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Microwave generator is the heart of microwave oven. The quality of a microwave oven mostly depends on the
quality of the microwave generator. A microwave generator is mainly composed of magnetron and its power
supply circuit, FIG.2-3 is the typical circuit diagram of the present used microwave oven's generator. The power
supply circuit is composed of rectifying circuit and filament circuit.
Usually, we adopt continuous wave magnetron. It can turn the direct energy which is applied to the magnetron
after being high voltage rectified to microwave energy, the power supply circuit supply a direct high voltage
between the cathode and anode of the magnetron, a filament voltage to the cathode filament of the magnetron.
The working process of the magnetron: When the anode volt-age gradually rises from zero, the anode current is
approx.zero, the power is very small as well. When the anode voltage rises to "THRESHOLD" voltage value, the
magnetron starts oscillating, and the anode current would increase obviously, provided the anode voltage rises a
little more, the anode current would increase a lot (FIG.2-4), and would reach the rate value quickly.
If the anode voltage is undulating, it would cause the anode
current swing fiercely, even made the magnetron stop oscillating.
For keeping a steady output, the power supply circuit of the
oven must supply a stead direct current voltage. The filament
voltage of the magnetron must be supplied by an alternating
current voltage. For simplifying the circuit, it would be supplied
by the same leakage magnetic transformer with the anode
power (high voltage power). The filament of the magnetron
which the present used microwave ovens are all treated through
some special technology, and all have the cold start character.
But when in cold start, there still is a very strong surge electric
field attached to the surface of the anode, and would be harmful
to the anode. In order to reduce the surge voltage, the filament
of the magnetron must be connected as the FIG.2-3 shown. In
this figure, when the anode current of the magnetron circulates,
the filament current should flow to the FA end from the lower
end.
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In the working process of the microwave oven, the magnetron often makes the anode temperature rise due to
anode loss
caused by the electronics strike the oven surface and the heat radiate of the cathode. To prevent the anode
temperature rise too high, thus affect the working steady and its life span, it is necessary to cool the magnetron.
According to the different models and rate output of the magnetron, compelling wind cooling and flowing
water-cooling can be adopted. Usually, the home used microwave oven adopts the compelling wind cooling
method, and all are fixed with cooling fin.
Cooling system includes fan motor, air duct, air entrance, air vent etc. The flowing direction of the cooling wind
should. Be parallel to the cooling fin of the magnetron. Generally, we adopt the method of air blast rather than air
absorb. And all the cabinet of the oven is with air entrance and air vent, the hot wind blowing through the
magnetron is guided with air tube to improve the cooling effect. In the technical parameter chart of the magnetron,
it usually will give out the requirement of cooling wind. A shortage of cooling wind would damage the character of
the magnetron, even burn out the magnetron. The amount that the fan blasted should not be less than the
requirement. When fix the fan, attention must be paid to prevent the cool wind from blowing directly to the glass
part of the magnetron to avoid blasting.
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To those mechanical control microwave oven, electric control system mainly composes of interlock switch, timer,
power distributor and thermal cutout, etc. The electric control system of those tough control microwave oven is
mainly composed of interlock switch, computer controller and thermal cutout, etc.
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anode current
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(mA)
b
Fig.2-4
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