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Model
R-28STM
Pages
23
Size
4.21 MB
Type
PDF
Document
Service Manual
Brand
Device
Home Appliance / Microwave Oven
File
r-28stm.pdf
Date

Sharp R-28STM Service Manual ▷ View online

 
 
 
THE HEATING PRINCIPLE OF MICROWAVE 
Microwave is one kind of radio wave whose wave length is very short, frequency is very high. Therefore, it is called ultrahigh 
frequency electromagnetic wave. Microwave can heat food mainly result in the mutual affect of the food in the microwave field and 
microwave field itself. 
Under the affect of microwave field, the thermal effect mechanism produced from the mutual affect of the microwave and the food 
includes two aspects: One is Dielectric loss of polar molecule, the other is conductive loss of ion. 
Usually, food is constitute of organism (plant and animal).The organism is formed by all kinds of polar water molecule, polar 
protein molecule, and all sorts of salt ion. The center of gravity of the positive and negative charge in the molecule is not coincide. 
In normal condition, the molecule is in irregular order due to its thermal action, thus the food do not appear polarity (FIG.1-
1a).Under the action of outer electric field, the positive end of the polar molecule trend to the negative electric field, the negative 
end of polar molecule trend to the positive electric field, and somewhat arrange in order through the direction of the electric 
field(FIG.1-1c).This phenomenon usually be called “TORQUE POLARITY”. When the outer electric field apply for the opposite 
polarity, the polar molecule then arrange an opposite direction order accordingly (FIG.1-1b).If the direction of the outer electric field 
changed repeatedly, the polar molecule would repeatedly sway accordingly. During the swaying, it understands that the polar 
molecule would produce heat due to somewhat similar friction among them.   
 
 
 
 
 
 
 
 
When the electric field is applied for ultrahigh frequent microwave field from the outside, its direction would change tens billion 
times per second, so do the molecule. This kind of molecule swaying producing similar frictional heat from the interference and 
block of the action strength among the molecule, and changed to microscopic microwave heating. Microwave heating not only 
concerned the nature of the matter itself, but also closely connected with the electric field strength and frequency. When the 
frequency is low, the molecule swaying rate and the acute degree of the mutual friction among the molecule is low, and would 
produce much heat. When the frequency is too high, as the swing of the polar molecule is with rotating inertia, it made the swing 
do not in line with the changing rhythm of the electric field because of the friction drag, thus, actually lowed the polar molecule 
swaying speed. The friction dragging degree is concerning about the magneto electric wave frequency, polar molecule shape, and 
the matter’s sticky degree. To different matter’s molecule, there are different special frequency zone. Those ado
 
absorb microwave 
energy from these zone are most capable to turn microwave energy to heat energy. 
Apart from the above said action, there is another action which is electric ion under the action of microwave field, act fiercely 
accompanied with the acceleration of electric field. The positive ion transfer to the negative polarity of the field while the negative 
ion do opposite. Accompanying with the changing electric field, the electric ion changing accordingly. During the transferring, heat 
produced with the crash among the ion. This kind of action take the main effect to those microwave heating of high salt molecule. 
No matter it is the polar molecule swaying or the ion transferring, they both are turning the microwave energy which the heating 
matter got from the microwave field to hear energy. From the analysis of theory, we can draw such a conclusion that the power 
which a unit of volume matter absorbed from the microwave field as the following formula: 
Pa=K·E· F·Er·tgδ 
Pa Stands for the power the heated matter absorbed from the microwave field.
 
K Stands for a constant 
E Stand for microwave field strength 
F Stands for the microwave frequency. 
Er Stands for relative dielectric constant of the heated matter.
 
tgδStands for loss angle tangent of the heated matter 
2-1
 
 
 
INSTALLATIONS 
 
INSTALLING 
1. Empty the microwave oven and clean inside it with 
a soft, damp cloth. Check for damage such as 
misaligned door, damage around the door or dents 
inside the cavity or on the exterior. 
 
2. Put the oven on a counter, table, or shelf that is 
strong enough to hold the oven and the food and 
utensils you put in it. (The control panel side of the 
oven is the heavy side. Use care when handling.) 
 
3. Do not block the vent and the air intake openings. 
Blocking vent or air intake openings can cause 
damage to the oven and poor cooking results. 
Make sure the microwave oven legs are in place to 
ensure proper air flow. 
 
