Power control device for the magnetron of microwave oven

A power control device for a microwave cooking oven includes a regulator of the anode voltage of the magnetron and a heater voltage source. The anode voltage regulator includes a d.c. voltage bridge-type converter, a transformer, a rectifier and a filter. The heater voltage source includes a half-bridge d.c. to a.c. voltage converter. The half-bridge and bridge converters are controlled each by signals coming from the respective generators of pulse sequences.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to microwave apparatus operated for heating 
dielectric materials, and more particularly it relates to a power control 
device for the mgnetron of a microwave cooking oven. 
The invention can be utilized in various microwave apparatus for domestic 
and public uses, and also in low-power industrial microwave installations 
of the kind operated for evaporation of fluid mixtures, heat treatment of 
dielectric materials, control of a predetermined moisture content of a 
medium, and the like. 
DESCRIPTION OF THE PRIOR ART 
Among the most essential requirements put before contemporary household 
appliances are their high performance reliability in combinationwith 
minimized weight and dimensions, which requirements also apply to a device 
for controlling the power of a microwave cooking oven that is expected to 
be compact and light-weight and produce no electromagnetic interference 
transmittable via power supply circuits to other household electronic 
apparatus. 
There is known a device for controlling the power of the magnetron of a 
microwave cooking oven (Collection--Elektronnaya tekhnika. Seriya 
Elektronika SVCh. Series, 5, 1984, Moscow, P. V. Batsev, "Sistema 
avtomaticheskogo upravleniya sovremennykh promyshlennykh ustanovok 
SVCh-nagreva. P.II Analiz rezhimov raboty dvukhchastotnogo magnetrona v 
sovremennykh istochnikakh SVCh energii dlya promylshlennogo nagreva", pp. 
50-54), comprising a regulator of the anode voltge of the magnetron, 
including a current sensor and a control unit, an a heater voltage souce 
for the magnetron, the magnetron anode voltage regulator being in the form 
of a thyristor regulator with a serially coupled transformer having a 
choke connected in its output circuits. 
This known device is relatively bulky and of a considerable weight, whereas 
its performance reliability is inadequate. 
These shortcomings arise from the conversion and control of the power for 
the anode circuit of the magnetron being carried out in the known device 
at a low frequency, i.e. at the mains frequency of 50 or 60 Hz, which 
results in an increased weight and size of the transformers and filters of 
the device. With the thyristor regulator associated with an electronic 
control unit having serially connected thereto a power transformer, a 
powerful electromagnet and a choke included in the anode circuit of the 
magnetron, this represents for the thyristors of the regulator a markedly 
inductive load, which affects the performance reliability of the 
thyristors. The pulsed control of the thyristors creates an intermittent 
(pulsed) current causing electromagnetic interference transmittable via 
the electric power supply circuits and mains to other electronic 
apparatus. 
There is further known a device for controlling the power of the magnetron 
of a microwave cooking oven (Collection--Elecktronnaya tekhnika. Seriya 
Elektronika SVCh. Series 4, 1981, Moscow, M. N. Molokhov, I. D. Maslakov, 
"Reguliruemyi stabilizator vykhodnoi moshchnosti magnetrona", pp. 56-58), 
comprising a regulator of the anode voltage of the magnetron, connected by 
its output to the anode circuit of the magnetron, and a source of the 
heater voltage of the magnetron, having its output connected to the 
heater-cathode filament of the magnetron. The anode voltage regulator in 
this device includes a thyristor with its control circuit and a 
transformer of which the secondary winding is connected to the anode 
circuit of the magnetron. The thyristor and the primary winding of the 
transformer are connected in series and adapted to be connected to the 
power mains. 
The heater voltage source is in the form of an individual transformer 
having its primary winding connected to the power mains and its secondary 
winding connected to the heater-cathode filament of the magnetron. The 
control circuit produces signals governing the duration of the energized 
state of the thyristor, i.e. of the voltage supply to the primary winding 
of the transformer. In this way there is varied the duration of the 
microwave energy pulses generated by the magnetron in each half-cycle of 
the supply voltage, while the mean power of the microwave energy is 
maintained at a preset level. 
Unlike the first-described known device, this device is devoid of the choke 
affecting the performance reliability of the thyristors. 
