A CONTROL DEVICE FOR A SILICON-CONTROLLED RECTIFIER

Abstract: A circuit arrangement for the control of AC power supplied to a load (10) comprises a silicon-controlled rectifier (11) forming with the load a series circuit which is connected to an AC power source via terminals (12, 13). A series circuit comprising a resistor (14) and a capacitor (15) is connected in parallel with the silicon-controlled rectifier. The connecting point between the resistor (14) and the capacitor (15) is connected to the gate of the silicon-controlled rectifier via a trigger device, for example an AC diode referred to as Diac. A capacitor (17), a diode (18) and a capacitor (19) form a series circuit which is connected in parallel with the capacitor (15). By means of a discharge circuit or in another suitable way the voltage over the capacitor (19) can be kept at a desired level. Thereby, the conduction angle of the silicon-controlled rectifier may be set within wide limits, practically between 0` and 180` in every controlled half-cycle of the supply voltage.

Description: A control device for a silicon-controlled rectifier The present invention refers to a circuit arrangement for controlling the AC power delivered to a load via a silicon-controlled rectifier. In known circuit arrangements of the kind indicated above there is provided an AC rectifier of the type referred to as Triac. In parallel with the Triac there is connected a resistor in series with a capacitor, and the connecting point between the resistor and the capacitor is connected to the gate of the Triac via a trigger device, for example an AC diode of the kind referred to as Diac. In the usual way the Triac is supplied with AC voltage, and the function of the circuit arrangement is as follows. The capacitor charges through the resistor until the voltage over the capacitor reaches the break-through voltage for the Diac. At that voltage the Diac starts to conduct abruptly generating a current pulse of sufficient ampli¬ tude so as to bring the Triac into conduction. In order to control the time at which the Triac starts to conduct, either the resistor or the capacitor may be varied. The most common method of controlling the starting time is to vary the resistance of the resistor. An increase in resistance results in an increase of the time period required for charging the capacitor to the break-through voltage of the Diac and the Triac will turn on with a corresponding delay. Hence, the angle of conduction of the Triac will diminish. A decrease in resistance will have the opposite effect. A variation in resistance may easily be achieved by choosing for the resistor a potentiometer. In certain applications, for instance in connection with the control of electric motors in vacuum cleaners, the use of variable resistors will involve problems. The parameter which is to control the variation in resistance very often is the vacuum in the suction system of the vacuum cleaner, and variations in this vacuum are to be transferred into motion performed by the movable contact of a potentiometer. The means for trans¬ ferring the variations in vacuum as well as the potentiometer- have to be designed and selected with care in order for the arrangement to function in a reliable way for a long time, and these requirements will involve an increased cost for the product. Another factor that may involve an increased cost is that often the control means comprise contacts which close and open. Connecting such control means to the resistor causes high-frequency interferences to develop, which may be of great amplitude because of the voltage over the resistor taking values of about 300 volts in case the vacuum cleaner is operated at a mains voltage of 220 volts. Contrary hereto, the voltage over the capacitor is comparatively low, usually of the magnitude of 10 volts. Accordingly, regulating contacts in a control arrange¬ ment connected to the capacitor will involve a considerable decrease of the level of the high-frequency interferences that may be generated. One possibility of regulation in the low voltage capacitor circuit is to connect a variable resistor in parallel with the capacitor. However, this solution has the disadvantage that control of the trigger time is possible only during the first half of each half-cycle of the supply voltage. When a conduction angle of the controlled rectifier is desired which exceeds 90 , the parallel connection of the capacitor and the variable resistor cannot be used. From the foregoing it appears to be advantageous to regulate on the capacitor instead of varying on the resistor in order to set the conduction angle of the silicon-controlled rectifier. Due to the fact that in practice it is impossible to continuously vary the capacitance of the capacitor, one solution is to provide several capacitors of different values which may stepwise be connected in parallel by a contact device. Thereby the conduction angle may be changed in steps. Such an arrangement is described in connection with a vacuum cleaner in the Swedish patent specification No. 349,742. The arrangement includes an additional capacitor normally connected in parallel with the capa¬ citor forming part of the trigger circuit. The additional capacitor is disconnect¬ ed when the dust bag has been clogged to a certain degree and the air flow in the suction system has decreased correspondingly. An object of the invention is to provide a circuit arrangement of the kind stated above wherein it will be possible to continuously control the power delivered to a load by varying the capacitance of the capacitor in dependence on a control parameter. By the invention the transfer between a sensor for the control parameter and the variable capacitor will be simplified, particularly in vacuum cleaners. Moreover, the invention makes it possible, as an alternative, to provide means for setting of different power levels instead of the continuous control. Another advantage is that the invention makes control of the conduction angle possible also during the second half of each half-cycle of