AU2242999A

AU2242999A - Controlled switching circuit

Info

Publication number: AU2242999A
Application number: AU22429/99A
Authority: AU
Inventors: Peter Birrell
Original assignee: HPM Industries Pty Ltd
Current assignee: HPM Industries Pty Ltd
Priority date: 1998-04-16
Filing date: 1999-03-26
Publication date: 1999-10-28
Anticipated expiration: 2019-03-26
Also published as: AU748268B2

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT 0* a c a Applicant: H.P.M. INDUSTRIES PTY LTD A.C.N. 000 102 661 Invention Title: Controlled Switching Circuit The following statement is a full description of this invention, including the best method of performing it known to me/us: H [K ~1 IIHH T 21W! >1 T IF F ~1F1]I 1 I ~11Hf B HI" K ~rr~T I II 2 Controlled Switching Circuit Field of the Invention This invention relates to an electrical circuit which facilitates electrical actuation of a switching device in a situation where access cannot conveniently be gained to wiring other than an active conductor. The invention has been developed in relation to residential lighting circuits and is hereinafter described in that context. However, it will be understood that the invention does have broader application.

Background of the Invention Lighting circuits in residential buildings typically employ above ceiling twin-core active and neutral wiring.

*'.Also, twin-core wiring normally is used for connecting wall 15 switches in circuit between the above-ceiling active conductor and the active side of ceiling or wall mounted -light fittings. That is, a neutral conductor is not normally available at a wall switch and, thus, no provision normally is made for taking power from the circuit at the light switch. Therefore, provision cannot conveniently be made for effecting electrically controlled switching of lighting, for example by using a relay which requires localised power to energise its coil. This problem exists especially in residences that have existing wiring that cannot conveniently be accessed.

Various so-called two wire switch circuits have been devised for effecting controlled switching of lighting, using only an active conductor. In one such circuit, for example that disclosed by Australian Patent 608416 (AU-B-30798/89) dated 28 February 1989, a triac is employed as a controlled switch, but this approach creates heat dissipation problems in confined spaces, particularly with relatively large currents in the order of 10 amps. In a practical approach to the problem, a capacitor has been used in circuit with a bistable relay and charged to its maximum level when the relay is open. The capacitor charge is then used to energise the relay ON coil, when the relay is to be actuated to a closed condition, but the relay may 33886 -3 be maintained in the closed condition only for such time as it takes for the capacitor to discharge to a level below that required to energise the relay OFF coil. Another approach has involved the use of a step-up transformer and reverse connected diodes for supplying latching current to a relay coil, but this not suitable for use in restricted space situations.

Sunumary of the Invention The present invention provides a different approach to the problem; one which facilitates sustained actuation of a controlled switching device, such as a relay, in a circuit in which an active conductor only is conveniently accessible.

V Broadly defined, the present invention provides a switching circuit which comprises: a) An electrically actuatable first switching device which is arranged to be connected in series with an active conductor and a load in a single phase ac circuit, b) A solid state second switching device connected in series with the first switching device, and C) A control circuit including an energy storage device connected across the second switching device and, also, ****across both the first and the second switching devices.

The control circuit is arranged to apply actuating signals ::25 to the first switching device and to apply periodic gating :signals to the second switching device in a manner to cause a predetermined voltage rise across the second switching device sufficient to effect charge replenishment in the energy storage device when the first switching device is in a conducting state.

In operation of the above defined switching circuit, the energy storage device (typically a capacitor) in the control circuit is employed as a source of energy for actuating and latching the first switching device. The energy storage device is charged initially to its full operating capacity when the first switching device is in a non-conducting condition. Thereafter, when the first switching device is actuated to a conducting condition, any 33886 __i^-ill 4 loss of charge from the energy storage device is replenished during voltage rise times that precede gating of the second switching device. Thus, the energy storage device is charged to its full capacity when the first switching device is actuated to its non-conducting condition and, due to the continuing periodic replenishment of the charge on the energy storage device when the first switching device is actuated into its conducting condition, the first switching device may be maintained in a conducting condition for indefinite periods of time.

Preferred Features of the Invention The first switching device may comprise a solid state switching device when employed in relatively low power applications, but it preferably comprises a relay having a coil which is energised by the actuating signal from the control circuit. That is, the relay coil is provided with actuating/latching current by way of the energy storage device in the control circuit.

In the interest of minimising unacceptable heat losses and/or in order to obviate the need for heat sinking, the solid state second switching device preferably comprises a low impedance device, that is one which, in its conducting state, exhibits an impedance that causes a voltage drop which is not greater than about 500 mV rms with a current flow of 10 amps rms. The second switching device most preferably comprises a metal oxide silicon field effect transistor (MOSFET) device.

