BIRRELL, Peter (8 Brindisi Place, Avalon, New South Wales 2107, AU)
THE CLAIMS
1. A switching circuit comprising: a) a thermally actuatable first switching device arranged to be connected in series with a load, b) a solid state second switching device connected in series with the first switching device, c) a control circuit connected and arranged to effect biasing of the second switching device to a condition whereby heat is generated in the switching circuit, and wherein the first switching device is arranged to be actuated thermally in response to heat being generated in the switching circuit.
2. The switching circuit as claimed in claim 1 wherein the second switching device comprises an element that dissipates and thereby generates said heat when biased to conduct at a predetermined level.
3. The switching circuit as claimed in claim 1 wherein a heat dissipating element is connected in circuit with the second switching device and is arranged to dissipate and thereby generate said heat when the second switching device is biased to conduct at a predetermined level equal to or greater than zero.
4. The switching circuit as claimed in claim 3 wherein the heat dissipating element comprises a solid state device.
5. The switching circuit as claimed in claim 4 wherein the heat dissipating element comprises a non-linear breakdown device connected in parallel with the second switching device.
6. The switching circuit as claimed in claim 2 wherein a heat conductor interconnects the first and the second switching devices and is arranged to conduct heat from the second to the first switching device.
7. The switching circuit as claimed in claim 2 wherein the second switching device is mounted in thermal contact with the first switching device whereby heat dissipated by the second switching device will conduct from the second to the first switching device.
8. The switching circuit as claimed in claim 3 wherein a heat conductor interconnects the heat dissipating element and the first switching device and is arranged to conduct heat from the heat dissipating element to the first switching device.
9. The switching circuit as claimed in claim 3 wherein the heat dissipating element is mounted in thermal contact with the first switching device whereby heat dissipated by the heat dissipating element will conduct from the heat dissipating element to the first switching device .
10. The switching circuit as claimed in any one of the preceding claims wherein the first switching device comprises a bimetallic switching device.
11. The switching circuit as claimed in claim 10 wherein the first switching device comprises a bistable bimetallic switching device.
12. The switching circuit as claimed in any one of the preceding claims wherein the second switching device comprises a low impedance solid state device.
13. The switching circuit as claimed in claim 12 wherein the second switching device comprises a metal-oxide semi-conductor field effect transistor (MOSFET) device.
14. The switching circuit as claimed in any one of the preceding claims wherein the control circuit comprises a microcontroller.
15. The switching circuit as claimed in any one of the preceding claims wherein a solid state, heat dissipating, third switching device is connected in circuit with the control circuit, wherein the third switching device is arranged to couple dissipated heat to the first switching device, and wherein the control circuit is connected and arranged to effect biasing of the third switching device to a condition whereby heat is dissipated by the third switching device.
16. The switching circuit as claimed in claim 15 wherein the third switching device comprises a triac.
17. The switching circuit as claimed in claim 15 or claim 16 wherein the first switching device is arranged to be actuated thermally in a first direction in response to heat being dissipated by the second switching device and in a second direction in response to heat being dissipated by the third switching device.
18. The switching circuit as claimed in claim 17 wherein the first switching device is arranged to be actuated thermally to an OFF condition in response to heat being dissipated by the second switching device and to an ON condition in response to heat being dissipated by the third switching device.
19. A switching circuit comprising first and second electrically parallel limbs and a control circuit; the first limb comprising a) a thermally actuatable first switching device arranged to be connected in series with a first load, and b) a solid state second switching device connected in series with the first switching device of the first limb; the second limb comprising c) a thermally actuatable first switching device arranged to be connected in series with a second load, and d) a solid state second switching device connected in series with the first switching device of the second limb; the control circuit being connected to, and arranged to effect biasing of, the second switching device in each of the first and second limbs to a condition whereby heat is generated in the second switching device; and wherein the first switching device in each of the first and the second limbs is arranged to be actuated thermally in response to heat being generated in the second switching device in the respective limbs.
