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4-1. Construction of microwave oven
Cavity
Cabinet locking
in microwave
and holding
food.
Door
Gate coming in
and out food. Magnetron
It has window Inject microwave
and seal to oven.
structure
OVEN CAVITY PARTS
Controller INTERIOR PARTS
Control and
supervise BASE PLATE PARTS
DOOR PARTS
operating state
of oven.
LATCH BOARD PARTS
CONTROL PANEL PARTS
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4-2. How does the microwave oven operate?
After plugging your oven into a standard household socket, press start button or turn the timer. And then switches
(primary, secondary, timer) close the circuit.
Eventually the input power will be applied to turntable motor, timer motor, fan motor, oven lamp and high voltage transformer.
A high voltage of 2,100 volts AC is generated in the second winging of H.V.Transformer. And the voltage is doubled by
H.V.Capacitor & H.V.Diode. The 4Kv DC voltage is applied to filament of magnetron, then magnetron start to produce microwave.
This microwave is injected to cavity.
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4-3. WARNING - High voltage circuit ( 4,000 volt )
* The half-wave voltage doubler circuit consists of the secondary winding of
the high voltage transformer, H.V. diode (rectifier), and H.V. capacitor. The
INPUT + - H.V. diode allows alternating current (AC) to flow in one direction only and
+ rectifies it to pulsating direct current (DC).
The H.V. capacitor is able to store energy on one half of the AC cycle and
release it on the other half cycle.
SENONDARY
WINDING AT * During the first half cycle of operation, the secondary winding of the
2000V
transformer supplies 2000 Volts to the capacitor current flows through the
MAGNETRON
diode and returns to the transformer for a complete circuit. This half-cycle
- of AC charges the capacitor to approximately 2000 Volts.
+ -
INPUT
* During the second half cycle of operation, the current flows in the opposite
-
direction, again supplies 2000 Volts to the circuit.
* This permits the capacitor to discharge its 2000 Volts on top of the 2000
SECONDARY Volts generated by the secondary winding, creating an approximate total
WINDING AT
2000V voltage of negative 4000 Volts D.C.
MAGNETRON
The negative 4000 Volts DC causes the magnetron to conduct current
+ and, to oscillate at 2450 MHz.
The first half cycle and the second half cycle become one complete cycle,
repeated input power frequency times per second .
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4-4. Microwave generation system_Magnetron (1/2)
GASKET ANTENNA (OUTPUT)
FILAMENT (CATHODE) MAGNET (MAGNETIC CIRCUIT)
ANODE
MAGNET (FERRITE)
VANE
FILAMENT TERMINALS
FEED THROUGH CAPACITOR
FILTER CASE
FILTER
* The magnetron is the energy source for the microwave oven.
ANTENNA The magnetron is a vacuum tube of special construction.
ELECTRONS CATHODE It is basically a diode with addition of a magnetic field.
It consists of a small, coiled heating element (filament) made of tungsten
which readily emits electrons when heated.
This element serves as the cathode (negative element) within the tube.
The anode (positive element of the tube) consists of a thick walled copper
cylinder with vertical vanes extending inward which surround but do not
touch the cathode.
To complete the magnetron, and make it operate distinctly different from
ANODE other vacuum tubes, two permanent magnets are mounted over each end
of the tube.
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4-4. Microwave generation system_Magnetron (2/2)
* In order to create an electron flow from cathode to anode, the cathode must
be heated and a potential difference must exist between the two.
2,450 MHz.
ANTENNA This is accomplished by heating the cathode with 3, 4 to 3,5 V AC. ( from the
filament winding of the high voltage transformer) and applying a negative
4000 V DC (from the voltage doubler circuit) to the cathode.
ELECTRONS CATHODE
* Originally the electrons would travel in a straight line from the cathode to the
anode. However, with the addition of a permanent magnet surrounding the
anode creating a magnetic field, the electrons travel an orbital path between
the cathode and anode. As the electrons approach the anode, their orbital
path takes them past small resonant cavities that are part of the anode.
The passing notion of the electrons induces electron current to oscillate in the
PERMANENT
ANODE
MAGNET resonant cavities at the very high frequency or 2,450 MHz.
This RF (Radio frequency) energy is then transferred to the antenna.
TESTING MAGNETRON TUBE
* Disconnect power, remove the wrapper, and discharge the capacitor.
* Remove the two leads from the magnetron terminals.
* Connect the ohmmeter between one terminal of the magnetron and the outer case of the magnetron. If the ohmmeter reads
infinity, go to below. If the ohmmeter reads less than infinity, the magnetron is shorted.
* Connect an ohmmeter across the terminals of the magnetron. The ohmmeter should read less than one ohm f the
ohmmeter reads over one ohm or infinity, the tube is defective.
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