However, the last-described known device likewise carries out the 
conversion and control of the power for the anode circuit of the magnetron 
directly at the mains frequency, i.e., the switching of the thyristors and 
the operation of the transformer take place at the mains frequency, which 
results in an increased weight and dimensions of the transformer and 
filters and also in increased power consumption due to a significant 
no-load current value, thus brining down the efficiency factor. The 
magnetron in this known device operates in a mode that cannot be 
considered optimal, as the voltage fed to its anode is shalped as 
half-sine waves impairing the efficiency of utilization of the full power 
capacity of the magnetron. This situation is due to the fact that, first 
and foremost, the frequency spectrum generated by the magnetron is 
extended, whereas the anode voltage values yielding the maximum power 
output of the magnetron would be maintained within but a brief interval of 
the supply voltage cycle. With no stabilization of the heater voltage of 
the magnetron, the service life of its heater-cathode is curtailed, as 
fluctuations of the input voltage would result in variations of the 
heater-cathode temperature in magnetron, affecting its optimized 
utilization in the magnetron and eventually causing untimely failures on 
account of either overheating or underheating. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to reduce the dimensions and 
weight of a device for controlling the power of the magnetron of a 
microwave oven. 
It is another object of the present invention to enhance the performance 
reliability of this device. 
It is still another object of the present invention to bring down the level 
of electromagnetic interference produced by a device for controlling the 
power of the magnetron of a microwave oven, transmittable via power supply 
circuits. 
It is yet another object of the present invention to step up the efficiency 
factor of the device. 
These and other objects are attained in a device for controlling the power 
of the magnetron of a microwave oven, comprising a regulator of the anode 
voltage of the magnetron, having its output connected to the anode circuit 
thereof, and a source of the heater voltage of the magnetron, having its 
output connected to the heater-cathode filament. In accordance with the 
invention, the anode voltage regulator comprises a series connection of a 
d.c. voltage to a.c. voltage bridge-type converter of which the input is 
the power input of the anode voltage regulator, a transformer, a rectifier 
and a filter of which the output is the first output of the anode voltage 
regulator intended for connection to the anode circuit of the magnetron, 
the anode voltage regulator further including a diode having its input 
connected to the power input of the anode voltage regulator and its output 
serving as the second output of the anode voltage regulator, the device 
further comprising a series connection of a d.c. voltage source, a current 
amplitude shaper having its control input connected to the output of the 
filter an a saturable magnetic element having its output connected to the 
primary winding of the transformer; the device still further comprising a 
generator of sequences of control pulses for the anode voltage regulator, 
having its input connected to the output of the master control of the 
microwave cooking oven operating mode and its output connected to the 
control inputs of the bridge-type converter; the heater voltage source 
including a series connection of a half-bridge d.c. to a.c. voltage 
converter having its power input connected to the second output of the 
anode voltage regulator and another transformer having at least two 
secondary windings, the terminals of its one secondary winding being the 
output of the heater voltage source intended for connection to the 
heater-cathode filament of the magnetron; the device still further 
comprising a generator of sequences of pulses for heater voltage 
stabilization, having its input connected to another secondary winding of 
the transformer of the heater voltage source and having its outputs 
connected to the control inputs of the half-bridge converter; the device 
also comprising a series connection of a mains voltage rectifier of which 
the input serves as the power input of the device, a starting current 
limiter, a capacitance filter and a choke having its output connected to 
the power input of the anode voltage regulator. 
It is expedient that in the device the magnetic saturable element should 
comprise two coaxially arranged toroidal cores, a control winding having 
its turns encompassing both toroidal cores and a working winding made of 
two oppositely connected half-windings of which one is carried by one of 
the toroidal cores and the other one is carried by the other toroidal 
core. 
Alternatively, it may be expedient for the magnetic saturable element to 
include a toroidal core defining an annular cavity coaxial therewith, and 
the control and working windings of which the control winding is 
accommodated in the annular cavity coaxially therewith and the working 
winding is wound externally onto the toroidal core so that the planes 
accommodating its turns are orthogonal with respect to the planes 
accommodating the turns of the control winding. 
It is reasonable for the current amplitude shaper of the device to include 
a power transistor of which the input and output are, respectively, the 
input and output of the current amplitude shaper, and a series connection 
of a comparison circuit of which one input is the control input of the 
current amplitude shaper and the other input is connected with the output 
of a reference voltage source, and a d.c. voltage amplifier having its 
output connected with the control input of the power transistor. 