the supply voltage, which means that the conduction angle may be set at any value in the range of 0 - 180°. The object of the invention and the advantages indicated above will be achieved in an arrangement which incorporates the characterizing features stated in the appending claims. Other objects and advantages will appear from the following description of three embodiments with reference to the accompanying drawings, in which Fig. 1 is a circuit diagram of an arrangement according to the invention including a thyristor and Fig. 2 is a circuit diagram of an arrangement including a Triac. Fig. 3 is a circuit diagram of a modified arrangement using a Triac. Fig. 4 is a diagram showing the variation of the trigger voltage in the prior art circuit referred to above, and Fig. 5 is a diagram showing the variation of the trigger voltage in the circuits of Figs. 1 and 3. In Fig. 1 a load comprising an AC commutator motor is designated by 10. A thyristor 11 is connected in series with a load and this series circuit is connected to the mains via terminals 12, 13. A resistor 14 is connected in series with a capacitor 15, and this circuit is connected in parallel with the thyristor. The connecting point between the resistor 14 and the capacitor 15 is connected via a trigger diode 16 to the gate of the thyristor. The capacitor 15 is bridged by a series circuit comprising a capacitor 17, a diode 18 and an additional capa¬ citor 19. The capacitor 17 is bridged by a diode 41 and the series circuit of the diode 18 and capacitor 19 is bridged by a further diode 20. A device, not shown, for discharging the capacitor 19 can be connected via terminals 21 and 22. The circuit shown in Fig. 1 is adapted for the control of the turning-on arm of the thyristor 11 during the positive half-cycles of the supply voltage. The circuit functions in the following way. As already described above, the capacitor 15 is charged by the current flowing through the resistor 14. When the capacitor voltage has reached the break-through voltage for the trigger diode 16 the thyristor is triggered and starts to conduct. Then, the voltage over the capacitor 15 drops to a low value and the capacitor is prepared for a new charging phase in the next following positive half-cycle. Assuming that the capacitors 17 and 19 are discharged, the charging current through the capacitor 15 is accompanied by a charging current through the circuit comprising the capacitor 17, the diode 18 and the capacitor 19. In this way, the capacitor 19, which is large in comparison with the capacitors 15 and 17, will be charged during successive positive half- cycles to a voltage mainly corresponding to the trigger voltage for the trigger diode 16. When in each positive half-cycle the trigger voltage has been reached, the capacitor 15 is discharged through the trigger diode 16 and the thyristor 11. Also the capacitor 17 will be discharged provided that the voltage on the capacitor 19 has a value lower than the trigger voltage. For this purpose the capacitor is connected to the cathode of the thyristor via the diode 20. The diode 41 ensures complete discharge of the capacitor 17 during the negative half-cycle of the supply voltage. However, the diode 18 prevents discharge of the capacitor 19 an hence the voltage over this capacitor will remain at a level corresponding to th trigger voltage of the trigger diode 16. Because the voltage over the capacito 19 is practically the same as the trigger voltage, every additional positiv half-cycle will cause charging of the capacitor 15 but not of the capacitors 1 and 19. This is illustrated in Fig. 4 and means that the conduction angle will b at a maximum. In order to decrease the conduction angle of the thyristor, the voltage o the capacitor 19 must be set to a value lower than the trigger voltage for th trigger diode 16. For instance, if the said voltage is set to half the trigge voltage, the trigger voltage will have a shape as illustrated in Fig. 5. During positive half-cycle, the voltage in the connecting point between the resistor 1 and the capacitor 15 will rise along a straight line, the slope of which i determined by the time constant of these two components. However, when th voltage on the capacitor 15 exceeds the voltage on the capacitor 19, current wil start flowing in the circuit comprising the capacitor 17, the diode 18 and th capacitor 19. This means that the capacitor 17 will be connected in parallel wit the capacitor 15 and the time constant for the voltage on the capacitor 15 wil now be determined by the resistor- 14 and the capacitors 15 and 17. Th capacitor 19 is so large that the voltage on it will remain essentially unchange during the charging in each half-cycle. In order to have the voltage on th capacitor 19 remain constant for an extended time, means must be provided fo controlling said voltage, for instance by periodical discharging of the capacitor From Fig. 5 it appears that the voltage on the capacitor 15 changes along a lin having a smaller slope. Accordingly, the trigger level will be reached later an the conduction angle of the thyristor will decrease. By having the discharge o the capacitor 19 controlled by a control parameter it will be possible t continuously regulate the power delivered through the thyristor. Fig. 2 shows the circuit of Fig. 1 modified to replace the thyristor 11 by Triac 42 and the trigger diode 16 by an AC trigger diode or Diac 43. Moreover the diode 41 has been omitted. This circuit arrangement functions mainly in th same way as the arrangement of Fig. 1 with the difference that the Triac i triggered in both positive and negative half-cycles of the supply voltage. In thi circuit, the voltage on the capacitor 19 will take a value which at its maximu equals twice the trigger voltage of the Diac 43. The explanation is that th capacitors 15 and 17 together with the diodes 18 and 20 form a voltage double circuit which multiplies the voltage on the capacitor 15 by a factor 2. O ^ W Fig. 3 shows a circuit comprising a Triac 30 connected to the mains via an electric motor 10 