Sharp R-880B Service Manual page 11

ASYMMETRIC RECTIFIER
The asymmetric rectifier is a solid state device that pre-
vents current flow in both directions. And it prevents the
temperature rise of the high voltage transformer by blowing
the fuse M10A when the high voltage rectifier is shorted.
D2 D1
ASYMMETRIC
RECTIFIER
The rated peak reverse voltage of D1 of the asymmetric
rectifier is 6 KV. The rated peak reverse voltage of D2 of
the asymmetric rectifier is 1.7 KV. D1 and D2 of the
asymmetric rectifier or high voltage rectifier are shorted
when the each peak reverse voltage goes beyond the
each rated peak reverse voltage. (The process of blowing
the fuse M10A.)
1. The high voltage rectifier is shorted by any cause when
microwave cooking.
2. The peak reverse voltage of D2 of the rectifier goes
beyond the rated peak reverse voltage 1.7 KV in the
voltage doubler circuit.
3. D2 of the rectifier is shorted.
4. The large electric currents flow through the high volt-
age winding of the high voltage transformer.
5. The large electric currents beyond 10A flow through
the primary winding of the high voltage transformer.
6. The fuse blows by the large electric currents.
7. The power supply to the high voltage transformer is cut
off.
THERMAL CUT-OUT 125˚C (MG)
This thermal cut-out protects the magnetron against over-
heating. If the temperature goes up higher than 125˚C
because the fan motor is interrupted or the ventilation
openings are blocked, the thermal cut-out will open and
line voltages to the high voltage transformer will be cut off
and the operation of the magnetron will be stopped. The
thermal cut-out will cut back in at 95˚C.
THERMAL CUT-OUT170˚C (OVEN)
The thermal cut-out located on the top of the oven cavity
is designed to prevent damage to the oven if the foods in
the oven catch fire due to over heating produced by
improper setting of the cooking time or failure of control
unit. Under the normal operation, the oven thermal cut-out
remains closed. However, when abnormally high tem-
perature are reached within the oven cavity, oven thermal
cut-out will open at 170˚C causing the oven to shut down.
The thermal cut-out will cut back in at 155˚C.
THERMISTOR
The thermistor is a negative temperature coefficient type.
The temperature in the oven cavity is detected through the
resistance of the thermistor, and then the control unit
causes the heating element relay to operate, thus the
current to the heating element is turned ON/OFF.
MONITOR RESISTOR
The monitor resistor prevents the fuse M10A bursting
when the fuse M10A blows due to the operation of the
monitor switch.
TURNTABLE MOTOR
The turntable motor drives the turntable roller assembly to
HIGH VOLTAGE
RECTIFIER
9
rotate the turntable.
FAN MOTOR
The fan motor drives a blade which draws external cool air.
This cool air is directed through the air vanes surrounding
the magnetron and cools the magnetron. This air is chan-
nelled through the oven cavity to remove steam and
vapours given off from heating food. It is then exhausted
through the exhausting air vents of the oven cavity.
CONVECTION MOTOR
The convection motor drives the convection fan and
provides the heated air.
NOISE FILTER
The noise filter assembly prevents radio frequency inter-
ference that might flow back in the power circuit.
TOP HEATERS
The top heaters are located on the top of the oven cavity
assembly. The top heaters send out heat to grill foods.
BOTTOM HEATER
The bottom heater is located on the floor of the oven cavity
assembly. The bottom heater sends out heat to grill foods.
ONVECTION COOKING SYSTEM
This oven is designed with a hot air heating system where
food is not directly heated by the top and bottom heaters,
but is heated by forced circulation of the hot air produced
by the top and bottom heaters. The air heated by the top
and bottom heaters is circulated through the convection
passage provided on the outer casing of the oven cavity by
means of the convection fan which is driven by the
convection motor. It then enters the inside of the oven
through the vent holes provided on the left side of the oven.
Next, the hot air heats the food on the turntable and leaves
the oven cavity through the vent in the oven cavity left side
wall. Without leaving the oven, this hot air is reheated by
the top and bottom heaters, passes through the convec-
tion passage and enters the inside of the oven cavity
again, in a continuing cycle. In this way, the hot air
circulates inside the oven cavity to raise its temperature
and, at the same time, comes into contact with the food
being cooked. When the temperature inside the oven
cavity reaches the selected temperature, the top and
bottom heaters are de-energized. When the temperature
inside the oven cavity drops below the selected tempera-
ture, the top and bottom heaters are energized again. In
this way, the inside of the oven cavity is maintained at
approximately the selected temperature. When the con-
vection time reaches 0, the top and bottom heaters are de-
energized and the convection fan stops operating and the
oven shuts off. Upon completion of the cooking time, the
audible signal will sound, and oven lamp, turntable motor,
cooling fan motor and convection motor are de-energized.
At the end of the convection cycle, if the cavity air tempera-
ture is above 120˚C, the circuit to RY5 will be maintained
(by the thermistor circuit) to continue operation of the
cooling fan motor until the temperature drops below 104˚C,
at which time the relay will be de-energized, turning off the
fan motor. Relay RY1 will however, open as soon as the
convection cycle has ended, turning off the convection fan
motor. This will now cool.
R-880B