It is expedient that the generator of sequences of control pulses in the 
device should include a series connection of a clock pulse generator, an 
AND gate, a first pulse counter, a second pulse counter another AND gate, 
a self-excited oscillator with synchronizing inputs of which the output is 
the output of the generator of control pulse sequences, and still another 
AND gate having its output connected with the other inputs of the 
first-mentioned AND gates and its input serving as the control input of 
the generator of control pulse sequences, and one more AND gate having its 
inputs connected, respectively, to the outputs of the second pulse counter 
and of the last-mentioned AND gate, and its output connected to the input 
of the self-excited oscillator. 
The device for controlling the power of the magnetron of a microwave oven, 
constructed in accordance with the present invention, provides for 
stabilization of the anode voltage of the magnetron at a present permanent 
level, which allows to utilize the power capacity of the magnetron to a 
higher efficiency, to step up the factor of utilization of the anode 
voltage and, hence, to enhance the efficiency factor of the device, as a 
whole. 
A device constructed in accordance with the present invention is adapted 
for operation at a high frequency, which has allowed to reduce its overall 
dimensions and weight, to bring down the level of electromagnetic 
interference produced by the device and transmittable via power supply 
circuits to other electronic apparatus, and to enhance the performance 
reliability of the device. 
The stability of the heater voltage of the magnetron throughout a broad 
range of eventual destabilizing factors, ensured by the device in 
accordance with the invention, steps up still further the performance 
reliability of the magnetron of a microwaave cooking oven.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The device for controlling the power of the magnetron of a microwave 
cooking oven schematically illustrated in FIG. 1 comprises an anode 
voltage regulator 1 including a series connection of a bridge-type d.c. 
voltage to a.c. voltage converter 2 of which the input is the power input 
3 of the anode voltage regulator 1, a transformer 4 having its secondary 
winding 5 connected to the input of a rectifier 6, and a filter 7 of which 
the output is the first output 8 of the anode voltage regulator 1, 
connected to the anode circuit 9 of the magnetron 10. The anode voltage 
regulator 1 further includes a d.c. source 11 having connecting to its 
output the power input 12 of a current amplitude shaper 13 of which the 
control input 14 is connected to the anode circuit 9 of the magnetron 10, 
while the output of the current amplitude shaper 13 is connected to the 
input 15 of a saturable magnetic element 16 connected in parallel with the 
primary winding 17 of the transformer 4. Furthermore, the anode voltage 
regulator 1 includes a generator 18 of sequences of control pulses for the 
operation of this regulator 1, having its outputs connected to the control 
inputs 19 of the bridge-type d.c. voltage to a.c. voltage converter and 
its input connected to the master control 20 operable to select the 
operating modes of the microwave cooking oven, and a diode 21 of which the 
input is connected to the power input 3 of the bridge-type converter 2, 
while the output of the diode 21 is the second output 22 of the anode 
voltage regulator 1, connected to the input of a heater voltage source 23. 
The heater voltage source 23 includes a series connection of a generator 
24 of pulse sequences for heater voltage stabilization, a half-bridge d.c. 
voltage to a.c. voltage converter 25 and a transformer 26 having a primary 
winding 27 and secondary windings 28, 29, 30. The generator 24 of 
stabilizing pulse sequences has its input connected to the terminals of 
the secondary winding 28 of the transformer 26, and its outputs connected 
to the control inputs 31 of the half-bridge converter 25. The terminals of 
the secondary winding 29 of the transformer 26 are the outputs of the 
heater voltage source 23, connected to the heater-cathode filament 32 of 
the magnetron 10. The terminals of the secondary winding 30 are connected 
to the inputs of an auxiliary rectifier 33 of which the output is 
connected with the control input 34 of a starting current limiter 35. The 
herein disclosed device for controlling the power of the magnetron of a 
microwave cooking oven further comprises a series connection of a mains 
voltage rectifier 36 of which the input is the power input of the power 
control device for the magnetron of a microwave cooking oven, connectable 
to power mains and the output is connected with the power input 37 of the 
starting current limiter 35, a capacitance filter 38 and a choke 39 having 
its output connected to the power input 3 of the magnetron anode voltage 
regulator 1. 