20. The switching circuit as claimed in claim 19 wherein the first switching devices in the first and second limbs are mechanically coupled and arranged to perform a changeover operation.
21. The switching circuit as claimed in claim 19 or claim 20 wherein a solid state, heat dissipating, third switching device associated with each of the two limbs is connected in circuit with the control circuit, wherein a first of the third switching devices is arranged to couple dissipated heat to the first switching device in the second limb, wherein a second of the third switching devices is arranged to couple dissipated heat to the first switching device in the first limb, and wherein the control circuit is connected and arranged to effect biasing of respective ones of the third switching device to a condition whereby heat is dissipated by the third switching device.
22. The switching circuit as claimed in claim 21 wherein the third switching device comprises a triac.
23. The switching circuit as claimed in claim 21 or claim 22 wherein the first switching device in each of the first and second limbs is arranged to be actuated thermally in a first direction in response to heat being dissipated by the second switching device in each of the first and second limbs respectively and in a second direction in response to heat being dissipated by the third switching device in the second and first limbs respectively,
24. The switching circuit as claimed in any one of the preceding claims wherein the second switching device comprises two series connected solid state switching devices, each of which is connected in circuit with the control circuit, each of which is arranged to be biased to a condition to dissipate heat and each of which is thermally coupled to the first switching device.
25. The switching circuit as claimed in claim 24 wherein the two solid state switching devices comprise source-to-source connected MOSFET devices.
26. The switching circuit as claimed in any one of the preceding claims wherein the control circuit is programmed to effect biasing of the second switching deviee(s) responsive to a program function.
27. The switching circuit as claimed in any one of the preceding claims wherein the control circuit is programmed to effect biasing of the second switching device(s) responsive to a predetermined input signal.
28. A controllable switching device incorporating a switching circuit as claimed in any one of the preceding claims. |
CONTROLLED SWITCHING FIELD OF THE INVENTION
This invention relates to an electrical circuit that provides for controlled two-wire switching of a load. The invention is hereinafter described predominantly in relation to load switching in circuits in which access is available only to active and switched-active conductors in circuit with the load, but it will be understood that the invention may equally have application to load switching in circuits where access is available only to neutral and switched-neutral conductors.
BACKGROUND OF THE INVENTION
In many single phase ac circuits the neutral conductor is connected directly to a load and the active conductor only is switched, this establishing active and switched-active conductors. A typical such circuit is employed in electric lighting when, for example, above-ceiling neutral connections are made directly to ceiling mounted lamps and active-only switching of the lamps is provided at accessible wall locations. However, an inherent problem with such circuits, where access to the neutral conductor is not conveniently available, is that controlled switching (for example timed-out or remote switching) is not generally feasible.
Australian Patent 2002214803, dated 9 November 2001, discloses an electrical circuit and identifies earlier prior art circuits that facilitate two-wire controlled switching of a load. The circuit of the referenced Patent employs two series-connected switching devices, an electrically actuated first switching device (typically a relay) and a solid state second switching device. First and second energy storage devices, typically capacitors, are provided for delivering actuating power to the first switching device and gating energy to the second switching device, and control circuitry is provided to enable controlled charging of the storage devices during portions of successive cycles of the ac supply.
The electrical circuit disclosed in the referenced Patent provides advantages over the various earlier prior art circuits. However, the advantages are achieved by the use of relatively large components, including an electro-mechanical relay and electrolytic capacitors. This results in a relatively large (and, in some embodiments, an unacceptably large) product "package"; a problem that would be exacerbated if "two-way" switching (requiring two switched-active conductors) were to be employed. Another problem with this prior art circuit is that the life expectancy of electrolytic capacitors is relatively short, particularly when operated at elevated temperatures.
The present invention has been developed in an attempt to meet a continuing demand for controlled switching (typically where a neutral conductor is not conveniently available) but with an alternative, potentially more-space-efficient, circuit.