The gating signals to the second switching device ideally are generated in each half-cycle of the supply voltage. However, when the second switch device comprises a MOSFET device, the gating signals will be generated during alternate half-cycles (ie, during each positive half-cycle) and be delayed to cause the predetermined voltage rise during intervals that precede gating in the respective half-cycles. In any case the gating preferably is delayed to such an extent as to cause a voltage rise of at least 10 volts.

The control circuit may be arranged to provide for 33886

I

5 manual or remote ON-OFF actuation or timed actuation of the first switching device, in the latter case with or without manual over-ride control. However, the control circuit preferably incorporates a processor to facilitate controlled actuation of the first switching device responsive to an input signal from a manual switching device, a proximity detector, a light level sensor or a remote control (IR or RF) signal sensor.

The invention will be more fully understood from the following description of a preferred embodiment of a switching circuit. The circuit is described with referred to the accompanying drawings.

Brief Description of the Drawings In the drawings: 15 Figure 1 contains a diagrammatic representation of the principal elements of the switching circuit, S"Figure 2 shows the principal elements in a slightly S"more detailed way, Figure 3 shows a schematic wiring diagram which incorporates components of the switching circuit as shown in Figures 1 and 2 and, additionally, incorporates other circuit components which are employed to facilitate operation of the switching circuit from a photo-electric detector, and Figure 4 shows the waveform of voltage across a second switching device (ie, a MOSFET device) as shown in the circuits of Figures 2 and 3.

Detailed Description of the Invention As illustrated, the switching circuit 10 is located in circuit with a load 11 and across a single phase alternating current supply incorporating an active conductor A and a neutral conductor N. The active and neutral conductors are located above a ceiling 12, and twin-core wiring 13 is employed to connect the aboveceiling active conductor A to the active side of the load 11. Thus, the wiring 13 has an active conductor 14 and a switched-active conductor 33886 6 The load typically comprises a ceiling mounted lamp which is connected directly to the neutral conductor N by a neutral loop 16.

The switching circuit 10 comprises an electrically actuatable first switching device 17 (which is connected in series with the active conductors 14, 15 and the load 11), a solid state second switching device 18 connected in series with the first switching device 17, and a control circuit 19. As shown in Figures 2 and 3, the first switching device 17 comprises a normally-open electromagnetic relay having contacts 20 and an actuating coil 21. Also, as again shown in Figures 2 and 3, the second switching device 18 comprises a MOSFET device.

9999 The control circuit 19 includes energy storage S. 15 capacitors 22 and 23 (which are shown schematically in Figure 2 and in relationship to other circuit components in S" Figure 3) and, although described in more detail in the S"following text, the control circuit 19 may be described in general terms as being arranged to: a) Apply actuating (energising) signals to the relay coil 21 when the relay contacts 20 are to be closed, and b) Apply gating signals to the MOSFET 18 at periodic intervals following actuation of the relay, for example during each positive-going half-cycle of the supply voltage 25 when the supply voltage rises to a predetermined S. (instantaneous) level.

b The energy storage capacitors 22 and 23 within the control circuit 19 are connected across the MOSFET device 18 and, also, across both the MOSFET device and the relay 17. These connections provide for charging of the capacitors 22 and 23 when the relay 17 is open, and the connections provide also for cyclical charge replenishment of the capacitors when the relay 17 is closed. That is, top-up charging of the capacitors is effected during the short interval (prior to gating of the MOSFET device 18) during which the supply voltage rises to the predetermined level in each positive-going half-cycle of the supply.

Actuation of the relay 17 may be effected by switch 33886 I_ l n-I i I 7 operation or by way of an input to terminal 24 of the control circuit 19. The latter form of actuation of the relay 17 may be effected responsive to a signal derived from a remote ON-OFF switch or from integrated circuitry that provides for light level sensing, movement sensing or timed activation control.

Circuitry which is applicable to photo-electric (light level) control of the switching circuit is shown in Figure 3 of the drawings, and reference is now made to that Figure for the purpose of describing the operation of principal aspects of the circuit. The other aspects of the circuit which are not described but which are illustrated in Figure 3 may be regarded as being incidental only to the present invention.

15 When the relay contacts RLI:B are open, during each positive half-cycle of the supply voltage the capacitor C7 is charged by way of resistor R12, capacitor C5 and diode D3 to a voltage level of approximately +24 volts.

Similarly, during each negative half-cycle of the supply, the capacitor C10 is charged by way of the resistor R12, capacitor C5 and diode D3A to a level of approximately -24 volts. The charge on the two capacitors C7 and C10 is used, as previously indicated, to provide closing and latching current for the coil RLI:A of the relay 17 when control switch SWI is closed.

.When the relay contacts RLI:A are closed and, as a consequence, no potential exists across the relay contacts to cause charging current to flow to the capacitors C7 and the charge on the capacitor C7 is replenished by delaying conduction of the MOSFET Q7(or 18) for a short time interval during the voltage rise time in each positive half-cycle of the supply voltage. This is achieved in the following manner.