FIG. 2 presents the circuit diagram of a version of the bridge-type d.c. to 
a.c. voltage converter 2. The converter 2 of this version comprises four 
controlled transistor gates 40, 41, 42, 43 wired into the respective four 
arms of the bridge circuit of which one diagonally opposite pair of 
junctions is connected to the power input 3 of the anode voltage 
regulator, and the other diagonally opposite pair of junctions has 
connected between them the primary winding 17 of the transformer 4. The 
control inputs of the transistor gates 40-43 are the respective control 
inputs 19 of the bridge-type converter 2. 
To minimize dynamic losses at the transistor gates 40-43, each gate is 
connected in parallel with a circuit 44 including a diode 45 connected 
across a parallel connection of a resistor 46 and capacitor 47. 
FIG. 3 presents the circuit diagram of a version of the half-bridge d.c. to 
a.c. voltage converter 25. the converter 25 includes a bridge circuit with 
controlled transistor gates 48, 49 wired into the respective arms of its 
one adjacent pair of arms, while capacitors 50, 51 are wired into the 
respective arms of the other pair. One pair of diagonally opposite 
junctions of the bridge circuit serves as the power input of the 
half-bridge converter 25, and the other pair of diagonally opposite 
junctions of the bridge circuit has connected between them the primary 
winding 27 of the transformer 26. The control inputs of the transistor 
gates 48, 49 are connected to the control inputs of the half-bridge 
converter 25. To minimize dynamic losses at the transistor gates 48, 49, 
they are connected in parallel with their own circuits 44 of the 
abovedescribed type. 
A version of the saturable magnetic element 16 illustrated in FIG. 4 has a 
pair of coaxial toroidal cores 52, 53, a control winding 54 whose turns 
encompass both cores 52 and 53 and whose ends are connected to the input 
terminals 15 of the saturable magnetic element 16, and a working winding 
55 including two oppositely connected half-windings 55' and 55" 
accommodated, respectively, on the cores 52 and 53, the terminal ends of 
the winding 55 being connected to the output terminals of the saturable 
magnetic element 16. 
Another version of the saturable magnetic element 16 illustrated in FIG. 5 
has a single toroidal core 56 defining a coaxial annular cavity 57 
accommodating a control winding 58 coaxial with the cavity 57, and a 
working winding 59 wound onto the toroidal core 56. The windings 58 and 59 
are so relatively oriented that planes accommodating the respective turns 
of the working winding 59 are arthogonal with respect to the planes 
accommodating the turns of the control winding 58. The terminal ends of 
the control winding 58 are connected to the input terminals 15 of the 
saturable magnetic element 16, and the terminal ends of the working 
winding 59 are the respecetive output terminals of the element 16. The 
toroidal core 56 is split at half its height to facilitate the assembling 
of the saturable magnetic element 16. 
The mains voltage rectifier 36 (its circuit diagram not shown) can be of 
any suitable known structure, e.g. a bridge circuit with diodes included 
in its arms and a pair of diagonally opposite junctions connectable to the 
mains supply. 
The capacitance filter 38 (its circuit diagram is not shown, either) can 
likewise be of any suitable known structure, e.g. as a set of capacitors 
connected in parallel. 
FIG. 6 presents the circuit diagram of the starting current limiter 35 
including a parallel connection of a resistor 60 and a controlled gate 61, 
the control input of the gate 61 being connected to the control input 34 
of the limiter 35. The power input 37 of the starting current limiter 35 
is connected to the junction 62 of one terminal of the resistor 60 and the 
first power electrode of the semiconductor gate 61, the junction 63 of the 
other terminal of the resistor 60 and the other power electrode of the 
semiconductor gate 61 is the output of the starting current limiter 35. 
FIG. 7 is an example of the circuitry (in a block-unit diagram) of the 
generator 18 of sequences of control pulses for the anode voltage 
regulator of the magnetron, including a series connection of a clock pulse 
generator 64, an AND gate 65, a first pulse counter 66, a second pulse 
counter 67, another AND gate 68 and a self-excited oscillator 69 with 
synchronizing inputs 70, 71. Furthermore, the generator 18 of control 
pulse sequences includes a third AND gate 72 having its output connected 
to the other inputs of the AND gates 65 and 68 and its input serving as 
the control input of the generator 18 of control pulse sequences, and a 
fourth AND gate 73 having its inputs connected, respectively, to the 
outputs of the counter 67 and of the AND gate 72 and its output connected 
to the other input 71 of the self-excited oscillator 69 whose outputs are 
the outputs of the generator 18 of control pulse sequences. 
