircuit.It accompanies a very
small error because it works on commercial frequency.
7) Door sensing Switch
A switch to “tell” the LSI if the door is open or closed.
8) Back Light Circuit
A circuit to drive the back light (Light emitting diodesLD1- LD4).
9) Absolute Humidity Sensor Circuit
This circuit detects moisture of the cooking food to allow its auto-
matic cooking.
[2] 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.
2. Operational Principle of Absolute Humidity Sen-
sor
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 150
°
C, 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 differ-
ence 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 opera-
tional amplifier.
3. Detector Circuit of Absolute Humidity Sensor Cir-
cuit
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 AN1 terminal.With this volt-
age 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 poten-
tial at both F-3 terminal of the absolute humidity sensor and AN0 termi-
nal of the LSI. The voltage of AN1terminal will indicate about -2.5V.
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 ter-
minal of theLSI.
Symbol
Valtage
Application
VSS
-5V
LSI (IC1)
Ventilation opening for sensing
Sensing part
(Open vessel)
Sensing part
(Closed vessel)
Thermistors
C
S
R 3
R 1
R 2
+
-
Operational
amplifier
Output
voltage
S : T hermis tor
open ves s el
C : T hermis tor
clos ed ves s el
2
Abs olute humidity (g/m )
O
u
tp
u
tv
o
lt
a
g
e
Abs olute humidity vs ,
output voltage characteris tic
