MICROWAVE OVEN APPARATUS FOR VARYING INTENSITY OF HEATING WAVES
United States Patent 3862390
A high frequency heating apparatus comprising a power source; a pulse signal generating circuit connected to the power source so as to generate pulse signals whose interval and width can be freely varied; a bidirectional thyristor; a high frequency oscillator connected to the bidirectional thyristor; and a triggering circuit for triggering the bidirectional thyristor upon receipt of pulse signals from the pulse signal generator so as to energize the high frequency oscillator.
US Patent References:
HIGH FREQUENCY,HIGH POWER SOURCE SOLID STATE INVERTER
DePrisco - August 1969 - 3460025
HIGH FREQUENCY POWER SUPPLY
Landis - February 1971 - 3566243
HIGH FREQUENCY,HIGH POWER SOURCE SOLID STATE INVERTER
DePrisco - August 1969 - 3460025
HIGH FREQUENCY POWER SUPPLY
Landis - February 1971 - 3566243
Application Number:
05/394679
Publication Date:
01/21/1975
Filing Date:
09/06/1973
View Patent Images:
Export Citation:
Assignee:
Tokyo Shibaura Electric Co., Ltd. (Kawasaki-shi, JA)
Primary Class:
Other Classes:
331/87
International Classes:
H05B6/68; H05B9/06
Field of Search:
219/10.55 331/86,87,113R,145 323/24,25,40 321/2,4
Primary Examiner:
Reynolds, Bruce A.
Attorney, Agent or Firm:
Flynn & Frishauf
Claims:
What is claimed is
1. A microwave oven apparatus comprising:
2. A microwave oven apparatus according to claim 1 wherein said pulse signal generator comprises an astable multivibrator.
3. A microwave oven apparatus according to claim 1 wherein said triggering circuit comprises:
4. A microwave oven apparatus according to claim 1 wherein said controlled rectifying circuit includes a bidirectional thyristor.
5. A microwave oven apparatus according to claim 1 wherein said pulse signal generator includes means for varying the pulse width of the pulses generated thereby.
6. A microwave oven apparatus comprising:
7. A microwave oven apparatus according to claim 6 wherein said pulse signal generator comprises an astable multivibrator.
8. A microwave oven apparatus according to claim 6 wherein said triggering circuit comprises:
9. A microwave oven apparatus according to claim 6 wherein said controlled rectifying circuit includes a bidirectional thyristor.
10. A microwave oven apparatus comprising:
11. A microwave oven apparatus comprising:
1. A microwave oven apparatus comprising:
2. A microwave oven apparatus according to claim 1 wherein said pulse signal generator comprises an astable multivibrator.
3. A microwave oven apparatus according to claim 1 wherein said triggering circuit comprises:
4. A microwave oven apparatus according to claim 1 wherein said controlled rectifying circuit includes a bidirectional thyristor.
5. A microwave oven apparatus according to claim 1 wherein said pulse signal generator includes means for varying the pulse width of the pulses generated thereby.
6. A microwave oven apparatus comprising:
7. A microwave oven apparatus according to claim 6 wherein said pulse signal generator comprises an astable multivibrator.
8. A microwave oven apparatus according to claim 6 wherein said triggering circuit comprises:
9. A microwave oven apparatus according to claim 6 wherein said controlled rectifying circuit includes a bidirectional thyristor.
10. A microwave oven apparatus comprising:
11. A microwave oven apparatus comprising:
Description:
BACKGROUND OF THE INVENTION
This invention relates to a high frequency heating apparatus such as an electronic oven. As is known, a high frequency apparatus such as an electronic oven thermally treats an object of heating using high frequency energy of about 2450 MHz. To date, however, there has not been provided any electronic oven whose high frequency output can be continuously controlled within broad bounds. Accordingly, it has been sometimes impossible to carry out the heat treatment of an object by electric waves of proper intensity, thus giving rise to considerable inconvenience. For example, where scrambled eggs are to be prepared, heating by too intense electric waves causes a stirred mixture of the yolks and whites to be explosively scattered, failing to attain proper cooking. Further, it is not desired to apply intense electric waves in melting frozen foodstuffs.
SUMMARY OF THE INVENTION
It is accordingly the object of this invention to provide a high frequency heating apparatus whose high frequency output can be controlled continuously or stepwise over a broad range.