SUMMARY OF THE INVENTION
Broadly defined, the present invention provides a switching circuit that comprises: a) a thermally actuatable first switching device which is arranged to be connected in series with a load, b) a solid state second switching device connected in series with the first switching device, c) a control circuit arranged when activated to bias the second switching device to a condition whereby heat is generated in the switching circuit, and wherein the first switching device is arranged to be actuated thermally in response to heat being generated in the switching circuit.
The invention may also be defined as providing a controllable switching device incorporating the above defined switching circuit.
The heat may be generated (in the switching circuit) by a heat dissipating element that is located in circuit with the second switching device; for example by a heat dissipating element in the form of a solid state device, such as a non-linear breakdown device, or a resistor, connected in parallel with the second switching device. However, in the interest of circuit simplification, the heat may be generated in the second switching device, in which case the second switching device per se will comprise the heat dissipating element.
In either case, the generated/dissipated heat may optionally be conducted (from the heat dissipating element) to the first switching device by way of a heat conductor or by direct thermal coupling. In the latter (preferred but non-essential) case the heat dissipating element may be mounted in direct thermal contact with the first switching device.
The first switching device optionally comprises or incorporates a bimetallic switching element, desirably a bistable bimetallic switching element, but it might alternatively comprise any other switching device that is arranged to switch between OFF and ON conditions with changes in temperature. For example, the switching device might comprise a heat sensitive solid state switching device, a re-settable fusible device incorporating a eutectic alloy, or an expanding fluid (gas or liquid) actuator. Also, the first switching device may optionally incorporate provision for a manual switching operation.
The second switching device may comprise at least one low impedance device, desirably (but not essentially) one that causes a voltage drop which is not greater than about 500 mV rms with a current flow of about 10 amps rms. The or, if more than one, each device may comprise a metal-oxide semi-conductor field effect transistor (MOSFET) device.
Activation of the control circuit (to effect biasing of the second switching device and consequential heat generation/dissipation) may optionally be effected in any one of a number of ways; for example, responsive to a program function (such as a programmed time-out setting) or responsive to an input signal to the control circuit. In the latter case the input signal might be derived, for example, as a power line modulated signal, a radio frequency or infra-red signal, or from a light, sound, temperature, power-level-consumption or proximity sensor. Also, the control circuit may comprise discrete circuit components, an integrated circuit or a micro-processor.
The first switching device may optionally be arranged to be connected to the load by way of a switched-active conductor of an ac circuit, in which case the second switching device will be arranged to connect in the ac circuit by way of an active conductor. However, the connecting arrangements may be reversed. Also, whilst the switching circuit in accordance with the present invention will more normally be employed in series with active and switched-active conductors in the absence of an available neutral conductor, the switching circuit will have equal application in a circuit using neutral and switched-neutral conductors in the absence of an available active conductor.
Also, the switching circuit as above defined may be arranged to provide intrinsic over-load circuit protection. When so arranged, if the load current exceeds a pre-determined level then the resultant heat generation/dissipation will cause the first switching device to trip.
The switching circuit, by controlling power delivered to a load, may make use of the (variable) power to provide a visual alert to a user prior to the occurrence of a switching function. Furthermore, a "master" switching device in an installation comprising a group of "slave"
switches may function to modulate the power to a load, so effecting generation of local power line signalling to the slave switches.
The invention will be more fully understood from the following drawing- related description of illustrative embodiments of a switching circuit that provides for controlled energisation of a load.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings- Figure 1 shows a block diagrammatic representation of a first embodiment of the switching circuit, one that provides for controlled single pole OFF switching operation,
Figure 2 shows a schematic representation of a realisable form of the switching circuit of Figure 1, Figure 3 shows a block diagrammatic representation of a second embodiment of the switching circuit, one that provides for controlled single pole ON/ OFF switching operations,
Figure 4 shows a schematic representation of a realisable form of the switching circuit of Figure 3, Figure 5 shows a block diagrammatic representation of a third embodiment of the switching circuit, one that provides for controlled single pole change-over switching operation,
Figure 6 shows a schematic representation of a realisable form of the switching circuit of Figure 5, Figure 7 shows a block diagrammatic representation of a fourth embodiment of the switching circuit, one that provides for controlled single pole change-over ON/ OFF switching operation,
Figure 8 shows a schematic representation of a realisable form of the switching circuit of Figure 7, Figure 9 shows a block diagrammatic representation of a fifth embodiment of the switching circuit, one that also provides for controlled single pole change-over switching operation but which
provides for symmetrical conduction during both positive and negative half-cycles, and
Figure 10 shows a schematic representation of a realisable form of the switching circuit of Figure 9.