FIG. 8 is an example of the circuitry (also in a block-unit diagram) of the 
generator 24 of heater voltage stabilizing pulse sequences, including a 
series connection of a generator 74 of clock pulses, an AND gate 75, a 
first pulse counter 76, a second pulse counter 77, an AND gate 78, and a 
self-excited oscillator 79 with synchronizing inputs 80, 81, of which the 
outputs are the outputs of the generator 24 of pulse sequences. The 
generator 24 of the pulse sequences further includes a third AND gate 82 
having its output connected to the other inputs of the AND gates 75 and 78 
and its input connected to the secondary winding 28 (FIG. 1) of the 
transformer 26, a fourth AND gate 83 (FIG. 8) having its inputs connected, 
respectively, to the outputs of the second pulse counter 77 and of the AND 
gate 82 and its output connected to the other input 81 of the self-excited 
oscillator 79. 
FIG. 9 is an example of the circuitry of the current amplitude shaper 13 
including a power transistor 84 of which the input and output are, 
respectively, the input 12 and output of the current amplitude shaper 13, 
and a series connection of a comparison circuit 85 of which one input is 
the control input 14 of the current amplitude shaper 13 and the other 
input is connected to the output of a reference voltage source 86, and a 
d.c. amplifier 87 having its output connected to the control input of the 
power transistor 84. 
The power control device for the magnetron of a microwave cooking oven 
embodying the invention operates as follows. 
With the device connected to the domestic a.c. power supply mains, the 
mains voltage rectifier 36 (FIG. 1) rectifies this voltage (the chart of 
time-related variation of this output voltage of the rectifier 36 is shown 
in FIG. 10a). Then this voltage is filtered by the capacitance filter 38, 
the latter being selected to have adequately high capacity so that the 
level of pulsation U.sub.c should not exceed 2-5% of the amplitude 
(rectified) E.sub.m of the voltage (FIG. 10b). As the total capacitance of 
the capacitors making up the filter 38 is considerable, to protect the 
household against excessive leaps of the elecrric current in the mains 
which could cause fires and malfunctioning of other electric and 
electronic household appliances (e.g. frame scan failures in television 
receivers) the starting current limiter 35 is included between the 
rectifier 36 and the capacitance filter 38. At the initial period, the 
capacitors of the capacitance filter 38 are charged through the resistor 
60 (FIG. 6), and then (generally, after 1 second), the controlled gate 61 
shunts the resistor 60. 
The d.c. voltage is fed from the capacitance filter 38 (FIG. 1) via the 
choke 39 to the input 3 of the anode voltage regulator. The regulator 1 
incorporates the bridge-type voltage converter 2 whose control inputs 19 
are fed with square-tooth voltage of the meander type (FIG. 10c). 
The transistor gates 40 (FIG. 2), 41, 42, 43 of the bridge-type 
transtorized voltage converter 2 are alternatingly in two quasi-stable 
states. The time of the stable state equals half the cycle of the control 
voltage coming to the inputs 19 of the converter 2 from the generator 18 
(FIG. 1) of sequences of control pulses. Connected in parallell with the 
primary winding 17 of the transformer 4 of the bridge-type voltage 
converter 2 is the working winding 55 (FIG. 4) or 59 (FIG. 5) of the 
saturable magnetic element 16. 
Let us consider the operation of the herein disclosed device with the 
version of the saturable magnetic element 16 illustrated in FIG. 5. The 
current flowing through the current amplitude shaper 13 (FIG. 1) and the 
control winding 58 (FIG. 5) of the saturable magnetic element 16 induces 
in the core 56 the transverse component B.sub.x of magnetic induction: 
EQU B.sub.x =.mu..mu..sub.o TW.sub.1 /l.sub.o, 
where 
B.sub.x is the component of magnetic induction saturating the core 56 in 
the direction of the X-axis; 
.mu. is magnetic permeability of the material of the core 56; 
.mu..sub.o is magnetic permeability of vacuum; 
T is the current flowing through the current amplitude shaper 13; 
W.sub.1 is the number of turns of the control winding 58; 
l.sub.o is the mean length of a magnetic line of force in the core 56. 
