US3646423A

US3646423A - Converter for changing alternating current into direct current

Info

Publication number: US3646423A
Application number: US46322A
Authority: US
Inventors: Kenzo Tatematsu; Teruhisa Kaneko
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1969-06-20
Filing date: 1970-06-15
Publication date: 1972-02-29
Anticipated expiration: 1989-02-28
Also published as: DE2029776B2; FR2046960B1; GB1323441A; DE2029776A1; DE2029776C3; FR2046960A1

Abstract

A converter for changing alternating current into direct current by charging a large-capacity capacitor through a full wave rectification circuit which, together with a transformer, is connected to an AC power supply. The output voltage of a voltage divider circuit consisting of the above-mentioned capacitor and a parallel-connected resistor is compared with a predetermined reference voltage by a comparator circuit. Then, if the former is lower than the latter, a gate circuit is triggered to put a selfoscillating circuit into a state where it is ready for oscillation, and the oscillating operation of the above-mentioned self-oscillating circuit is triggered through the gate circuit connected to an intermediate point of a closed loop made up of a rectification circuit for rectifying an AC source voltage and of a part of the aforesaid voltage divider circuit, the output of the self-oscillating circuit providing a trigger signal for a thyristor means connected to the aforementioned transformer.

Description

United States Patent Tatematsu et al. 51 Feb. 29, 1972 [54] CONVERTER FOR CHANGING 3,507,096 4/1970 Hall et al ..321/18 X ALTERNATING CURRENT INTO 3,524,102 8/1970 Michalski et al ....3l5/240 X DIRECT CURRENT 3,037,147 5/1962 Genuit et al. ....315/240 X 3,538,418 11/1970 Allmgton ..321/18 [72] lnventors: Kenzo Tatematsu, Amagasaki; Teruhisfl 3,383,579 5/ 1968 Hung ..321/24 X Kaneko, Kadoma, both of Japan [73] Assignee: Matsushita Electric Industrial Co., Ltd., Primary Beha Osaka, Japan Attorney-Stevens, Davis, Miller 8!. Mosher [22] Filed: June 15, 1970 [57] ABSTRACT [21] APPl- 46,322 A converter for changing alternating current into direct current by charging a large-capacity capacitor through a full wave [30] Foreign Application priority Dam rectification circuit which, together with a transformer, is connected to an AC power supply. The output voltage of a voltage June 20, 1969 Japan ..44/49863 divider circuit consisting f the above memioned capacitor June Japan-W "44/49864 and a parallel-connected resistor is compared with a predeter- Aug. 29, 1969 Japan ..44/69483 mined reference voltage by a comparator circuiL Then, if the former is lower than the latter, a gate circuit is triggered to put [52] US. Cl ..321/14, 315/240, 321/ 1 8, a semoscmating circuit into a State where it is ready for oscil SI I t g 3 lation, and the oscillating operation of the above-mentioned d 383 self-oscillating circuit is triggered through the gate circuit con- 1 0 nected to an intermediate point of a closed loop made up of a rectification circuit for rectifying an AC source voltage and of 56] Reierences Cited a part of the aforesa d voltage dlvrder circu t, the output of the self-oscillating circuit providing a trigger signal for a thyristor UNITED STATES PATENTS means connected to the aforementioned transformer.

3,375,403 3/1968 Flieder ..323/24 14 Claims, 10 Drawing Figures RES/$722? L040 /8 W L /4 5b 0 0-! R66 IMR H 7 l3 mar/ 5mm 1 u l I M? RES/570R Ear/FER 6011345470? I56 RJWER TRANS 22%, GATE [MR PHASE 1E7 P-q V50 RES/57D? 7 C PATENTEDFEB29 m2 SHEET 2 BF 7 CONVERTER FOR CHANGING ALTERNATING CURRENT INTO DIRECT CURRENT The present invention relates to a converter for changing alternating current into direct current, or more particularly to a converter in which a DC output voltage is impressed on a large-capacity capacitor to temporarily store energy which is repeatedly supplied to a load at frequent intervals during a short period of time.

As an AC to DC converter of this sort, a copying machine employing a xenon lamp light source is known. Although it can produce a large luminous flux, a xenon lamp draws considerable electric current and it is more advantageous to employ such a lamp through which a large current flows during a very short period of time.

It is also known that, in those machines and apparatus in which a large-capacity capacitor is impressed with a DC output, a large inrush current flows from the power source to the machines and apparatus during the initial stage of charging the capacitor. This surge current has a very great impact on the components of the machines and apparatus, resulting in their damages or, in worst cases, destruction. As a means for controlling the surge current, a phase control with a thyristor is commonly used.

An object of the present invention is to provide an AC to DC converter to which is commercially applied a novel controlling means for controlling the firing angle of a thyristor in accordance with the charging state of a large-capacity capacitor to effectively prevent the surge current from flowing into the capacitor.

Another object of the present invention is to provide an AC to DC converter capable of manually regulating the time to end the charging of a large-capacity capacitor.

