4
anode current
I
b
(mA)
a
n
o
d
e
v
ol
t
a
g
e
e
b
(
k
V
)
m
Fig.2-4
1
1
.
.
2
2
M
M
I
I
C
C
R
R
O
O
W
W
A
A
V
V
E
E
G
G
E
E
N
N
E
E
R
R
A
A
T
T
E
E
R
R
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 voltage 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 has
been treated with special techniques and has the ability to be
"cold booted". But when started in cold, 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.
1
1
.
.
3
3
C
C
O
O
O
O
L
L
I
I
N
N
G
G
S
S
Y
Y
S
S
T
T
E
E
M
M
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.
1
1
.
.
4
4
E
E
L
L
E
E
C
C
T
T
R
R
I
I
C
C
C
C
O
O
N
N
T
T
R
R
O
O
L
L
S
S
Y
Y
S
S
T
T
E
E
M
M
In mechanical control microwave ovens, electric control systems is mainly composed 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.
