Monitor Switch (Australia Model); Powertransformer; Voltage Doubler Circuit; Magnetrontube - Sharp R-8320E Service Manual

MONITOR SWITCH (AUSTRALIA MODEL)

The monitor switch mounted on the upper latch hook is
activated (the contacts opened) by the latch head on the
door while the door is closed. This switch is intended to
render the oven inoperative by means of blowing the fuse
when the contacts of the upper latch switch fail to open
when the door is opened.
Functions;
1. When the door is opened, the monitor switch contacts
close (to ON condition) due to their being normally
closed. At this time the upper and the lower latch
switches are in OFF condition (contacts open) due to
their being normally open contact switches.
2. As the door goes to a closed position, the monitor
switch contacts are first opened and then the upper and
lower latch switches contacts close. (In case of opening
the door, each of these switches operate inversely.)
3. If the door is opened and the upper latch switch con-
tacts fail to open, the fuse blows simultaneously with
the closing of the monitor switch contacts.
CAUTION: BEFORE REPLACING A BLOWN FUSE,
T E S T T H E U P P E R L A T C H S W I T C H
AND MONITOR SWITCH FOR PROPER
OPERATION. (REFER TO THE TEST
PROCEDURE ON PAGE 56.)
POWER TRANSFORMER
The transformer consists of three windings: primary,
filament and high voltage.
During a cook cycle, the 220/240 volts A.C. applied to the
primary winding of the transformer through the cook relay
contacts is converted to 3.15 volts A.C. on the filament
winding and approximately 2400 volts A.C. on the high
voltage winding. The 3.15 volts A.C. voltage heats the
magnetron filament. This causes the tube cathode to
readily emit the electrons necessary for tube conduction
whenever the negative 4000 D.C. voltage is applied to the
cathode.
WITHOUT
MAGNETIC
FIELD
CATHODE
ELECTRONS
Figure 10. Basic Magnetron
without Magnetic Field
The 2400 volts A.C. voltage is fed to the voltage doubler
circuit.

VOLTAGE DOUBLER CIRCUIT

The voltage doubler circuit consists of a rectifier and a
capacitor. The 2400 volts A.C. from the high voltage wind-
ing of the power transformer is applied to the voltage
doubler circuit, where it is rectified and converted to
approximately 4000 volts negative D.C. needed for
magnetron operation.
Rectifier: The rectifier is solid state device that allows cur-
rent flow in one direction, but prevents current flow in the
opposite direction. This acts as rectifier changing alternat-
ing current into pulsating D.C.
High voltage capacitor: The capacitor is able to store
energy on one half of the power cycle and release it along
with the transformer output to produce approx. 4000
negative D.C. volts to the magnetron.
MAGNETRON TUBE
The basic magnetron tube is a cylindrical cathode within a
cylindrical anode surrounded by a magnetic field. When the
cathode is heated by the filament winding of the power
transformer, electrons are given off by the cathode. These
negatively charged electrons are attacted to the more
positive anode of the tube when the negative 4000 D.C.
voltage is applied to the cathode.
Ordinarily, the electrons would travel in a straight line from
the cathode to the anode as shown in Figure 10. But the
addition of a magnetic field, provided by permanent
magnets surrounding the anode, causes the electrons to take
an orbital path between the cathode and anode, Figure 11.
As the electrons approach the anode, they travel past the
small resonant cavities that are part of the anode. Interac-
tion occurs, causing the resonant cavities to oscillate at the
very high frequency of 2450 megahertz. This RF energy is
radiated from the magnetron antenna into the waveguide
into the cooking cavity feedbox, and finally into the
cooking cavity where food has been placed to be cooked.
WITH
MAGNETIC
. FIELD
ANODE--/
Figure 11. Basic Magnetron with Magnetic Field