A further object of the present invention is to provide an AC to DC converter which, in the event of malfunction of its means for setting a capacitor-charging voltage, prevents the charging to an excessive voltage and secures operator safety without affecting other components.

A still further object of the present invention is to furnish an AC to DC converter which is capable of stopping the capacitor charging operation when a thyristor for smoothly starting the machine breaks down and which is thus capable of preventing an extension of the trouble.

A still further object of the present invention is to provide an AC to DC converter suitable for a device to turn on a discharge lamp which has a high starting voltage and requires a large current for actuation.

The above and other objects, features and advantages will be made apparent by the detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a drawing showing a construction of the present invention, as based on its principle;

FIG. 2 illustrates an embodiment of the present invention;

FIG. 3 is a diagram showing the operation of the embodiment shown in FIG. 2;

FIG. 4 shows another modification of the invention; and

FIGS. 5a to 5f indicate modifications of the essential parts of the above.

A detailed explanation of the present invention, starting with the principle of its construction, is given below.

FIG. 1 shows a construction based on the principle of this invention, wherein a main circuit consists of a main power source transformer l, a thyristor 2 and a choke coil 3 connected in series with the primary coil of the transformer, a full wave rectifier 4 made up of diodes connected with the secondary coil of the transformer, a large-capacity capacitor 5 impressed with the output of the full wave rectifier and a load 6 supplied with the energy stored in the capacitor 5.

The phase control circuit of the thyristor 2 is constructed in such a way that a self-oscillator circuit 10 producing a trigger signal for the thyristor is controlled by a gate circuit 9 which in turn is controlled by three voltages, i.e., the voltage of a voltage divider circuit consisting of resistors 7a, 7b and 7c"which divide the charging voltage of the capacitor 5, the voltage obtained by shifting the phase of the source voltage by a certain angle and the output voltage of a comparator circuit 8 which compares the charging voltage of the capacitor 5 or its equivalent with a reference voltage.

The voltage whose phase is shifted by a certain angle with respect to the phase of the source voltage can be obtained by adding a phase shifting circuit 12 to the secondary side of a power transformer 11 for the control circuit. A rectification circuit 13 is also added to the secondary side of the aforementioned power transformer 11 to supply the power required for the control circuit.

On the other hand, the comparator circuit 8 is connected with an overcharge-protecting circuit 14 which is energized when a voltage appearing at the sliding terminal of the resistor 7b as a result of dividing the charging voltage of the capacitor 5 exceeds a predetermined value, thereby stopping the charging action. The capacitor 5 is thus controlled so that its rated breakdown voltage is not exceeded. Incidentally, 15a, 15b and show impedance elements.

The operation of the aforesaid components is explained below. The power transformers l and 11 are connected across the power source 18 through a switch 16 and a fuse 17. Immediately after the switch 16 is closed, the capacitor 5 has not been charged and no voltage appears across the resistors 7a, 7b and 70. When the input voltage of the comparator circuit is lower than a reference voltage, the gate circuit 9 makes the self-oscillating circuit 10 ready to oscillate.

The output voltage of the phase-shifting circuit 12, on the other hand, is supplied to the gate circuit 9 to trigger the selfoscillating circuit 10 which is ready to oscillate. The time to start oscillation can be controlled and consequently the conduction can angle of the thyristor 2 determined by constructing the gate circuit 9 is such a manner that the self-oscillating circuit 10 is triggered at a certain level of the phase-shifted voltage.

When conduction through the thyristor 2 begins at a small conduction angle, an exciting current flows into the main power transformer I, inducing in its secondary coil a boosted AC voltage, which after being rectified by the rectification circuit 4 is impressed on the capacitor 5 to charge it. When the capacitor 5 starts to be charged, the comparator circuit 8 and the gate circuit 9 are impressed with a voltage or control signal through the resistors 7a, 7b and 7c. Across the resistor 70 comes toexist a DC voltage, which changes the level of the output supplied by the phase shifter circuit 12 to the gate circuit 9, thereby advancing the firing phase of the thyristor 2 and enlarging its conduction angle. As a result, the electric current flowing into the main power transformer l is increased, expediting the charging of the capacitor 5.

When the voltage which is obtained by dividing the charging voltage of the capacitor 5 and which is impressed on the comparator circuit 8 reaches the level of its reference voltage, the comparator circuit 8 reverses its output condition,'actuating the gate circuit 9 to stop oscillation of the self-oscillating circuit 10.

By the way, it is for the following reason that the phase shifter circuit 12 is added. With the increase in energy supplied to a load, the capacity of the main power transformer l is enlarged, resulting in an increased exciting current. Therefore, an inrush exciting current several to 10 or more times as large as the current under normal conditions flows through the primary coil of the main power transformer 1, depending on the conduction phase of the thyristor 2. Also, the charging current of the large-capacity capacitor 5 combines with the aforesaid exciting current to cause a very large surge current.