According to an aspect of this invention, there is provided a high frequency heating apparatus which comprises a pulse signal generator for generating pulse signals whose interval or width can be freely varied; a controlled rectifying circuit; a high frequency oscillator connected to the controlled rectifying circuit; and a triggering circuit for triggering the controlled rectifying circuit upon receipt of pulse signals from the pulse signal generator to energize the high frequency oscillator.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of a high frequency heating apparatus embodying this invention; and
FIG. 2 is a circuit diagram of switching means used with the high frequency heating apparatus of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
The circuit of a high frequency heating apparatus of FIG. 1 embodying this invention comprises an AC source 1; a switching circuit 5 connected to the power source through the contacts 4b and 4c of an electromagnetic contactor 4 with a fuse 3; and a high frequency oscillator 2 such as a magnetron for supplying high frequency energy to a heating chamber (not shown). Connected between the junction of the fuse 3 and the contact 4b and the junction of the power source 1 and the contact 4c is a series circuit comprised of a power supply switch 6, cooking switch 7, the motor 9 of a timer 8, the time limit contact 10 of the timer 8 and a heating chamber door interlock switch 11. Connected in parallel with the cooking switch 7 is the normally open contact 4a of the electromagnetic contractor 4. Further, connected in parallel with the motor 9 is the exciting coil 4L of the electromagnetic contactor 4.
There will now be described by reference to FIG. 2 the switching circuit 5. This switching circuit 6 includes a rectifying circuit 24 formed of a capacitor 23 and diode 22 and connected to the input terminal I 1 of the switching circuit 5 through a resistor 21. Connected to both ends P and N of the capacitor 23 is a pulse signal oscillator 25 for generating pulse signals whose interval (i.e., the interval between successive pulses) or pulse width can be freely varied. The oscillator 25 comprises an astable multivibrator including, for example, NPN transistors 26 and 27. The emitters of the NPN transistors 26 and 27 are jointly connected to one end N of the capacitor 23. The collectors of said transistors 26 and 27 are connected to the other end P of the capacitor 23 through resistors 28 and 29. The base of the NPN transistor 26 is connected to the other end P of the capacitor 23 through a variable resistor 30 and also to the collector of the NPN transistor 27 through a capacitor 31. The base of the NPN transistor 27 is connected to the other end P of the capacitor 23 through a variable resistor 32 and also to the anode of the diode 34 through a capacitor 33. The cathode of the diode 34 is connected to the collector of the NPN transistor 26. The junction of the diode 34 and capacitor 33 is connected to the other end P of the capacitor 23 through a resistor 35.
Variations in the resistance of the variable resistor 30 give rise to changes in the width of pulse signals from the oscillator 25. Namely, where the variable resistor 30 has a resistance approaching zero, an output pulse signal has its width decreased. Conversely where the variable resistor 30 presents a maximum value, an output pulse signal has a maximum width. In either case, the interval or spacing between successively generated pulses of the output pulse signal is not affected by variations in the value of the variable resistor 30 but by those in the value of another variable resistor 32. The reference level of the output pulse signal is set at zero volts and the amplitude thereof at Vy volts.
Connected to the output terminal of the pulse signal oscillator 25 is an energizing circuit 36 for rendering the later described bidirectional thyristor 45 conducting upon receipt of an output signal from the oscillator 25. The energizing circuit 36 includes a transistor 37 whose emitter is connected to said one end N of the capacitor 23 and whose base is connected to the collector of the transistor 26 through a resistor 38 and also to said one end N of the capacitor 23 through a resistor 39. The collector of the transistor 37 is connected between the diodes D 1 and D 2 of a rectifying circuit consisting of bridge-connected diodes D 1 , D 2 , D 3 and D 4 . The junction of the diodes D 3 and D 4 is connected to said one end N of the capacitor 23. Further, a capacitor 41 is connected between the junction A of the diodes D 1 and D 3 and the junction B of the diodes D 2 and D 4 . The junction A is connected to the output terminal O 2 of the switching means of FIG. 2 through a resistor 42, and the junction B is connected to the input terminal I 2 of the switching circuit 5. Connected to both ends of the capacitor 41 is a series circuit including a triggering element, for example, a bidirectional thyristor 43 and the primary winding 44a of a pulse transformer 44. Connected to the energizing circuit 36 is a controlled rectifier 45, for example, a bidirectional thyristor. Namely, the bidirectional thyristor 45 is connected between the input terminal I 2 and the output terminal O 1 of the switching circuit 5 through the secondary winding 44b of the transformer 44. A capacitor 46 is connected in parallel with a series circuit comprised of the secondary winding of the transformer 44 and thyristor 45. The output terminal O 1 of the switching circuit 5 is connected to the input terminal I 1 thereof.
