A METHOD OF CONTROLLING AN ELECTRONIC SWITCH AND AN ELECTRONIC SWITCH

Abstract: The present invention relates to a method of controlling an electronic switch and an electronic switch for carrying out the method. The electronic switch comprises an electromagnetic relay (1) and a controllable bidirectional semi-conductor switch (2), such as a triac, connected parallel with the switch (1b) of the relay and a control unit (3) for controlling the relay and the semi-conductor switch so that when connecting the load to a power source the semi-conductor switch is first turned on at the zero point of the AC voltage, subsequent to which and after a delay the switch of the relay is turned on, and when disconnecting the load from the power source, the switch of the relay is first turned off, subsequent to which and after a delay the semi-conductor switch is turned off at the zero point of the alternating current. According to the invention the control unit (3) also comprises means for turning off the semi-conductor switch (2) subsequent to turning on the switch (1b) of the relay (1) when connecting the load to a power source, and for turning on the semi-conductor switch (2) prior to turning off the switch (1b) of the relay (1) when disconnecting the load from the power source.

Description: A METHOD OF CONTROLLING AN ELECTRONIC SWITCH AND AN ELECTRONIC SWITCH The present invention relates to a method of controlling an electronic switch as defined in the pre¬ amble of claim 1. The invention also relates to an electronic switch according to the preamble of claim 2. An electronic switch comprising a relay and a parallelly arranged semi-conductor switch is previously known. This kind of switch enables the load to be con¬ nected to an electrical network without spart at the zero point of AC voltage and, accordingly, disconnect the load without spark at the zero point of AC current. When connecting the load to an electrical network this is carried out by turning the semi-conductor switch, such as a triac, on at the zero point of AC voltage and, subsequently, after a delay, the switch of the re¬ lay is turned on. Accordingly, when disconnecting the load from the network, the relay is first turned off and then, after a delay, the control of the semi-con¬ ductor switch, such as a triac, is removed, whereby the semiconductor switch disconnects the load from the electrical network, preferably at the zero point of the phase of the current. A drawback of the present electronic switch and the method of controlling thereof is that at time contact points of the switch of the relay are fouled and the voltage across the closed switch of relay is increased. This is caused by the fact that during the moment of connecting and/or disconnecting, the voltage across the contact points of the switch of the relay is only about 2 volts. This voltage is not sufficient to clean the contact points of the switch of the relay, whereby the voltage across the closed switch is increa¬ sed. The increased voltage results in that the semicon¬ ductor switch, such as a triac, conducts although the contact points of the switch of the relay are closed. This in turn leads to over-heating and destruction of the semi-conductor switch, such as a triac. The object of the invention is to disclose a new method of controlling an electronic switch and an electronic switch, by means of which the above-men¬ tioned problem can be avoided. A characterizing feature of the method of con¬ trolling an electronic switch according to the inventi- on is what is claimed in claim 1. Electronic power is connected to the load from an AC network or a corresponding AC source by means of an electronic switch and the electronic power is dis¬ connected accordingly. The electronic switch comprises an electromagnetic relay and a controllable bidirec¬ tional semi-conductor switch, such as a triac, connec¬ ted parallel with the switch of the relay. In the met¬ hod according to the invention the relay and the semi¬ conductor switch are controlled so that when connecting the load to a power source the semi-conductor switch is first turned on at the zero point of the AC voltage, subsequent to which and after a delay the switch of the relay is turned on, and when disconnecting the load from the power source, the switch of the relay is tur- ned off, subsequent to which and after a delay the semi-conductor switch is turned off at the zero point of the alternating current. According to the invention, the method also comprises turning the semi-conductor switch off subsequent to turning the switch of the re- lay on when connecting the load to a power source, and the semi-conductor switch is turned on prior to turning the switch of the relay off when disconnecting the load from the power source. The characterizing features of the electronic switch are described in claim 2. The electronic switch for connecting elect¬ ronic power from an AC network or a corresponding AC source to a load and, correspondingly, disconnecting electric power according to the invention comprises an electromagnetic relay and a controllable bidirectional semi-conductor switch, such as a triac, connected pa- rallel with the switch of the relay, and a control unit for controlling the relay and the semi-conductor switch so that when