The longitudinal component B.sub.y of magnetic induction produced by the 
current flowing through the working winding 59 of the saturable magnetic 
element 16 is defined by the ratio: 
##EQU1## 
where 
E.sub.m is the amplitude of voltage at the output of the rectifier 36; 
f is the frequency of control pulses at the inputs 19 of the converter 2; 
Q is the cross-sectional area of the magnetic material of the core 56 in a 
plane including the Y-axis; 
W.sub.2 is the number of turns of the working winding 59. 
The total induction B in the core 56 of the saturable magnetic element 16 
is defined as: 
##EQU2## 
where B.sub.s is the saturation induction of the material of the core 56 
of the saturable magnetic element 16. 
With no current flowing through the current amplitude shaper 13, the value 
of induction B in the core 56 would vary within a range: 
EQU +B.sub.s &gt;B&gt;-B.sub.s 
With current starting to flow in the control winding 58 of the saturable 
magnetic element 16, within a part of a half-cycle of the control voltage 
coming to the inputs 19 the induction B in the core 56 would be permanent 
and would equal B.sub.s, i.e. the saturable magnetic element 16 would 
acquire the state of saturation (see FIG. 10d). The time of the element 16 
being in the state of saturation is t.sub.1, and the time during which the 
induction B in the element 16 varies linearly is t.sub.2. During the time 
t.sub.1 the output of the bridge-type voltage converter 2 is shunted by 
the low output impedance of the element 16. The current flowing through 
the transistor gates 40-43 (FIG. 2) does not grow, as the choke 39 is 
included in the power supply circuit of the bridge-type converter 2. 
During the time t.sub.1 the voltage applied across the choke 39 equals 
E.sub.m. Following the switching of the transistor gates 40-43, the 
magnetic saturable element 16 leaves the state of saturation, but the 
choke 39 inverts the voltage at its winding, and this voltage is added to 
the voltage across the capacitance filter 38 and fed to the input 3 of the 
bridge-type voltage converter 2 (FIG. 10). The amplitude E.sub.m of the 
voltage across the choke 39 is defined by an expression: 
EQU E.sub.m t.sub.1 -E.sub.L t.sub.2 =0. 
EQU E.sub.L =E.sub.m t.sub.1 /t.sub.2 
The voltage applied to the input 3 of the bridge-type voltage converter 2 
equals: 
EQU E=E.sub.L +E.sub.m with O.ltoreq.t.ltoreq.t.sub.2, 
EQU or E=O with t.sub.2 .ltoreq.t.ltoreq.t.sub.1. 
The shape of the varying voltage across the coke 39 can be seen in FIG. 
10e. 
With the inductance L of the choke 39 properly selected, the current 
i.sub.L flowing through the choke 39 would be practically constant (FIG. 
10f), which is attained by selected the value of L from an inequality: 
##EQU3## 
i.e. q is the full time to saturation time ratio of the saturable magnetic 
element 16; and 
I.sub.min is the minimum current at the input 3 of the voltage converter 2. 
The secondary winding 5 of the transformer 4 shapes pulses of alternating 
polarity with intervening pauses (see FIG. 10g), to be subsequently 
rectified into unipolar pulses (see FIG. 10h) by the rectifier 6 and 
filtered out by the capacitance filter 7 (FIG. 10k). The capacitors of the 
capacitance filter 7 are charged to a value equalling: 
EQU (E.sub.m +E.sub.L)n=U.sub.a, 
where 
U.sub.a is the voltage across the output of the filter 7, 
n is the transformation ratio of the transformer 4. 
The capacitance of the capacitor of the filter 7 is selected for pulsation 
of voltage at the output of the filter 7 to equal zero. This permanent 
voltage is fed to the anode circuit 9 of the magnetron 10 and to the 
control input 14 of the current amplitude shaper 13. A part of the output 
voltage U.sub.a is compared with the standard or reference voltage from 
the source 86 (FIG. 9) in the comparison circuit 85, and a mismatch signal 
is fed to the power transistor 84 controlling the current through the 
control winding 58 (FIG. 5) of the saturable magnetic element 16. 
Considering that: 
##EQU4## 
with the value E.sub.m varying, the value of t.sub.1 is varied so as to 
maintain the permanence of U.sub.a. 
In case of voltage fluctuations in the supply mains, e.g. when the voltage 
at the input of the mains rectifier 36 (FIG. 1) drops, the current flowing 
through the control winding 58 (FIG. 5) is increased, which means that the 
core 56 of the saturable magnetic element 16 remains longer in the state 
of saturation (FIG. 10e). The saturable magnetic element 16 with two 
toroidal cores, illustrated in FIG. 4, operates similarly, the only 
difference being that the vector B of induction produced by the current 
flowing through the control winding 54 and by the currents flowing through 
the half-windings 55' and 55" of the working winding 55 is one and the 
same plane. 