Before cooking is started, the power supply switch 6 is thrown in, an object of heating is placed in a heating chamber (not shown) and the door of a heating apparatus, for example, an electronic oven is closed to actuate a switch 11. Later when a push button 7 is depressed with the timer 8 set to operate for a prescribed length of time, the time limit contact 10 is closed and the timer 8 commences the operation to energize the electromagnetic contactor 4. This energization causes the normally open contact 4 a of the electromagnetic contactor 4 to be closed in a selfholding state and also a contacts 4b and 4c to be closed. As the result, the voltage of the power source 1 is impressed on the high frequency oscillator 2 (FIG. 2) through the switching means 5 to commence the operation of the high frequency oscillator and consequently the heating of an object. When the set time of the timer 8 passes, the time limit contact 10 of the timer 8 is opened to deenergize the electromagnetic switch 4. The high frequency oscillator stops to end thermal cooking.
There will now be described the operation of the switching circuit 5. When the voltage of the power source 1 is impressed across the input terminals I 1 and I 2 , the pulse signal oscillator 25 starts operation with a time constant defined by the resistors 30 and 32 and the capacitors 31 and 33 so as to supply the transistor 37 with pulse signals having a reference level of volts and an amplitude of Vy volts. The transistor 37 is rendered conducting when a positive voltage Vy impressed on the base-emitter circuit of said transistor 37. The capacitor 41 is bypassed by the rectifying circuit 40 and the actuated transistor 37. Since voltage is not supplied to the bidirectional thyristor 43 nor to the primary winding 44a of the transformer 44, the bidirectional thyristor 45 is not ignited, namely, remains nonconducting, thereby preventing the impression of any voltage across the output terminals O 1 and O 2 of the switching circuit 5. When the base-emitter voltage of the transistor 37 falls from Vy volts to 0 volts, the transistor 37 is turned off. As the result, a capacitor 41 is supplied with voltage and gradually charged with electric energy. When the voltage across both ends of the capacitor 41 exceeds the breakover voltage of the bidirectional thyristor 43, then said thyristor 43 becomes nonconducting, allowing the primary winding 44a of the transformer 44 to be impressed with voltage, and in consequence the secondary winding 44b of said transformer 44 to be supplied with high voltage, with the resultant actuation of the bidirectional thyristor 45. Thus a prescribed voltage is impressed across the output terminals O 1 and O 2 of the switching means. The above-mentioned operation is repeated per half the AC cycle, and the bidirectional thyristor 45 is turned on and off alternately at an exceedingly small interval (i.e., a high repetition rate) to impress high frequency voltage on the high frequency oscillator 2.
High frequency output from the oscillator 2 can be controlled by varying the ON and OFF times of the switching circuit 5. According to the foregoing embodiment, the ON and OFF times vary with the values of the variable resistors 30 and 32 of the pulse signal oscillator 25. For example, where the resistance of the resistor 30 gradually increases from zero with that of the resistor 32 fixed, then the OFF time of the switching circuit 5 is lengthened, though its ON time remains unchanged, continuously decreasing a high frequency output (expressed on average) from the high frequency oscillator 2.
An average output P from the high frequency oscillator 2 per prescribed time t may be expressed as follows:
P = Ptx . (t . tx/Tx + ty )
where;
tx = ON time of the switching circuit 5
ty = OFF time of the switching circuit 5
Ptx = an average output from the high frequency oscillator 2 per unit time when the oscillator 2 is directly connected to a power source 1
It is generally demanded that tx + ty be chosen to have a fully smaller value than t.