connecting the load to a power source the semi-conductor switch is first turned on at the zero point of the alternating current, subsequent to which and after a delay the switch of the relay is turned on, and when disconnecting the load from the power source, the switch of the relay is turned off, subsequent to which and after a delay the semi-conductor switch is turned off at the zero point of the alternating cur- rent. According to the invention the control unit also comprises means for turning the semi-conductor switch off subsequent to turning the switch of the relay on when connecting the load to a power source, and for turning the semi-conductor switch on prior to turning the switch of the relay off when disconnecting the load from the power source. The control unit can be realized in numerous ways as control logic that can comprise, e.g., a micro¬ processor, or as a simple non-intelligent circuit as- sembled of suitable components. The control unit itself is controlled by means of a simple on/off-switch. An embodiment of the electronic switch comp¬ rises an optical switch indicating the zero phase angle for controlling the semi-conductor switch. Advanta- geously this kind of an optical switch combines in the same component an AC voltage phase angle indicator and a switch, which additionally is optically controlled. This kind of component enables the number of components used in the electronic switch to be decreased. An embodiment of the electronic switch comp¬ rises a time constant circuit, advantageously a resis¬ tor-capacitor (RC) circuit, a reference voltage source for forming three reference voltages and a comparison unit for comparing the control voltage coming via the time constant circuit with the reference voltages and for carrying out the turning of the switch of the relay and the semi-conductor switch on and off in correct time. DC voltage is advantageously used as control voltage, the voltage being connected to the control terminal of the control unit when it is desired to connect the load to electrical network by means of the electronic switch. Thus, the time constant circuit causes the internal control voltage to increase to near the maximum level of the control voltage during the determining time constant of the circuit and to exceed the levels the reference voltages. When the internal control voltage exceeds the predetermined reference voltage levels the semi-conductor switch is first tur¬ ned on, thereafter the switch of the relay is turned off and finally the semi-conductor switch is turned off. Accordingly, disconnecting the control voltage causes a reverse sequence. In one embodiment of the electronic switch the comparison unit comprises three voltage comparators, the first and third of which are used for controlling the semi-conductor switch and the second of which is used for controlling the relay. In one embodiment of the electronic switch the first voltage comparator comprises a field effect tran¬ sistor, the threshold voltage of which is used as the first reference voltage, with which the control voltage coming from the time constant circuit is compared for turning the semi-conductor switch on when the load is connected to a power source and for turning the semi¬ conductor switch off when disconnecting the load from the power source, the third voltage comparator being connected in front of the field effect transistor in series therewith for turning the semi-conductor switch off subsequent to the load having been connected via the switch of the relay to the power source and for turning the semi-conductor switch on prior to discon¬ necting the switch of the relay and the load from the power source. Thus, the first voltage comparator is utilized for turning the semi-conductor switch on, while the next, third voltage comparator is utilized for correspondingly turning the semi-conductor switch again off after the relay has been turned on and the load has been connected via the switch of the relay to the electrical network. In one embodiment of the electronic switch the reference voltage source comprises a resistor chain for forming the first and third reference voltages from a predetermined DC voltage. This is a simple and effi- cient way of realizing a number of desired reference voltages. An advantage of the invention is that it can replace the relay switch and thereby avoid the disad¬ vantages caused by the use of the relay. Further, an advantage of the invention is that the electronic switch comprises a controllable bidirectional semi¬ conductor switch, such as a triac, connected parallel with the switch of the relay, the semi-conductor switch being kept turned on only during changes, i.e. when the switch is being turned on and off. Thereby the semi¬ conductor switch, such as a triac, will not overheat and the self-cleaning of the contact points of the switch of the relay will be accomplished in a previous¬ ly known way. In the following, the invention is described in more detail by way of reference to the following pictures, of which Fig. 1 illustrates a simple block diagram of the electronic switch; Fig. 2 illustrates the control signals of the electronic switch of Fig. 1 and the network voltage acting across the load; Fig. 3 illustrates, in the form of a cleft diagram, another