DETAILED DESCRIPTION QF EXEMPLARY EMBODIMENTS OF THE INVENTION
As illustrated in Figure 1 the switching circuit 10 effectively comprises a single pole switch that may be activated by way of a control circuit 11 to perform an OFF operation following prior manual actuation (or separate switching) to a closed state.
The switching circuit 10 as shown in Figure 1 and subsequent figures will typically comprise or be incorporated in a controllable switching device having a plastics material body and mounting plate and, thus, the numeral 10 may also be regarded as designating a complete such controllable switching device.
The switching circuit 10 includes a thermally actuatable first switching device 12 and a solid state second switching device 13 connected in series with the first switching device, and the two switching devices are, as shown, connected in series with an external load 14 in a single phase circuit having active (A) and neutral (N) conductors. The active and neutral conductors and the load terminals may be located above the ceiling (not shown) of a building room, typically when the load 14 comprises a lamp.
The first switching device 12 comprises a bistable switch having an active (moveable) contact 12a mechanically coupled to a bimetal actuator. The second switching device 13 comprises a MOSFET device having its gate connected to the control circuit 11. As also described below with reference to Figure 2, when the switching circuit 10 is to be
actuated to an OFF state (for example under a time-out function programmed into the control circuit 11), the gate drive to the second switching device 13 is controlled to cause the second switching device to operate at a level that consumes a high (but less than destructive) level of power, whereby it is caused to generate (i.e., dissipate) heat at a level sufficient to cause actuation of the first switching device 12. The second switching device 13 is itself caused to function as a heat dissipating element, and the dissipated heat is coupled into the first switching device 12, to cause that device to snap open and so to break the series load circuit.
The generated heat may be conducted from the second switching device/ heat dissipating element 13 to the first switching device 12 by way of a heat conductor 15 (as shown diagrammatically in Figure 1) or, more preferably, by direct thermal coupling. In the latter case the second switching device 13 may be mounted in direct thermal contact with the first switching device 12, for example by engaging the first switching device with a heat sink (i.e., heat conductor 15) to which the second switching device is mounted.
Figure 2 shows a possible implementation of the switching circuit of Figure 1 and in which the second switching device 13 comprises a MOSFET having a low-on-resistance so as to not dissipate significant heat when in the saturated ON state. Zener diode 16 provides over- voltage protection for the MOSFET 13.
When the first switching device 12 is actuated (e.g., manually) to its closed state, diode 17 conducts a small amount of power into storage capacitor 18 for a short time at the beginning of every positive half-cycle of the mains supply. This power is determined to maintain continuous operation of the control circuit 11 during whatever time the first switching device is closed.
The control circuit 11 as illustrated in Figures 2, 4 and 8 comprises a PIClOF series microcontroller, whereas that which is illustrated in Figure 6 comprises a PIC 12F series microcontroller and that which is illustrated in Figure 10 comprises a PIC 16F series microcontroller. In relation to the switching circuit shown in figures 1 and 2, the control circuit 11 provides a gating signal to the second switching device 13 via resistor 19 whereby the second switching device is held in the saturated ON state for most of the positive half-cycle and all of the negative half- cycle of each cycle of the supply (during which the first switching device is closed), and is turned OFF only for the period at the beginning of each positive half-cycle during which the capacitor 18 is charging.
For operation of the switching circuit, feedback for mains cycle timing is provided by resistor 20, and resistor 21 provides supplementary feedback of the voltage drop across the second switching device 13 and, hence, a measure of the load current. From the measure of the load current and the ON state saturation resistance of the second switching device, a derivation may be made in the control circuit 11 as to the point in the linear region at which the second switching device should be biased for heat dissipation at a level determined to effect opening of the first switching device 12.