A timer customarily used with an electronic oven has a maximum operating time of 10 to 30 minutes. Therefore, where the ON time tx of the switching circuit 5 is set at about 2 seconds and the OFF time ty thereof is made variable within the range of about 0 to 20 seconds, then the high frequency oscillator 2 will give forth a maximum output when the OFF time ty is set at zero and produce an output equal to about one-tenth of said maximum output when the OFF time is set at 20 seconds. Where the sum of the ON time tx and OFF time ty of the switching circuit 5 is chosen to be fully small relative to a required cooking time, then an output from the high frequency oscillator 2 can be continuously controlled, saving the practical operation of a high frequency heating apparatus from any obstruction due to errors in said output.
In the heating apparatus of this invention, the switching circuit 5 is provided, as previously described, between the power source 1 and high frequency oscillator 2, permitting the free change of an output from the high frequency oscillator. This means that various objects of heating can be properly cooked by electromagnetic waves of suitable intensity. For example, scrambled eggs can be satisfactorily prepared without causing a stirred mixture of the yolks and whites to be explosively scattered. Further, where the high frequency oscillator 2 is made to emit weak electric waves by setting the value of the variable resistor 30 at a suitable level, then frozen foods can be properly melted.
There has been described this invention by reference to the aforesaid embodiment. It will be understood, however, that the invention is not limited to the illustrated embodiment. For example, the pulse signal oscillator 25 which comprised an astable multivibrator in said embodiment may be replaced by a different type of oscillator. Further, though the variable resistors 30 and 32 were used for the free control of the ON and OFF times of the switching circuit 5, yet one of these resistors may be replaced by a fixed type resistor so as to define the ON or OFF time of the switching circuit 5 at a prescribed level. The variable resistors 30 and 32 were made continuously variable, but it is possible to use the type whose resistance can be changed stepwise. Moreover, the controlled rectifying element 45 which was shown as a bidirectional thyristor may be replaced by another type formed of, for example, two silicon controlled rectifiers.
This invention relates to a high frequency heating apparatus such as an electronic oven. As is known, a high frequency apparatus such as an electronic oven thermally treats an object of heating using high frequency energy of about 2450 MHz. To date, however, there has not been provided any electronic oven whose high frequency output can be continuously controlled within broad bounds. Accordingly, it has been sometimes impossible to carry out the heat treatment of an object by electric waves of proper intensity, thus giving rise to considerable inconvenience. For example, where scrambled eggs are to be prepared, heating by too intense electric waves causes a stirred mixture of the yolks and whites to be explosively scattered, failing to attain proper cooking. Further, it is not desired to apply intense electric waves in melting frozen foodstuffs.
SUMMARY OF THE INVENTION
It is accordingly the object of this invention to provide a high frequency heating apparatus whose high frequency output can be controlled continuously or stepwise over a broad range.
According to an aspect of this invention, there is provided a high frequency heating apparatus which comprises a pulse signal generator for generating pulse signals whose interval or width can be freely varied; a controlled rectifying circuit; a high frequency oscillator connected to the controlled rectifying circuit; and a triggering circuit for triggering the controlled rectifying circuit upon receipt of pulse signals from the pulse signal generator to energize the high frequency oscillator.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of a high frequency heating apparatus embodying this invention; and
FIG. 2 is a circuit diagram of switching means used with the high frequency heating apparatus of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
The circuit of a high frequency heating apparatus of FIG. 1 embodying this invention comprises an AC source 1; a switching circuit 5 connected to the power source through the contacts 4b and 4c of an electromagnetic contactor 4 with a fuse 3; and a high frequency oscillator 2 such as a magnetron for supplying high frequency energy to a heating chamber (not shown). Connected between the junction of the fuse 3 and the contact 4b and the junction of the power source 1 and the contact 4c is a series circuit comprised of a power supply switch 6, cooking switch 7, the motor 9 of a timer 8, the time limit contact 10 of the timer 8 and a heating chamber door interlock switch 11. Connected in parallel with the cooking switch 7 is the normally open contact 4a of the electromagnetic contractor 4. Further, connected in parallel with the motor 9 is the exciting coil 4L of the electromagnetic contactor 4.