In a system for controlling the load by using the thyristor 2 at the primary side of the main power transformer l, as the load frequency becomes higher, a large exciting current flows more frequently. Since this surge exciting current is delayed by about 1r/2 with respect to the voltage phase, it is necessary to select the triggering phase of the thyristor 2 at zero or a very ,small value of the current phase. The phase shifter circuit 12 is for determining the triggering phase of the thyristor 2.

lt is undesirable from the viewpoint of both performance and economy not to control the initial triggering phase of the thyristor 2. In the first place, it is required that the excess current capacity of the thyristor 2 be made large so that it can sufficiently withstand the surge of the exciting current. This calls for an electric current capacity larger than needed for each element and contributes to a higher cost of the apparatus. Secondly, the flow of a large exciting current temporarily puts the devices connected to the AC power source 18 in a state of low impedance or short circuit, which has a bad effect on not only the power supply, but also other devices connected to it. Thirdly, the protective means such as a thermal nofuse breaker and a fuse which are inserted between the AC power source and other devices are liable to be energized even by an instantaneous excess current. For this reason, it often happens that the devices are inconveniently cutoff from the AC power source by the protective means. Fourthly, not only the thyristor 2 but also other components must be improved in their characteristics to withstand the surge current, resulting in a still higher cost.

In view of the above, the initial triggering phase of the thyristor 2 should be controlled.

The gate circuit 9 is provided for the following reason: As described above, the inrush magnetizing current into the main power transformer 1 lags behind the phase of the AC source voltage by about 11/2. Therefore, by regulating the phase shifting circuit 12 in such a way that the electric current flows through the thyristor 2 in the above-mentioned phase condition, the exciting current of the power transformer 1 and the charging current to the capacitor can be controlled. When the conduction angle of the thyristor 2 is maintained at a fixed level, however, the ratio of the firing period to one cycle of the output of the AC power source 18 is small, holding the charging current of the capacitor 5 at a small value. Because of the small value of the charging current, it takes a long time until the charging voltage of the capacitor 5 reaches the predetermined voltage value. This reduces the operating efficiency of the apparatus. There is also a way of providing a phase control at the first half cycle, supplying electricity without any subsequent phase control. Although this method is advantageous in that the charging time of the capacitor can be shortened, there is still room for improvements to be made, because only a small amount of charge can be stored at the first half cycle and a large current flows at the next half cycle.

The problems mentioned above are soluble by increasing the firing angle of the thyristor 2 at each half cycle. For this purpose, the level of the output voltage of the phase shifting circuit 12 must be changed with the increase in the charging voltage of the capacitor 5, and also a means must be provided so as to control the oscillation-starting phase of the self-oscillating circuit 10 in accordance with a certain relationship between the above two voltages. The gate circuit) is required to simultaneously control the self-oscillating circuit 10 by means of a plurality of control voltages mentioned above.

The charging voltage of the capacitor 5 is determined by the positionof the sliding terminal of the resistor 7b. Therefore, the resistor 7b is used with relative frequency and its structural reliability is lower than that of other component parts. Because this resistor 7b is a factor determining the charging voltage of the capacitor 5, some other means in place of it must be secured in case of a failure. Here in this invention, it is so constructed that the voltage across the terminals of the resistor 7c is led to the overcharging protecting circuit 14 by the impedance element 15b to cut off the apparatus from the AC power source 18 when the charging voltage of the capacitor 5 reaches an allowable limit.

FIG. 2 is a diagram showing an electric circuit of a copying machine embodying the present invention. Closing the switch SW, causes the electric current to flow through the power transformer T, for a controlling circuit to which DC power is supplied through the rectification circuit D and the capacitor C The phase of the secondary voltage of the transformer T is advanced by approximately 1r/2 with respect to the voltage of the power source S by the capacitor C It is then rectified by the full wave rectifying circuit D,, to be converted into a signal for controlling the thyristor TH. In addition, the capacitor C,, a component used in generating an exciting voltage for the lamps La, and La, is charged through the diode D,,.

At this juncture, a switch SW closed to the side of the terminal a causes the base and emitter of the transistor 0-,, to be short-circuited, thus keeping it in a cutoff state. At the same time, the transistor Q, is maintained in a cutoff state, the base bias voltage being supplied to the transistor 0,, to permit the current to pass it. Since the capacitor C is short-circuited by the transistor Q the self-oscillating circuit consisting of the capacitor C resistor R, and the unijunction transistor 0,, is not actuated. As a result, no gate signal is impressed on the bidirectional triode thyristor Tl-l through the pulse transformer T and the capacitor C, is not charged.

If the switch SW is closed to the side of the terminal b, the transistor 0;, is made ready to operate. The capacitor C, is not charged immediately after the closing of the switch SW and therefore the voltage obtained from the resistors R,, VR,, VR VR, and VR, and the diode D, equals zero. That is to say, the sliding terminal of the resistor VR,, the contact between the resistors VR, and VR, and the sliding terminal of the resistor VR, are maintained at ground potential. Because the sliding terminal of the resistor VR, and the node of the resistors VR, and VR, are maintained at ground potential, the bases of the transistors Q and Q, are kept at ground potential through the diodes D and D and the resistor R respectively. Thus, the transistors Q and Q, maintain a cutoff state. The transistor Q which is kept at a cutoff state by the transistor 0,, causes the current to flow through the transistor Q putting the transistor 0, into a state ready for conduction.