As indicated previously, the gate drive level to the second switching device 13 is determined to shift operation of the device from the saturated ON state to a point in its linear range where it produces sufficient dissipating heat loss to cause actuation of the first switching device 12 in the manner previously described.
A significant feature of the above described circuit is that, when in the OFF state, the load 14 is completely isolated and no power is consumed in the control circuit. This is to be contrasted with other (prior art) two- wire switching circuits (referred to previously) in which some quiescent
current is drawn by the control circuit through the load, whether the switch be open or closed.
Figures 3 and 4 show a second embodiment of the switching circuit 10 and one which effectively comprises a single pole switch that may be activated for ON/ OFF operation by way of the control circuit 11. The switching circuit is similar in configuration and operation to that shown in Figure 1 and like reference numerals are employed to identify like components. However, in the second embodiment, a solid state third switching device 22 is provided to actuate the first switching device 12 to a closed state, following its prior actuation to an open state, (or vice versa) by controlled gating of the second switching device 13.
When the switching circuit 10 is to be actuated to a closed state, in this embodiment by the control circuit 11 rather than manually, the gate drive to the third switching device 22 is controlled to gate the device into conduction and, with sufficiently high current through the third switching device 22 it will dissipate heat at a level sufficient to cause actuation of the first switching device 12 to the closed state. Thus, in this embodiment the third switching device 22 is also caused to function as a heat dissipating element, and the dissipated heat is coupled to the first switching device 12 by a heat conductor 23, causing that device to snap closed and provide a parallel path for load current.
Gate drive to the third switching device 22 may also be controlled by the control circuit 11 to provide for phase control of the power to the load. Thus, the switching circuit 10 may be controlled to function as a dimmer control switch.
Figure 4 shows a circuit implementation of the switching circuit 10 of Figure 3 and in which the third switching device 22 comprises a triac.
The circuit is similar to that shown in Figure 2 and control that is exercised to actuate the first switching device 12 to an OFF condition is substantially the same as that described with reference to Figure 2. The following description is applicable to actuation of the first switching device 12 to an ON condition.
Resistors 24 and 25, combined with diode 26, supply power to the control circuit storage capacitor 18 when the first switching device 12 is in its OFF state. When current is to be delivered to the load 14 (as determined by a program function of the control circuit 11 or by a relevant input to the control circuit), the control circuit turns on transistor 27 via resistor 28. This causes current to be drawn from the gate of the triac 22 via resistor 29, triggering the triac into conduction; and this triggering is repeated during both positive and negative half- cycles of the supply for sustained current flow to the load via the triac 22 and the second switching device 13. Should the load current be sufficiently low as to be carried by the triac 22 alone, then no further change will occur in the switching circuit until gating of the triac is discontinued. However, should the load current demand be sufficiently high, heat dissipated by the triac 22 and conducted to the first switching device 12 via the heat conductor 23 will cause the first switching device to be actuated to its closed state to establish a parallel load current path.
Figures 5 and 6 show a third embodiment of the switching circuit 10 and one which effectively comprises a single pole change-over switch that may be activated for OFF operation (in similar manner to the first embodiment) from either an open or a closed state by way of the control circuit 11.
The switching circuit is similar in configuration and operation to that shown in Figures 1 and 2 but it incorporates parallel (A and B) sub-
circuits and like reference numerals are coded accordingly to identify- like components. Also, the two first switching devices 12A and 12B, which respectively may be actuated into either the open or closed state manually, are mechanically coupled by linkage 30 so as to perform a change-over operation (equivalent to a change-over switch) when actuated.