electronic switch; Fig. 4 illustrates voltage levels and changes at different points of the electronic switch of Fig. 3; and Fig. 5 illustrates, in the form of a circuit diagram, an embodiment of the electronic switch. The electronic switch of Fig. 1 comprises an electromagnetic relay 1 having a control coil la and a switch lb. A controllable bidirectional semi-conductor switch 2, such as a triac, is connected parallel with the switch lb of the relay. The terminals of. the elect¬ ronic switch are marked by LI and L2. One terminal is connected to the electrical network and the other to load (not shown) . The control unit 3 is utilized for controlling the electronic switch and especially the relay 1 and the semi-conductor switch 2. The electronic switch is controlled as fol¬ lows. Reference is made to Fig. 2. The relay 1 of the electronic switch and the semi-conductor switch 2 are controlled by means of the control unit 3 so that the semi-conductor switch 2 is first turned on by a control signal To at point ta of the zero point of a phase of the network current. At this point the load is connec- ted to the AC network. After a delay, at point tc, the control coil lb of the relay is fed the control signal Ro, whereby the switch lb of the relay turns on. The semi-conductor switch 2 is turned off at point tc, i.e. after the switch lb of the relay 1 has been turned on, by interrupting the feed of the control signal To. Thus the alternating current flows to the load first via the conducting semi-conductor switch, but after the relay has been turned on and the semi-conductor switch has been turned off, and after the point tb the current flows only via the switch lb of the relay 1. When the load is disconnected from the AC net¬ work the semi-conductor switch 2 is first turned on by feeding it the control signal To at point td, subse¬ quent to which and after a delay the control signal Ro is removed at point te from the control coil la of the relay 1, whereby the switch lb of the relay is turned off. After this, at point tf, the semi-conductor switch 2 is turned off at the zero point of the phase of the network current by removing the control signal To. The load is simultaneously disconnected from the electrical network. The network current to the load thereby flows first via the switch lb of the relay 1, then via the switch of the relay and the semi-conductor switch, and, finally, only via the semi-conductor switch prior to disconnecting the load from the electrical network. The network voltage UL acts across the load from point ta to point tf. Another electrical switch according to the in¬ vention is disclosed in Fig. 3. In this figure, the same reference numbers are used to denote the same parts as in Fig. 1. The electronic switch also comp- rises an electromagnetic relay 1 having a control coil la and a switch lb. A controllable bidirectional semi¬ conductor switch 2, such as a triac, is connected in parallel with the switch lb of the relay. The terminals of the electronic switch are marked by LI and L2, as previously in Fig. 1. One terminal is connected to the electrical network and the other to load (not shown) . The control unit 3 is utilized for controlling the electronic switch and especially the relay 1 and the semi-conductor switch 2. In the embodiment of Fig. 3 the control unit 3 comprises an optical switch 4 indicating the zero phase angle for controlling the semi-conductor switch 2. The optical switch 4 comprises a light emitter 4a, such as an LED, a light indicator 4b and a zero phase angle in- dicator 4c. The control unit 3 further comprises a time constant circuit 5, such as an RC circuit, a reference voltage source 6 for forming three reference voltages Ul, U2, U3 and a comparison unit 7 comprising voltage comparators 8, 9, and 10. The voltage comparators 8, 9, and 10 of the comparison unit 7 are utilized for compa¬ ring the control voltage Uin coming via the time cons- tant circuit 5 with the reference voltages Ul, U2 and U3. The first and second voltage comparators 8, 10 are utilized for controlling the semi-conductor switch 2 via the optical switch 4, and the second voltage compa¬ rator 9 is utilized for controlling the relay 1, i.e. the switch lb of the relay via the control coil la. In Fig. 3, the electronic switch according to the invention operates as follows. Reference is also made to Fig. 4. The control voltage Uo is connected to the control input Uin of the control unit 3 at point to. The control voltage Uo is a DC voltage of a sui¬ table level, small in proportion to the network volta¬ ge. In the output of the time constant circuit 5 the circuit effects a relatively slow increase of the in¬ ternal control voltage Uos. One of the inputs of each of the voltage comparators 8, 9, and 10 of the voltage comparison unit 7 is fed the internal control voltage Uos which is compared in the first voltage comparator 8 to the voltage Ul and in the second voltage comparator 9 to the voltage U2, which is higher than Ul, and in the third voltage comparator 10 to the voltage U3, which is higher than U2. When the internal control voltage Uos is increased to level Ul at time tl the output voltage Uol of the voltage comparator 8 is inc¬ reased to a predetermined level and it gives a control voltage to the