The base potential of the transistor Q, varys with the output of the phase shifting circuit consisting of the capacitor C and the full wave rectifying circuit D,,.

Referring to FIG. 3 where the waveforms of the voltage and current at the various parts are shown, a full wave rectifying voltage E, which is advanced in phase by about 11/2 ahead of the voltage E, of the AC power supply S appears at the ungrounded terminal of the full wave rectifying circuit D,,. As the terminal on the positive side of the full wave rectifying circuit D,, is grounded, the voltage E, becomes negative as shown in the drawing.

Watching the half cycle of the AC voltage E, immediately after closing the switch SW to the side of the terminal b, the voltage E, stands at zero in the neighborhood of the phase rr/2 of the AC voltage. In a circuit including the resistors R and R and the variable resistor VR, which are connected in series with the full wave rectifying circuit D,,, a voltage E or a division of the voltage E,, appears at the node of the resistors R and R The voltage E is also elevated to zero in the neighborhood of the phase 11/2 of the source voltage E Consequently, the cathode potential of the diode 5 is highest at the aforesaid phase, and the base potential of the transistor 0., is also increased.

As described above, since the current flows through the transistor 0,, by changing the position of the switch SW the current also passes through the transistor 0, while the cathode potential of the diode D is about zero. Due to the conduction of the transistor 0., as mentioned above, the transistor O is reversed to remove the short circuit across capacitor C As a result, a charging current flows into the capacitor C through the resistor R When the voltage across the terminals of the capacitor C reaches the firing voltage for the unijunction transistor 0,, conduction begins through the transistor Q permitting a pulse-shaped current to flow in the primary coil of the pulse transformer T The pulse voltage E is induced in the secondary coil of the pulse transformer T and this pulse voltage is impressed on the gate electrode of the bidirectional triode thyristor TH as a trigger signal, thereby enabling the current to flow through the thyristor TH.

rent I flows through the main power transformer T, to start the charging of the capacitor C,. The voltage E, across its terminals comes up a little as shown in the drawing.

On the other hand, the oscillating circuit consisting of the unijunction transistor 0,, the resistor R and the capacitor C oscillates at a frequency determined by the time constant C R,

and the firing voltage of the unijunction transistor 0,. The voltage E is reduced in accordance with the output voltage E, of the full wave rectifying circuit D,,, lowering the cathode potential of the diode D Therefore, the bias voltage between the base and the emitter of the transistor Q, is also reduced, cutting off the transistor. The reversing of the transistor 0., causes the conduction of the transistor terminating the oscillation of the aforesaid oscillating circuit. Conduction through the thyristor TH continues until the current I becomes zero even if the voltage E ceases to exist. When the current I becomes zero, the thyristor TH is restored to a cutoff state.

When the capacitor C, is charged, the voltage corresponding to the charging voltage appears at the sliding terminals of the resistors VR, and VR,. The voltage generated at the sliding terminal of the resistor VR, is impressed on the transistor Q, through the resistor R and, in the transistor 0,, is compared with the breakdown voltage of the zener diode ZD. The same can be said of the transistor 0-,.

Also, the potential of the sliding terminal of the resistor VR, is elevated a little higher than the ground potential, increasing the conduction angle of the thyristor TH at the next half cycle of the AC source voltage E,.

In .other words, while the phase of voltage E, is between 1r and Zr, the voltage IE at the node of the resistors R and R is made higher in the neighborhood of the phase 3rr/2 than at the phase 1r/2. Hence the firing of the transistor 0., is advanced and consequently, the time to start the oscillation of the oscillating circuit consisting of the unijunction transistor Q61 the resistor R, and the capacitor C is advanced. This increases the conduction angle of the thyristor TH, lengthening the period during which the charging current for the capacitor C, flows.

Each time the capacitor C, is charged, the conduction angle of the thyristor is enlarged and the current I is increased, while being so controlled that it does not become excessively large, thereby increasing the charging voltage E, of the capacitor C According as the charging voltage E, increases, the voltage at the sliding terminal of the resistor VR, is heightened. If this voltage goes higher than the sum of the breakdown voltage of the zener diode ZD, the voltage drop at the base-emitter junction of the transistor 0, and the voltage drop in the diode D conduction through the transistor Q, begins. Then, conduction takes place in the transistor 0,, the transistors Q and Q, are cutoff, the current starts to flow through the transistor 0,, and the capacitor C,, is short-circuited, terminating the oscillation. The above gating operation of the transistor Q, is secured by the existence of the diode D Changing the position of the switch SW from the terminal a to the terminal b on completion of charging the capacitor C,, the discharging of the capacitor C causes the gate current to flow into the silicon controlled rectifier SCR,, starting the flow of electric current through it. Then, the capacitor C,, which hasbeen charged through the diode D is discharged through the primary coil of the pulse transformer T inducing a pulse voltage in its secondary coil.