There will now be described by reference to FIG. 2 the switching circuit 5. This switching circuit 6 includes a rectifying circuit 24 formed of a capacitor 23 and diode 22 and connected to the input terminal I 1 of the switching circuit 5 through a resistor 21. Connected to both ends P and N of the capacitor 23 is a pulse signal oscillator 25 for generating pulse signals whose interval (i.e., the interval between successive pulses) or pulse width can be freely varied. The oscillator 25 comprises an astable multivibrator including, for example, NPN transistors 26 and 27. The emitters of the NPN transistors 26 and 27 are jointly connected to one end N of the capacitor 23. The collectors of said transistors 26 and 27 are connected to the other end P of the capacitor 23 through resistors 28 and 29. The base of the NPN transistor 26 is connected to the other end P of the capacitor 23 through a variable resistor 30 and also to the collector of the NPN transistor 27 through a capacitor 31. The base of the NPN transistor 27 is connected to the other end P of the capacitor 23 through a variable resistor 32 and also to the anode of the diode 34 through a capacitor 33. The cathode of the diode 34 is connected to the collector of the NPN transistor 26. The junction of the diode 34 and capacitor 33 is connected to the other end P of the capacitor 23 through a resistor 35.
Variations in the resistance of the variable resistor 30 give rise to changes in the width of pulse signals from the oscillator 25. Namely, where the variable resistor 30 has a resistance approaching zero, an output pulse signal has its width decreased. Conversely where the variable resistor 30 presents a maximum value, an output pulse signal has a maximum width. In either case, the interval or spacing between successively generated pulses of the output pulse signal is not affected by variations in the value of the variable resistor 30 but by those in the value of another variable resistor 32. The reference level of the output pulse signal is set at zero volts and the amplitude thereof at Vy volts.
Connected to the output terminal of the pulse signal oscillator 25 is an energizing circuit 36 for rendering the later described bidirectional thyristor 45 conducting upon receipt of an output signal from the oscillator 25. The energizing circuit 36 includes a transistor 37 whose emitter is connected to said one end N of the capacitor 23 and whose base is connected to the collector of the transistor 26 through a resistor 38 and also to said one end N of the capacitor 23 through a resistor 39. The collector of the transistor 37 is connected between the diodes D 1 and D 2 of a rectifying circuit consisting of bridge-connected diodes D 1 , D 2 , D 3 and D 4 . The junction of the diodes D 3 and D 4 is connected to said one end N of the capacitor 23. Further, a capacitor 41 is connected between the junction A of the diodes D 1 and D 3 and the junction B of the diodes D 2 and D 4 . The junction A is connected to the output terminal O 2 of the switching means of FIG. 2 through a resistor 42, and the junction B is connected to the input terminal I 2 of the switching circuit 5. Connected to both ends of the capacitor 41 is a series circuit including a triggering element, for example, a bidirectional thyristor 43 and the primary winding 44a of a pulse transformer 44. Connected to the energizing circuit 36 is a controlled rectifier 45, for example, a bidirectional thyristor. Namely, the bidirectional thyristor 45 is connected between the input terminal I 2 and the output terminal O 1 of the switching circuit 5 through the secondary winding 44b of the transformer 44. A capacitor 46 is connected in parallel with a series circuit comprised of the secondary winding of the transformer 44 and thyristor 45. The output terminal O 1 of the switching circuit 5 is connected to the input terminal I 1 thereof.
Before cooking is started, the power supply switch 6 is thrown in, an object of heating is placed in a heating chamber (not shown) and the door of a heating apparatus, for example, an electronic oven is closed to actuate a switch 11. Later when a push button 7 is depressed with the timer 8 set to operate for a prescribed length of time, the time limit contact 10 is closed and the timer 8 commences the operation to energize the electromagnetic contactor 4. This energization causes the normally open contact 4 a of the electromagnetic contactor 4 to be closed in a selfholding state and also a contacts 4b and 4c to be closed. As the result, the voltage of the power source 1 is impressed on the high frequency oscillator 2 (FIG. 2) through the switching means 5 to commence the operation of the high frequency oscillator and consequently the heating of an object. When the set time of the timer 8 passes, the time limit contact 10 of the timer 8 is opened to deenergize the electromagnetic switch 4. The high frequency oscillator stops to end thermal cooking.