4. The oven should not be installed in any area where 
heat and steam are generated, because they may 
damage the electronic or mechanical parts of the 
unit. 
Do not install the oven next to a conventional 
surface unit or above a conventional wall oven. 
 
5. Use microwave oven in an ambient temperature 
less than 104°F(40°C). 
 
6. Place the microwave oven on a sturdy and flat 
surface at least 10 cm(4 inches) from the wall. 
 
7. Place the microwave oven as far away as possible 
from TV, RADIO, COMPUTER, etc., to prevent 
interference. 
 
 
BEFORE YOU BEGIN, READ THE FOLLOWING INSTRUCTIONS COMPLETELY 
EARTHING INSTRUCTIONS 
This microwave oven is designed to be used in a fully 
earthed condition. 
It is imperative, therefore, to make sure it is properly 
earthed before servicing 
WARNING-   
THIS  APPLIANCE 
MUST BE EARTHED 
IMPORTANT
 
 
The wires in this mains lead are colored in 
accordance with the following code: 
Green-and-yellow:              Earth 
Blue:                                      Neutral 
Brown:                                  Live
 
As the colors of the wires in the mains lead of this 
appliance may not correspond with the colored 
markings identifying the terminals in your plug, 
proceed as follows. 
 
The wire which is colored 
green-and-yellow must be 
connected to the terminal in the plug which is marked 
with the letter 
or by the earth symbol (
 ) or 
colored 
green or green-and-yellow
 
The wire which is colored 
blue must be connected to 
the terminal in the plug which is marked with the letter 
or colored black
 
The wire which is colored 
brown must be connected 
to the terminal in the plug which is marked with the 
letter 
or colored red
3-1 
SCHEMATIC DIAGRAM
TCO.
 
 
4-2 
 
 
CIRCUIT DESCRIPTION
 
 
 
 
 
 
 
GENERAL DETAILS
• The low voltage transformer supplies the necessary
voltage to the micom controller when power cord is
plugged in.
• When the door is closed, the primary switch is ON, the
secondary switch is ON, and the monitor switch opens
(contact COM and NO).
WHEN SELECTING COOKING POWER
LEVEL AND TIME
• The micom controller memorizes the function you set.
• The time you set appears in the display window.
• Each indicator light turns on to indicate that the stage
has been set.
WHEN TOUCHING THE START PAD
• The coil of the relay is energized by the micom
controller.
• Power input is supplied to the high voltage transformer
through the fuse to the primary switch and relay 2.
• Turntable rotates.
• The fan motor rotates and cools the magnetron by
blowing the air (coming from the intake on the
baseplate)
• The air is also directed into the oven to exhaust the
vapor in the oven through the upper plate.
• Cooking time starts counting down.
• 3.2 volts AC is generated from the filament winding of
the high voltage transformer. This 3.2 volts is applied
to the magnetron to heat the magnetron filament
through two noise preventing choke coils.
• A high voltage of approximately 2100 volts AC is
generated in the secondary of the high voltage
transformer which is increased by the action of the high
voltage diode and charging of the high voltage
capacitor.
• The negative 4,000 Volts DC is applied to the filament
of the magnetron.
WHEN THE OVEN IS SET AT ANY LEVEL
EXCEPT MAXIMUM.
• The micom controller controls the ON-OFF time of relay
2 by the applied signal to vary the average output 
power of microwave oven as POWER LEVEL.
WHEN THE DOOR IS OPENED DURING
COOKING
• Both the primary switch and relay 2 are cut off primary
winding voltage of the high voltage transformer.
• ON-OFF of relay 2 is coupled electrically with opening
and closing of the secondary switch.
• When the door is opened, the secondary switch is
opened and when the door is closed, the secondary
switch is closed.
• The cooking time stops counting down.
• Relay stops functioning.
• As the door is opened, if the contact of primary switch
and relay 2 and/or secondary switch fails to open, the
fuse opens due to the large current surge caused by
the monitor switch activation, which in turn stops
magnetron oscillation.
L
FUSE
TRANS-
FORMER
RELAY 2
MICOM CONTROLLER
SECONDARY
SWITCH
SWITCH
N
MONITOR
SWITCH
PRIMARY
MONITOR
SWITCH
L
FUSE
RELAY 2
MICOM CONTROLLER
SECONDARY
SWITCH
PRIMARY
SWITCH
N
TRANS-
FORMER
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