The voltage induced in the pulse transformer T, is impressed on the trigger coil of the lamps La, and La,, causing discharge thereof. As soon as a faint discharge occurs across the trigger coil and the electrode of the lamps La, and L11 a main discharge starts, causing the capacitor C, to be discharged through the lamps La, and La and the choke coil CH The lamps La, and La, then radiate, consuming the energy stored in the capacitor C Due to the discharge of the capacitor C,, the voltage E becomes approximately zero, repeating the above-mentioned charging operation.

At the time when the charging voltage E, of the capacitor C, reaches a predetermined value and conduction through the transistor Q, starts, the transistor 0, which has an emitter circuit common to the transistor 0, is in a cutoff state. This is because of the function of the diode D between the sliding terminal of the resistor VR, and the base of the transistor 0-,, and the firing voltage of the transistor 0 is substantially elevated by the amount of voltage drop in the diode D The following is an explanation of the above apparatus in the case of a failure of the above means. The resistor VR, is provided for the purpose of determining the charging voltage E of the capacitor C,, and is used on relatively many occasions. Therefore, contacts troubles are liable to develop between its sliding terminal and the resistance material, frequently resulting in the loss of the resistor function.

When the resistor VR, fails, the comparison operation by the transistor Q, stops and the charging of the capacitor C, continues with the result that the charging voltage is apt to exceed an allowable limit thereof. Under this condition, the voltage at the node of the resistors VR, and VR, is impressed upon the transistor 0 through the diode D Conduction of the transistor 0, causes another conduction of the transistor 0,, thereby triggering the silicon controlled rectifier SCR,. When current starts to flow through the silicon controlled rectifier SCR, the relay R, is excited to open the switch SW, between the power transformer T, and the power source S, separating the apparatus from the power source S. At the same time, the collector current of the transistor Q, passes from the diode D, through the resistor R,,, triggering the silicon controlled rectifier SCR,. Thus, the capacitor C, is discharged through the lamps La, and L11 Even if the resistance material of the resistor VR, is worn out or disconnected due to the movement of the sliding terminal, entirely the same protective function as mentioned above is secured by the variable resistor VR, connected in parallel with the resistor VR,. Needless to say, an ordinary fixed resistor instead of the variable resistor VR, may be used.

Moreover, contacts faults or other breakdowns occurring in the resistor VR, causes the electric current from the full wave rectifying circuit D, to flow through the ground, diodes D and D and the resistor R in that order. The transistor Q, is reversely biased by the voltage across the terminals of the diode D and cutoff, making the current flow through the transistor Q When the phase of the source voltage E, equals 1r/2 or 11-12 times an odd number, the output voltage of the full wave rectifying circuit D goes to zero, which cuts off the diode D and reverses the transistor 0 As a result, in the neighborhood of the above phase, the oscillating circuit consisting of the unijunction transistor 0 the resistor R, and the capacitor C is energized, delivering a gate signal to the thyristor TI-[ to start its conduction. Since this sequence is repeated in every half cycle of the source voltage E,, no excess current flows into the apparatus from the AC power source S even though the charging time is extended.

Thus, the above means prevents the surge of exciting current which is otherwise observed immediately after the power supply is switched on. This enables the large-capacity capacitor C, to be charged in a short period of time. The charging voltage E, and the charging time for the capacitor C, can be regulated by the resistors VR, and VR, respectively. In addition, since capacitor C, is not charged to an voltage, greater safety is assured.

FIG. 4 is a drawing which illustrates a similar circuit to that shown in FIG. 2, and in which the control of the firing angle of the thyristor TH is revised. The following is an explanation of the revision.

The full wave rectifying circuit D is connected at the secondary side of the transformer T to charge the capacitor C,,, through the resistor R,,,, and the variable resistor VR,,,,. Until the capacitor C, reaches a predetermined value of voltage, the transistor Q,,,,corresponding to the transistor O in FIG. 2is in a cutoff state. Also, no voltage appears at the sliding terminal of the resistor VR,,, -corresponding to the resistor VR, in FIG. 2and the transistor Q|02 is cutoff. Therefore, a smoothing circuit consisting of the resistor R and the capacitor C is electrically isolated from the output terminals of the full wave rectifying circuit D and a full wave rectified voltage is directly impressed on the capacitor rot- At this time, if the breakover voltage of the unidirectional diode thyristor D inserted between the capacitor C and the primary coil of the pulse transformer T is set at a value equivalent to or a little lower than the crest value of the full wave rectification voltage, the capacitor C is discharged at the phase of about 1r/2 later than the AC voltage through the aforesaid unidirectional diode thyristor D and the primary coil of the pulse transformer T This discharge induces a pulse voltage in the pulse transformer T triggering the thyristor TH.