optical switch 4. The light emitter 4a starts to emit optical radiation which is detected by its light indicator 4b. The zero phase indicator 4c detects the next zero phase of the network voltage Uv at point t2 and then connects a suitable control to the input of the semi-conductor switch 2, whereby the semi¬ conductor switch is turned on. Thus, current starts to flow through the semi-conductor switch 2, and the load is connected to the electrical network and the network voltage Uk acts across the load. The internal control voltage Uos still increa¬ ses constantly and reaches the reference voltage U2 at point t3. Thereby the control voltage Uo2 is received at the output of the voltage comparator 9 and control current is fed to the control coil la of the relay 1. This causes the relay 1 to pull and its switch lb to close. Thereby both the switch lb and the semi-conduc- tor switch 2 are turned on. The internal control voltage Uos still increa¬ ses and reaches the reference voltage U3 at point t4. Thereby the voltage comparator 10 gives the control voltage Uo3 to the optical switch 4. The control volta- ge Uo3 reverses the control voltage Uol of the optical switch 4, whereby the light emitter of the optical switch 4 ceases to emit optical radiation and the semi¬ conductor 2 no more receives a control signal to its control input. Semi-conductor switch 2 is turned off at point t4. Thereby current flows to the load only via the switch lb of the realy 1. When disconnecting the load from the network the control voltage Uo being fed to the control unit 3 is interrupted and the control terminal Uin is connec- ted to the ground. In Fig. 4 this is effected at point t5. Immediately after this the time constant circuit 5 causes the internal control voltage Uos to decrease towards zero. At point tβ the control voltage Uos dec¬ reases below the reference voltage U3 which causes the output voltage Uo3 of the voltage comparator 10 to drop and the optical switch 4 regains its control Uol, which causes the semi-conductor switch 2 to be turned again on at point tβ. The internal control voltage Uos still decreases and at point t7 it is equal to the reference voltage U2, which causes the output voltage Uo2 of the voltage comparator 9, i.e. the control of the relay, to drop to zero. This causes the switch lb of the relay 1 to open. The load is still, nevertheless, connected to the electrical network via the semi-conductor switch 2. The internal control voltage Uos still decrea- ses and reaches the reference voltage Ul at point t8. The voltage Uol that acted at the output of the voltage comparator 8 is reset which causes the light emitter 4c of the optical switch 4 be turned off. Nevertheless, the combination of the zero point detector 4c and semi- conductor switch 2 are still on and stops conducting at the next zero point t9 of the load current Iv. Thereby the semi-conductor 2 is turned off, whereby also the load is disconnected from the electrical network. The electronic switch is in its initial stage and ready to be re-connected. Fig. 5 discloses a circuit diagram for an electronic switch according to the invention. In this figure the same reference numbers are used for the same parts as in figures 1 and 3. The time constant circuit 5 is realized by means of a resistor 5a and a capacitor 5b. The first voltage comparator 8 comprises a field effect transistor 11, the threshold voltage Uh of which is utilized as the first reference voltage Ul. The third voltage comparator 10 is connected in series with the field effect transistor 11, on the input side of the transistor. The reference voltage source 6 comp¬ rises both a DC voltage source from which a predeter¬ mined DC voltage Uc is received and a chain of resis¬ tors 12, 13, 14 for forming the second and third refe- rence voltages U2, U3. The electronic switch of Fig. 5 functions as was disclosed in connection with Figs. 1 and 3. The following is nevertheless stated in short, with refe¬ rence also to Fig. 4. When the control voltage Uo is connected to the input of the control unit 3, the ca¬ pacitor 5b starts to charge and its voltage Uos starts to increase. When the voltage Uos of the capacitor increases so as to exceed the threshold voltage Uh = Ul of the field effect transistor 11, the field effect transistor 11 is turned on. The optical switch 4 re¬ ceives its control and it fires, i.e. turns on the semi-conductor switch 2 at the zero point of the next phase of the network voltage. When the voltage Uos of the capacitor 5b further increases to the level of the reference voltage U2 the relay 1 is turned on. When the voltage Uos of the capacitor 5b has increased to the level of the reference voltage U3, the control of the optical switch 4 is removed and the semi-conductor switch 2 is turned off, i.e. it is disconnected. The load is then connected to the electrical network via the switch lb of the relay 1. Accordingly, when the control voltage Uo is connected to zero, the voltage Uos of the capacitor 5b starts to decrease and the above-disclosed connecting sequence (cf. Fig. 4) is repeated in reversed order. The invention is not limited to the above-men- tioned embodiment, but many modifications thereof are possible within the scope of the inventive step defined in the appended claims.