and terminals 12, 13, as in the embodiments of Figs. 1 and 2. A resistor 31 and a capacitor 32 are connected in parallel with the Triac. The connecting point between the resistor 31 and the capacitor 32 is connected to the gate of the Triac 30 via a Diac trigger diode 33. A series circuit comprising a capacitor 40 and one diagonal of a diode bridge 44 is connected in parallel with the capacitor 32. The opposite diagonal of the bridge 44 is connected to a series circuit comprising a resistor 34 and a capacitor 35. A resistor 36 and a series circuit of a resistor 38 and a contact 37 form two parallel branches to the capacitor 35. The contact 37 may be controlled by a pressure-responsive sensor provided in the suction system of a vacuum cleaner, not shown. The resistor 34, which has a low value (< 100 ohms) decreases the risk of generation of oscilla¬ tions in the regulating system. The resistor 36, which has a high value, allows the regulation to start with the capacitor 35 being discharged. Hence, the conduction angle of the Triac will be small and the start current of the motor 10 will take a low value. The resistor 38 limits the discharging current via the contact 37 and thereby prevents the contact from being burned. The circuit arrangement of Fig. 3 functions in principle in the same way as the circuit according to Fig. 1, and the diode bridge allows the control of the Diac also during the negative half -cycles. The embodiments described above and shown in the drawings are not intended to delimit the invention, the scope of which will be determined solely by the appending claims. For example, the invention may be applied to arrangements including other types of loads. Moreover, the means controlling the voltage on the capacitor 19 (Figs. 1 and 2) and the capacitor 35 (Fig. 3), respect¬ ively, may be modified within the scope of the invention. , gυs_£ ^ O PI

Claims: Claims 1. A circuit arrangement for controlling the AC power delivered tι via a silicon-controlled rectifier (11;30;42), a first resistor (14;31) < ' ■ capacitor 0-502) forming a series circuit which is connected in parallel said rectifier, the connecting point between the first resistor (14;31) first capacitor (15;32) being connected to the gate of the rectifier (11;30;- a trigger device (16;33;43), and a second capacitor (17;40) forming a s. circuit with means (18,19; 35,44) controlling the connection of the series cir in parallel with the first capacitor (15;32), characterized in that i means (18,19; 35,44) for controlling the connection of the second capacitc (17;40) comprise means (19;35) for applying on the second capacitor a predete mined potential equal to or less than twice the trigger voltage of the trigg device (16;33;43). 2. A circuit arrangement according to Claim 1, characterized i that the predetermined potsntial is adjustable. 3. A circuit arrangement according to Claim 1, characterized i that the pretermined potential is continuously adjustable in dependence on control parameter. 4. A circuit arrangement according to any preceding claim, character ized in that the means for applying the predetermined poteπtional comprise first diode (18) and a third capacitor (19), the third capacitor being large i comparison with the first and second capacitors (15,17), wherein the secon capacitor (17), the first diode (18) and the third capacitor (19) form a serie circuit which is connected in parallel across the first capacitor (15), the anode o the diode being connected to the second capacitor (17). 5. A circuit arrangement according to Claim 4, characterized by second diode (20), the cathode of which being connected to the anode of the firs diode (18) and the anode of which being connected to the connecting poin between the silicon-controlled rectifier (11) and the first capacitor (15), wherei the second capacitor (17) is bridged by a third diode (41), the anode of whic being connected to the connecting point between the first diode (18) and th second diode (20). 6. A circuit arrangement according to Claim 1, 2 or 3 comprising a Tria (42), characterized in that the means for applying the predetermine potential comprises a first diode (18) and a third capacitor (19) being large i comparison with the first and second capacitors (15,17), wherein the secon capacitor (17), the first diode (18) and the third capacitor (19) form a serie circuit which is connected in parallel with the first capacitor (15), the anode o the diode (18) being connected to the second capacitor (17), and a second diode (20) being provided, the cathode of which being connected to the anode of the first diode (18) and the anode of which being connected to the connecting point between the Triac (42) and the first capacitor (15). 7. A circuit arrangement according to Claim 1, 2 or 3, comprising a Triac (30), characterized in that the means for applying the predetermined potential comprises a diode bridge (44), one diagonal of which being connected in series with the second capacitor (40) and the opposite diagonal of which being connected to a circuit comprising a fourth capacitor (35), the capacitance value of which being large in comparison with the other capacitors (32,40). 8. A circuit arrangement according to Claim 7, characterized in that the fourth capacitor (35) is connected in parallel with a contact device (37) controlled by a control parameter. 9. A circuit arrangement according to Claim 8, characterized by a second resistor (38) connected in series with the contact device (37). 10. A circuit arrangement according to Claim 8 or Claim 9, character¬ ized by a third resistor (34) connected in series with the fourth capacitor (35). 11. A circuit arrangement according to any of Claims 8 - 10, character¬ ized in that the load comprises an electric motor being part of a vacuum cleaner, wherein the contact device comprises a contact (37) which is controlled by a pressure responsive sensor provided in the suction system of the vacuum cleaner.

Inventors: LINDMARK M
Applicants: LINDMARK M
Application year: 1981
Main classification: H02M5
IPC codes: H02M1