The discharging of the capacitor C, remarkably reduces the voltage between its terminals and restores the unidirectional diode thyristor D to a cutoff state. This oscillation frequency being determined by the capacitor C the pulse transformer T and the resistors R and VR the charging and discharging are repeated.

At the initial stage of switching on the power supply, the transistor Q is in a cutoff state and no smoothing operation is performed by the capacitor C and the resistor R Therefore, the voltage impressed on the capacitor C is in the form of a pulsating current and is reduced lower than the breakover voltage of the unidirectional diode thyristor D in a short time, thereby stopping the oscillating action. On the other hand, the thyristor TH continues conduction until the current fed by the AC power source S comes to zero, but due to the smallness of its conduction angle, an excessive surge current can be prevented.

After the capacitor C is charged because of the conduction of the thyristor TH, a voltage presents itself at the sliding terminal of the resistor VR This voltage improves the conductive state of the transistor O with a result of electrically inserting the capacitor C and the resistor R between the output terminals of the full wave rectifying circuit D Hence, the smoothness of the rectification voltage is improved, the oscillation-starting phase is advanced, and the conduction angle of the thyristor TH is increased.

In other words, early in the stage of charging the capacitor C,, the transistor is cut off or in a nearly cutoff state and the unidirectional diode thyristor D is impressed with a voltage which increases with the full wave rectification voltage. The unidirectional diode thyristor D is thus put into a conductive state in the neighborhood of a crest value of the impressed voltage. Due to this conduction, the voltage across the terminals of the capacitor C drops and conduction again takes place near the crest value of the next half cycle. However, the charging of the capacitor C, improves the conduction of the transistor Q electrically inserting the capacitor C, and the resistor R Hence the DC portion of the voltage impressed on the capacitor C is increased, which advances the firing phase of the unidirectional diode thyristor D increasing the conduction angle of the thyristor TH.

When the charging voltage of the capacitor C, reaches a prescribed value, conduction of the transistor Qrm begins and the capacitor C is short-circuited. When the charging of the capacitor C is stopped, no voltage is impressed on the unidirectional diode thyristor D, without generating any signal for triggering the thyristor TH. Therefore, the charging operation of the apparatus stops.

FIG. 5 shows examples of a partial modification, in which portions corresponding to those of the apparatus shown in FIG. 2 have the same designation. First of all, FIG. 5a shows a modification of the thyristor 2 for controlling the surge current, using the silicon controlled rectifiers SCR and SCR connected in parallel in opposite directions. As a load for the unijunction transistor Q,;, the pulse transformer T, with three coils is used, one of the coils being connected to the unijunction transistor 0 and the other two to a point between the gate and the cathode of the silicon controlled rectifiers SCR and SCR through the resistors R and R respectively.

The silicon controlled rectifiers SCR and SCR are made conductive when a forward direction is given at every half cycle.

FIG. 5!: illustrates an instance where the two-terminal bidirectional diode thyristor TH is provided at the primary side of the transformer T,. Taking advantage of the breakover of the unidirectional diode thyristor D in place of the unijunction transistor Q a gate signal is impressed across the terminals of the above-mentioned thyristor TH from the pulse transformer T through the DC blocking capacitor C The operation of this device, as in the case of the device shown in FIG. 5a, is equivalent to that of the device shown in FIGS. 2 and 4.

FIG. 5c shows a modification of the input section of the gate circuit or the comparator circuit 8 and the preceding stages for detecting the charge voltage of the capacitor 5. To begin with, the charging voltage of the capacitor C, is divided by the resistors R and R and then the voltage division is impressed on the transistor Q in a emitter-follower circuit. The

emitter circuit of the transistor Qm Consists of the resistors VR,, VR and VR;, and the diode D,. Voltage impression is made from the sliding terminal of the resistor VR, through the resistor R on the base of the transistor O which has a zener diode ZD as an emitter load. This transistor Q together with the transistor Q makes up a Schmitt circuit, controlling the conduction of the transistor Q and subsequent elements connected from the output of the circuit.

The employment of the emitter-follower circuit makes it possible to prevent an erroneous actuation due to noises, even if the design of the apparatus requires the resistor VR. to be connected with a lead wire from a control circuit which is located remote from the resistor.

It goes without saying that the Schmitt circuit consisting of the transistors Q40: and 040:; may be replaced by a construction employing the transistor 0, as shown in FIG. 2.

FIG. 5d shows the case in which the reverse blocking triode thyristor TH is used instead of the silicon controlled rectifier SCR in FIG. 2, without the transistor 0,, and an accompanying circuit.

FIG. 52 illustrates a modification of a combination of the gate circuit 9 and the self-oscillating circuit 10, wherein, instead of the transistor 0;, in parallel with the capacitor C the transistor 0 connected in series with the resistor R is controlled by the collector output of the transistor Q The operation in this case is equivalent to that in FIG. 2.