Additionally, the respective pairs of first and second switching devices 12A-13A and 12B-13B are thermally coupled and operation of each of the sub-circuits A and B is identical to the above described operation of the switching circuit shown in Figures 1 and 2. The mechanical coupling (by linkage 30) of the two first switching devices 12A andl2B ensures that one only of the two current paths (through the two second switching devices 13A and 13B) is active at any one time. Also, by the existence of signals in either the A or B sub-circ!uits, The control circuit 11 may, as required, exercise control over the appropriate second switching device 13 A or 13B.
Figures 7 and 8 show a fourth embodiment of the switching circuit 10 and one which effectively comprises a single pole change-over switch that may be activated for ON and OFF operation (in similar manner to the second embodiment) from either an open or a closed state by way of the control circuit 11,
The switching circuit in this embodiment is similar in configuration and operation to that shown in Figures 3 and 4 but, as in the case of the third embodiment, it incorporates parallel (A and B) sub-circuits and like reference numerals are coded accordingly to identify like components. The two first switching devices 12A and 12B are mechanically coupled so as to perform a change-over operation
(equivalent to a change-over switch) and the respective pairs of first and second switching devices 12A-13A and 12B-13B are thermally coupled,
as in the case of the embodiment shown in and described with reference to Figures 5 and 6. However, in this case "cross-over" thermal couplings 3 IA and 3 IB are made between the triac 22A and the first switching device 12B and between the triac 22B and the first switching device 12A.
Figures 9 and 10 illustrate a fifth embodiment of the switching circuit 10 and one that effectively comprises a single pole change-over switch that may be activated for OFF operation from either state by way of the control circuit 11. However, unlike previously described embodiments, the second switching device 13 in this case comprises two series source-connected MOSFETS 13a and 13b and the second switching device thus provides for symmetrical conduction in both positive and negative half-cycles of the supply.
The switching circuit of Figures 9 and 10 is similar in configuration and operation to that shown in and described with reference to Figures 5 and 6, but it provides for symmetrical conduction in both positive and negative half-cycles of the supply. The first switching device 12 is arranged to be operated in either direction from the on state to the off state by the control circuit 11 applying an appropriate biasing signal to the second switching device 13. Various arrangements (not illustrated) may be employed to provide for dual direction bi-stable mechanical operation of the first switching device 12 as a result of the heating via thermal conduction 15 from the second switching device 13. For example, two bi-metal discs positioned back-to-back and operating lever contacts or a single bi-metal disc operating in a transverse direction through a dual cam arrangement might be employed for this purpose.
The switching circuit of Figures 9 and 10 may be employed in multi-way switching of a load and, when employing the switching circuit to adapt a pre-existing typical multi-way switching installation, may be
connected with a manually operable change-over switch 31. Then, only one of the pre-existing bi-stable switches would need to be replaced with the circuit of Figures 9 and 10 in order to provide for full OFF control of the installation.
Also, with the employment of the two MOSFETS 13a and 13b as the second switching device and dissipation of power in both the positive and negative half-cycles, a higher power level will be dissipated for a given load during biasing of the MOSFETS by the control circuit 11. This results in greater symmetry of current in the load as well as a wider range of loads that can be switched by the device. A single bidirectional over-voltage breakdown device 16 is employed to protect the two MOSFETS 13a and 13b.
A further advantage of using the two MOSFETS 13a and 13b is that power is supplied to storage capacitor 18 via the dual diode 17 at the beginning of every positive and every negative half-cycle. Thus power is supplied twice per cycle and so a smaller amount is required during each conduction and the symmetry of the current is improved. This power is determined to maintain continuous operation of the control circuit 11 during whatever time the first switching device is closed. Furthermore, the provision of two MOSFETS 13a and 13b facilitates phase control or dimming of the load should the MOSFETS have sufficient voltage rating.
A further feature of the Figures 9,10 embodiment is that it may include an amplifier 32 (Figure 10) to provide for amplification of a controlling power line carrier signal such as a Decabit or Telenerg ripple control signal. Such amplified signal may be digitally filtered and processed in control circuit 11 to facilitate the control of switching by energy supply utilities.
Variations and modifications falling within the scope of the appended claims may be made in respect of the embodiments of the invention as above described.
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