Claims: CLAIMS 1. A method of controlling an electronic switch, by means of which electronic power is connected to the load from an AC network or a corresponding AC source and the electronic power is disconnected accor¬ dingly, the electronic switch comprising an electromag¬ netic relay (1) and a controllable bidirectional semi¬ conductor switch (2), such as a triac, connected pa- rallel with the switch (lb) of the relay, in which method the relay (1) and the semi-conductor switch (2) are controlled so that when connecting the load to a power source the semi-conductor switch is first turned on at the zero point of the AC voltage, subsequent to which and after a delay the switch of the relay is first turned on, and when disconnecting the load from the power source, the switch of the relay is turned off, subsequent to which and after a delay the semi¬ conductor switch is turned off at the zero point of the alternating current, characterized in that the method also comprises turning the semi-conductor switch (2) off subsequent to turning the switch (lb) of the relay (1) on when connecting the load to a power source, and the semi-conductor switch (2) is turned on prior to turning the switch (lb) of the relay (1) off when dis¬ connecting the load from the power source. 2. An electronic switch for connecting elect¬ ronic power from an AC network to a load and, corres¬ pondingly, disconnecting electronic power comprising an electromagnetic relay (1) and a controllable bidirec¬ tional semi-conductor switch (2), such as a triac, connected parallel with the switch (lb) of the relay (1), and a control unit (3) for controlling the relay and the semi-conductor switch so that when connecting the load to a power source the semi-conductor switch is first turned on at the zero point of the AC voltage, subsequent to which and after a delay the switch of the relay is turned on, and when disconnecting the load from the power source, the switch of the relay is first turned off, subsequent to which and after a delay the semi-conductor switch is turned off at the zero point of the alternating current, characterized in that the control unit (3) also comprises means for turning the semi-conductor switch (2) off subsequent to turning the switch (lb) of the relay (1) on when connecting the load to a power source, and for turning the semi-con- ductor switch (2) on prior to turning the switch (lb) of the relay (1) off when disconnecting the load from the power source. 3. An electronic switch according to claim 2, characterized in that the control unit (3) comprises an optical switch (4) indicating the zero phase angle for controlling the semi-conductor switch (2). 4. An electronic switch according to claim 2 or 3, characterized in that the control unit (3) comp¬ rises a time constant circuit (5), advantageously a re- sistor-capacitor (RC) circuit, a reference voltage source (6) for forming three reference voltages (Ul, U2, U3) and a comparison unit (7) for comparing the control voltage (Uin) coming via the time constant cir¬ cuit (5) with the reference voltages (Ul, U2, U3) and for carrying out the turning of the switch (lb) of the relay (1) and the semi-conductor switch (2) on and off in correct time. 5. An electronic switch according to claim 4, characterized in that the comparison unit (7) comprises three voltage comparators (8, 9, 10), the first (8) and third (10) of which are used for controlling the semi¬ conductor switch (2) and the second (9) of which is used for controlling the relay (1). 6. An electronic switch according to claim 5, characterized in that - the first voltage comparators (8) comprises a field effect transistor (11), the threshold voltage (Uh) of which is utilized as the first reference voltage (Ul), with which the control voltage (Uin) from the time constant circuit (5) is compared for turning the semi¬ conductor switch (2) on when connecting the load to a power source and for turning it off when disconnecting the load from the power source; and - the third voltage comparator (10) is connected in front of the field effect transistor (11), in series therewith for turning the semi-conductor switch (2) off after the load has been connected via the switch (lb) of the relay to the power source and for turning the semi-conductor switch (2) on prior to disconnecting the switch (lb) of the relay and the load from the power source. 7. An electronic switch according to claim 4, 5 or 6, characterized in that the reference voltage source (6) comprises a resistor chain (12, 13, 14) for forming the second and third reference voltages (U2, U3) from a predetermined DC voltage (Uc) .

Inventors: RIUTTALA RAIMO, SAIRANEN MARTTI, RIUTTALA, RAIMO, SAIRANEN, MARTTI
Applicants: AHLSTROEM OY, A. AHLSTROM CORPORATION
Application year: 1993
Main classification: H01H9
IPC codes: H01H9