FIG. 5f is an instance in which an astable multivibrator is used for the self-oscillating circuit H0. The stable multivibrator consists of the transistors Q and O and its power supply is controlled by the transistor Q which in turn is driven by the transistor Q The output of the astable multivibrator energizes the switching transistor O and a gate signal is delivered to the thyristor TH through the pulse transformer T What we claim is:

1. A converter for changing alternating current into direct current comprising, a transformer for changing an AC source voltage, a reactor and a thyristor means attached to said transformer, a first rectifier circuit for rectifying the output of said transformer, a capacitor which is charged by the output of said rectifier circuit and which produces a DC voltage, a voltage divider circuit which produces a voltage corresponding to the charging voltage of said capacitor, a comparator circuit for comparing the output voltage of said voltage divider circuit with a predetermined reference voltage, a gate circuit which is energized in accordance with the output of said comparator circuit as a first controlling signal, an oscillator circuit whose oscillating action is controlled by said gate circuit and which supplies a trigger signal to said thyristor and a second rectifier circuit which rectifies the AC source voltage to obtain a second controlling signal which the gate circuit combines with a third signal to change the oscillation starting phase of said oscillator circuit, said third controlling signal being obtained from said voltage divider circuit and corresponding to the charging voltage of said capacitor, said gate circuit being opened by the first controlling signal when the charging voltage of said capacitor is lower than a predetermined reference voltage of said comparator circuit, said second controlling signal being led to said oscillator circuit in combination signal with the third controlling signal which is subject to change with the increase in the charging voltage of said capacitor, thereby advancing the oscillation starting phase of said oscillator circuit with respect to the AC source voltage to increase the conduction angle of said thyristor means.

2. A converter for changing alternating current into direct current according to claim 1, wherein a phase shifter circuit is provided by adding a capacitor to said second rectifier circuit and the phase of the second controlling signal is shifted about 7r/2 from the phase of the AC source voltage.

3. A converter for changing alternating current into direct current according to claim 2, in which is employed said voltage dividing means having as a voltage divider circuit resistor in parallel with the capacitor charged by said first rectifier circuit, said phase shifter circuit and a part of said voltage dividing means forming a closed loop from an intermediate point of which a combined signal of said second and third controlling signals is impressed on said gate circuit.

4. A converter for changing alternating current into direct current according to claim 2, wherein said oscillator circuit has a unijunction transistor, a resistor inserted between the emitter and one of the bases of said transistor, a capacitor inserted between the emitter and the other of the bases of said transistor and a pulse transformer which is a load of said transistor; said gate circuit having a base circuit including a transistor of said comparator circuit which produces the first controlling signal, a closed loop consisting of said phase shifter circuit and a part of said voltage dividing means, and also a transistor switching means in parallel with the capacitor of said oscillator circuit; and wherein the second and third controlling signals change, with the increase in the DC output voltage, the base bias voltage of the transistor switching means of said gate circuit impressed with the first controlling signal.

5. A converter for changing alternating current into direct current according to claim 2, wherein said comparator circuit is further equipped with an overcharge-protecting circuit for disconnecting the AC power supply when the output voltage of said voltage divider circuit is compared with and exceeds the reference voltage by said comparator circuit; said voltage divider circuit consisting of a resistor with a sliding terminal and a variable resistor connected in parallel; a diode being inserted between the sliding terminal of said resistor and the input terminal of said overcharge-protecting circuit; and another diode being inserted between one of the fixed terminals of said resistor and the input terminal of said overcharge-protecting circuit.

6. A converter for changing alternating current into direct current according to claim 1, wherein said voltage dividing means includes resistors in parallel with said capacitor; and said second rectifier circuit and a part of said voltage dividing means forming a closed loop, from an intermediate point of which a combination of said second and third controlling signals is supplied to said gate circuit.

7. A converter for changing alternating current into direct current according to claim 6, wherein said oscillator circuit consists of a capacitor connected to the output terminal of the second rectifier circuit, a unidirectional diode thyristor connected in series with a pulse transformer and a smoothing circuit connected in series with a transistor whose conduction is controlled by the second and third controlling signals; conduction of said transistor being improved with the increase in the charging voltage of the capacitor connected with the first rectifier circuit; said smoothing circuit making the output of said second rectifier circuit smoother; and thereby controlling the conduction angle of the thyristor connected to said pulse transformer.

8. A converter for changing alternating current into direct current according to claim 1, wherein said comparator circuit is further equipped with an overcharge-protecting circuit which cuts off the connection with the AC power supply when the output voltage of the voltage divider circuit exceeds the reference voltage of said comparator circuit after making a comparison between the two; said voltage divider circuit consisting of a voltage dividing means including a resistor with a sliding terminal connected in parallel with a variable resistor; a diode being inserted between the sliding terminal of said resistor and the input terminal of said overchargeprotecting circuit; and another diode being inserted between a fixed terminal of said resistor and the input terminal of said overcharge-protecting circuit.

9. A converter for changing alternating current into direct current according to claim 1, wherein a controlled rectifying means comprising two silicon controlled rectifiers connected in parallel in opposite directions is used as a thyristor means; and the output of said oscillator circuit being supplied to said silicon controlled rectifiers as a gate signal.