At the commencement of the positive half-cycle of the supply, the MOSFET Q7 is off as a result of input to the non-inverting input of caparator IC2:B having been driven negative in the previous half-cycle and having reset the output of caparator IC2:A to a low state.

33886 111 -~11~11_ Itl- 1- 1~ 8 When the potential across capacitor C6 reaches a value of 13 volts, breakdown occurs in the Zener diodes ZD3 and ZD7, resulting in current flow through resistor R16. As a consequence, the voltage at the non-inverting input of comparator IC2:A goes high relative to that at the inverting input of the comparator. The output of comparator IC2:A then goes high and the MOSFET Q7 is gated into conduction. Positive feedback through resistor R19A maintains the comparator output at a high level for the remainder of the positive half-cycle of the supply.

At the commencement of each negative half-cycle of the supply, the non-inverting input of the comparator IC2:B goes negative, to establish a low impedance level between the comparator IC2:B and the non-inverting input of the 15 comparator IC2:A. That is, the non-inverting input of comparator IC2:A is "pulled" low relative to the inverting S- input of the comparator, and the inverting input is maintained slightly higher due to the pull-up effect of resistor R17, sufficient to cause the comparator IC2:A to switch into a low output condition. This condition is maintained for the remainder of the negative half-cycle of the supply, due again to the positive feedback of the 999" comparator IC2:A through resistor RI9A.

When the MOSFET Q7(or 18) is gated into conduction, the voltage drop across the MOSFET will typically have a value of less than 500 mV with a current level in the order of 10 amps. Figure 4 shows the waveform of the voltage across the MOSFET Q7 over one and one-half cycles of the supply voltage, although the relative magnitudes of the MOSFET and supply voltages are not shown to scale.

A Schottky diode SDl is located in parallel across the MOSFET Q7 (that is, effectively in parallel with the intrinsic MOSFET diode) for the purpose of reducing power dissipation in the MOSFET during negative half-cycles of the supply voltage. Also, a capacitor C11 is connected in parallel with the MOSFET device for the purpose of minimising electromagnetic radiation resulting from switching of the MOSFET device.

33886 9 Variations and modifications falling within the scope of the following claims may be made in respect of the circuit as described above and illustrated in the drawings.

*oo C. 38 *C ft Co *3*

Claims

1. A switching circuit which comprises: a) an electrically actuatable first switching device which is arranged to be connected in series with an active conductor and a load in a single phase ac circuit, b) a solid state second switching device connected in series with the first switching device, and c) a control circuit including an energy storage device connected across the second switching device and, also, across both the first and the second switching devices, the control circuit being arranged to apply actuating signals to the first switching device and to apply periodic gating signals to the second switching device in a manner to cause a predetermined voltage rise across the second switching device sufficient to effect charge replenishment in the *.*energy storage device when the first switching device is in a conducting state.

S"2. The switching circuit as claimed in claim 1 wherein the energy storage device comprises at least one capacitor.

3. The switching circuit as claimed in claim 1 or claim 2 wherein the first switching device comprises an *...electromagnetic relay having a coil to which "..."actuating/latching current is fed by way of the energy storage device. 25

4. The switching circuit as claimed in any one of claims 1 to 3 wherein the second switching device comprises a low impedance solid state switching device.

The switching circuit as claimed in claim 4 wherein the second switching device comprises a MOSFET device.

6. The switching circuit as claimed in any one of the preceding claims wherein the control circuit is arranged to apply gating signals to the second switching device during each half-cycle of the supply voltage of the ac circuit.

7. The switching circuit as claimed in any one of claims 1 to 5 wherein the control circuit is arranged to apply gating signals to the second switching device during each half-cycle of the supply voltage of the ac circuit and wherein the gating signals that are applied in alternative 33886 _I 11 half-cycles are delayed to cause the predetermined voltage rise to occur during intervals that precede gating in the respective half-cycles of the supply.

8. The switching circuit as claimed in any one of the preceding claims wherein the control circuit is arranged to provide for a predetermined voltage rise of not less than volts.

9. The switching circuit as claimed in any one of the preceding claims wherein the control circuit provides for switch-operated generation of the actuating signals to be applied to the first switching device.

The switching circuit as claimed in any one of the preceding claims wherein the control circuit incorporates a processor which is arranged to provide for controlled actuation of the first switching device responsive to an input signal from a manual switching device, a proximity S- detector, a light level sensor or a remote control signal sensor.

11. A switching circuit substantially as shown in the accompanying drawings and substantially as hereinbefore described with reference thereto. Dated this 22nd day of March 1999 H.P.M Industries Ptv Limited By their Patent Attorneys GRIFFITH HACK 33886