There will now be described the operation of the switching circuit 5. When the voltage of the power source 1 is impressed across the input terminals I 1 and I 2 , the pulse signal oscillator 25 starts operation with a time constant defined by the resistors 30 and 32 and the capacitors 31 and 33 so as to supply the transistor 37 with pulse signals having a reference level of volts and an amplitude of Vy volts. The transistor 37 is rendered conducting when a positive voltage Vy impressed on the base-emitter circuit of said transistor 37. The capacitor 41 is bypassed by the rectifying circuit 40 and the actuated transistor 37. Since voltage is not supplied to the bidirectional thyristor 43 nor to the primary winding 44a of the transformer 44, the bidirectional thyristor 45 is not ignited, namely, remains nonconducting, thereby preventing the impression of any voltage across the output terminals O 1 and O 2 of the switching circuit 5. When the base-emitter voltage of the transistor 37 falls from Vy volts to 0 volts, the transistor 37 is turned off. As the result, a capacitor 41 is supplied with voltage and gradually charged with electric energy. When the voltage across both ends of the capacitor 41 exceeds the breakover voltage of the bidirectional thyristor 43, then said thyristor 43 becomes nonconducting, allowing the primary winding 44a of the transformer 44 to be impressed with voltage, and in consequence the secondary winding 44b of said transformer 44 to be supplied with high voltage, with the resultant actuation of the bidirectional thyristor 45. Thus a prescribed voltage is impressed across the output terminals O 1 and O 2 of the switching means. The above-mentioned operation is repeated per half the AC cycle, and the bidirectional thyristor 45 is turned on and off alternately at an exceedingly small interval (i.e., a high repetition rate) to impress high frequency voltage on the high frequency oscillator 2.
High frequency output from the oscillator 2 can be controlled by varying the ON and OFF times of the switching circuit 5. According to the foregoing embodiment, the ON and OFF times vary with the values of the variable resistors 30 and 32 of the pulse signal oscillator 25. For example, where the resistance of the resistor 30 gradually increases from zero with that of the resistor 32 fixed, then the OFF time of the switching circuit 5 is lengthened, though its ON time remains unchanged, continuously decreasing a high frequency output (expressed on average) from the high frequency oscillator 2.
An average output P from the high frequency oscillator 2 per prescribed time t may be expressed as follows:
P = Ptx . (t . tx/Tx + ty )
where;
tx = ON time of the switching circuit 5
ty = OFF time of the switching circuit 5
Ptx = an average output from the high frequency oscillator 2 per unit time when the oscillator 2 is directly connected to a power source 1
It is generally demanded that tx + ty be chosen to have a fully smaller value than t.
A timer customarily used with an electronic oven has a maximum operating time of 10 to 30 minutes. Therefore, where the ON time tx of the switching circuit 5 is set at about 2 seconds and the OFF time ty thereof is made variable within the range of about 0 to 20 seconds, then the high frequency oscillator 2 will give forth a maximum output when the OFF time ty is set at zero and produce an output equal to about one-tenth of said maximum output when the OFF time is set at 20 seconds. Where the sum of the ON time tx and OFF time ty of the switching circuit 5 is chosen to be fully small relative to a required cooking time, then an output from the high frequency oscillator 2 can be continuously controlled, saving the practical operation of a high frequency heating apparatus from any obstruction due to errors in said output.
In the heating apparatus of this invention, the switching circuit 5 is provided, as previously described, between the power source 1 and high frequency oscillator 2, permitting the free change of an output from the high frequency oscillator. This means that various objects of heating can be properly cooked by electromagnetic waves of suitable intensity. For example, scrambled eggs can be satisfactorily prepared without causing a stirred mixture of the yolks and whites to be explosively scattered. Further, where the high frequency oscillator 2 is made to emit weak electric waves by setting the value of the variable resistor 30 at a suitable level, then frozen foods can be properly melted.
There has been described this invention by reference to the aforesaid embodiment. It will be understood, however, that the invention is not limited to the illustrated embodiment. For example, the pulse signal oscillator 25 which comprised an astable multivibrator in said embodiment may be replaced by a different type of oscillator. Further, though the variable resistors 30 and 32 were used for the free control of the ON and OFF times of the switching circuit 5, yet one of these resistors may be replaced by a fixed type resistor so as to define the ON or OFF time of the switching circuit 5 at a prescribed level. The variable resistors 30 and 32 were made continuously variable, but it is possible to use the type whose resistance can be changed stepwise. Moreover, the controlled rectifying element 45 which was shown as a bidirectional thyristor may be replaced by another type formed of, for example, two silicon controlled rectifiers.