10. A converter for changing alternating current into direct current according to claim 1, wherein said thyristor means is a bidirectional diode thyristor; the output of said oscillator circuit being supplied to a point between the terminals of said bidirectional diode thyristor through a DC blocking condenser.

11. A converter for changing alternating current into direct current according to claim I, wherein said thyristor means is a bidirectional triode thyristor; the output of said oscillator circuit being supplied to the gate electrode of said bidirectional triode thyristor.

12. A converter for changing alternating current into direct current according to claim 1, wherein said comparator circuit is a Schmitt circuit; and a constant-voltage conducting diode being connected to emitter circuits of two transistors making up said Schmitt circuit.

13. A converter for changing alternating current into direct current according to claim 1, wherein said oscillator circuit is an astable multivibrator; a current-controlling means being inserted between said astable multivibrator and the DC power supply; and conduction of said current-controlling means being controlled by the gate circuit to thereby control the start and stop of the oscillating action of said astable multivibrator.

14. A converter for changing alternating current into direct current comprising; a transformer for changing an AC source voltage; a thyristor having a gate electrode and connected in series with a reactor to the primary side of said transformer; a first full wave rectifier circuit connected to the secondary side of said transformer; a first capacitor which is charged by said ,rectifier circuit and which produces a DC voltage; a voltage divider circuit consisting of resistors which divide the charging voltage of said capacitor, a comparator circuit which has a zener diode as it reference voltage source and which compares said reference voltage with the voltage obtained from said voltage divider circuit and corresponding to the charging voltage of said capacitor, discharge trigger circuit which jointly uses said reference voltage source and which releases to a load the charges stored in said capacitor when the voltage obtained from said voltage divider circuit reaches the reference voltage; a phase shifter circuit which produces a pulsating current having a phase about 1r/ 2 shifted from that of the AC source voltage through the second capacitor and the second full wave rectifier circuit; a gate circuit energized by the output of said comparator circuit and the output obtained by combining the output voltage of said phase shifter circuit with the output voltage of said voltage divider circuit which increases according as the charging voltage of said first capacitor increases; a self-oscillator circuit whose oscillating action is controlled by said gate circuit and which sends its output to the gate electrode of said thyristor to trigger the same; the output of said phase shifter circuit impressed on said gate circuit being increased by the output voltage of said voltage divider circuit as said capacitor is charged; the phase, in which oscillation of said self-oscillator circuit is started by said gate circuit, thereby being gradually advanced to increase the conduction angle of said thyristor.

Claims

2. A converter for changing alternating current into direct current according to claim 1, wherein a phase shifter circuit is provided by adding a capacitor to said second rectifier circuit and the phase of the second controlling signal is shifted about pi /2 from the phase of the AC source voltage.

8. A converter for changing alternating current into direct current according to claim 1, wherein said comparator circuit is further equipped with an overcharge-protecting circuit which cuts off the connection with the AC power supply when the output voltage of the voltage divider circuit exceeds the reference voltage of said comparator circuit after making a comparison between the two; said voltage divider circuit consisting of a voltage dividing means including a resistor with a sliding terminal connected in parallel with a variable resistor; a diode being inserted between the sliding terminal of said resistor and the input terminal of said overcharge-protecting circuit; and another diode being inserted between a fixed terminal of said resistor and the input terminal of said overcharge-protecting circuit.

11. A converter for changing alternating current into direct current according to claim 1, wherein said thyristor means is a bidirectional triode thyristor; the output of said oscillator circuit being supplied to the gate electrode of said bidirectional triode thyristor.

14. A converter for changing alternating current into direct current comprising; a transformer for changing an AC source voltage; a thyristor having a gate electrode and connected in series with a reactor to the primary side of said transformer; a first full wave rectifier circuit connected to the secondary side of said transformer; a first capacitor which is charged by said rectifier circuit and which produces a DC voltage; a voltage divider circuit consisting of resistors which divide the charging voltage of said capacitor, a comparator circuit which has a zener diode as it reference voltage source and which compares said reference voltage with the voltage obtained from said voltage divider circuit and corresponding to the charging voltage of said capacitor, discharge trigger circuit which jointly uses said reference voltage source and which releases to a load the charges stored in said capacitor when the voltage obtained from said voltage divider circuit reaches the reference voltage; a phase shifter circuit which produces a pulsating current having a phase about pi /2 shifted from that of the AC source voltage through the second capacitor and the second full wave rectifier circuit; a gate circuit energized by the output of said comparator circuit and the output obtained by combining the output voltage of said phase shifter circuit with the output voltage of said voltage divider circuit which increases according as the charging voltage of said first capacitor increases; a self-oscillator circuit whose oscillating action is controlled by said gate circuit and which sends its output to the gate electrode of said thyristor to trigger the same; the output of said phase shifter circuit impressed on said gate circuit being increased by the output voltage of said voltage divider circuit as said capacitor is charged; the phase, in which oscillation of said self-oscillator circuit is started by said gate circuit, thereby being gradually advanced to increase the conduction angle of said thyristor.