With the amplitude E.sub.m (1+t.sub.1 /t.sub.2) of the voltage at the input 
3 of the bridge-type converter 2 being stabilized, the voltage across the 
capacitors 50, 51 (FIG. 3) of the half-bridge voltage converter 25 would 
likewise be stabilized. The transistor gates 48, 49 of the half-bridge 
voltage converter 25 are controlled by the generator 24 (FIG. 1) of pulse 
sequences, the input of the generator 24 being connected to the winding 28 
of the transformer 26 so as to maintain a stable acting value of the 
heater voltage. The microwave power output of the oven is controlled by 
varying the on off time ratio of the magnetron 10. This is obtained by the 
input of the generator 18 of control pulse sequences for the anode voltage 
regulator 1 being connected to the output of the oven operating mode 
master control 20 which predetermines the periodicity of the feed of 
control pulse sequences to the inputs 19, offering the following choice of 
operating modes: 
"warm", "low heat", "defreezing", "stewing", "boiling", "baking", "frying", 
"warming-up", "fast warming". 
With no control pulses fed to the inputs 19, the transistor gates 40-43 of 
the bridge-type voltage converter 2 become non-conductive, which causes a 
discharge of the capacitor of the filter 7 and termination of the anode 
voltage and current of the magnetron 10 (FIG. 10l, m). 
In case of a failure of the heater voltage source 23 (FIG. 1) or elase of a 
failure or burning-out of the heater-cathode filament circuit (which is 
the major type of magnetron failures), the voltage at the output of the 
auxiliary rectifier 33 would cease, which results in the controlled gate 
61 (FIG. 6) becoming non-conductive in the next successive half-cycle of 
the a.c. voltage fed to the input of the current limiter 35 (owing to the 
current flowing through the controlled gate 61 falling to zero). Thus, 
with the controlled gate 61 rendered non-conductive, the current flows 
through the resistor 60, which limits the overload current and the 
increase of the voltage across the anode-cathode gap of the magnetron 10 
in the no-load mode. This protection prevents failures associated with 
electrical breakdowns caused by elevated voltages in the anode circuit. 
Otherwise such failures could have harmed irreversibly the transformer, 
diodes,capacitors and the entire structure of the device. 
Let us consider in some more detail the operation of the generator 18 (FIG. 
1) of pulse sequences. The generator 64 of clock pulses (preferably, a 
quartz-type oscillator with a 10-20 MHz frequency) is connected to the 
pulse counters 66, 67 through the AND gate 65, while the outputs of the 
pulse counters 66, 67 are likewise connected through the AND gates 68, 73 
to the synchronizing inputs 70, 71 of the self-excited oscillator 69. The 
AND gate 72 forms a signal at its output when both its inputs receive the 
respective signals from the oven master control and from the heater 
voltage source when the heater-cathode of the magnetron is fully warmed 
up. The outputsignal of the AND gate 72 comes to the other inputs of the 
AND gates 65, 68 and 73, opening these gates and letting clock pulses from 
the clock pulse generator 64 pass to the conters 66, 67 and then to the 
synchronizing inputs 70, 71 of the self-excited oscillator 69, is which 
way the generating of a control pulse sequence is started. With no signal 
coming to either input of the AND gate 72, the counters 66, 67 become 
disconnected from the clock pulse generator 64 and from the synchronizing 
inputs of the self-excited oscillator 69 by the AND gates 65, 68, 73, 
respectively. The pulse counters 66, 67 store their attained state. With 
signals coming once again to both inputs of the AND gate 72, the pulse 
counters 66, 67 resume the counting, and a new sequence is formed as a 
continuation of the preceding sequence of control pulses (FIG. 11). This 
structure of the generator 18 of control pulse sequences provides for 
maintaining equality of the areas of the positive (S.sub.+) and negative 
(S.sub.-) half-cycles, and for avoiding a current rise in one of the 
initial half-cycles of an acting sequence of pulses, thus preventing 
voltage leaps at the choke 39 and unsteady performance of the anode 
voltage regulator 1 and of the heater voltage source 23.