GB2324662A

GB2324662A - Automatic load circuit changeover switch; limiting total current from a supply

Info

Publication number: GB2324662A
Application number: GB9808676A
Authority: GB
Inventors: Keith Harry Stokes
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-04-25
Filing date: 1998-04-24
Publication date: 1998-10-28
Also published as: GB9808676D0

Abstract

An automatic changeover switch 104 has a first state in which a first load 103 is directly connected to a supply 101 and a second load 102 is connected to the supply via a current limiting resistor R1, and in response to a demand for current by the second load 102, the switch 104 switches to a second state in which the first load 103 is disconnected and the second load 102 is supplied via a bypass around the resistor R1. The arrangement may be used in a caravan or chalet with a limited supply, the first load 103 being a low priority appliance, such as a heating or cooling appliance, and the second load 102 being a higher priority appliance such as a kettle or microwave oven. The resistor R1 allows sufficient current flow to power a clock of the microwave oven. An override switch (S2, Fig.3) may be provided to turn on the second load, and this switch may be operated manually or by a thermostat or a timer clock. Demand for current in the second circuit 102 may be detected by a current transformer 106. Alternatively, demand for current by the second circuit 102 may be detected from the voltage drop across the current limiting resistor (20, Fig.4) resulting in drop-out of a relay (4) and thereby of a relay (5) which effects the load circuit changeover. When a current transformer (22) senses a fall in the current demand from the second load, another relay (6) resets relays (4, 5) to reconnect the first load.

Description

AN AUTOMATIC CIRCUIT CHANGEOVER SWITCH The present invention relates in general to an automatic electrical circuit changeover device and in particular but not exclusively to a changeover device which is able to switch a supply of mains power between two electrical circuits in response to changes in the power demand of one of the two circuits.

There are many instances where the power demands of two or more load circuits together potentially exceed the power available from an electrical supply. It is therefore necessary to apportion power supply between the respective load circuits. One situation, which will be discussed by way of example only, relates to holiday chalets and caravan sites. Typically, each chalet or caravan is provided with an individual mains electrical supply and a circuit breaker is used to limit the individual current available to a predetermined value, for example of 16 amps. If the current limited power supply to the caravan or chalet is exceeded this will cause the circuit breaker to trigger, turning off the power supply and causing great inconvenience to the occupants until the circuit breaker is re-set.

A 16 amp supply is generally sufficient to power relatively high consumption items of electrical equipment such as toasters, electric kettles, microwave ovens and the like. However, it is also desired to provide electric heating appliances in the caravan or chalet. Whilst a 16 amp supply would power a heater with an output of approximately 4 kilowatts, in practice the heater must be down rated in order to take account of the potential power demands of other electrical equipment within the chalet or caravan which may operate at the same time, in order that the total load does not exceed the limited 16 amp supply.

In general, it is left to the user to monitor the total load being placed on the power supply and, for example, to manually switch off an electric heating appliance before switching on a kettle.

In the example discussed above, a current limiting circuit breaker such as an MCB or RCB is generally designed to give a very fast response time and trip as soon as the total current goes above the preset limit. It is therefore desired to always keeps the total load within the preset limit and, in particular, to avoid current spikes. Also, it is desired to permanently supply power to at least one circuit, in order, for example, to power the clock of a microwave oven.

It is an aim of the present invention to provide an automatic changeover switch addressing at least some of the problems described above.

It is an aim of at least preferred embodiments of the present invention to provide an automatic changeover switch for switching a limited power supply between at least two circuits which may be, for example, a first circuit coupled to an electric heating appliance and a second circuit coupled to kitchen equipment such as an electric kettle or microwave.

According to the present invention there is provided an automatic changeover switch having a first position for coupling a power supply input to a first load circuit and for coupling said input via a current limiting means to a second load circuit; and a second position for decoupling said first load circuit from said input and coupling said input to said second load circuit other than via said current limiting means.

Preferably, said changeover switch comprises switch means for coupling a power supply input to either a first load circuit or a second load circuit; a series resistor for coupling said input to said second load circuit; and demand sensing means for sensing demand from the second load circuit and controlling the switch means accordingly.

Preferably, the switch means provides a low resistance path from the input to either of the load circuits. That is, the switch means provides full power from the input to a selected one of the load circuits. Preferably, the switch means comprises a relay.

Preferably, the series resistor provides a relatively high resistance path between the input and the second load, by comparison with the switch means. That is, the series resistor provides a current limited path. In use, the series resistor provides a permanent low current supply to one of the load circuits. Preferably, the series resistor has a value of the order of 200 ohms to 5K ohms.

Preferably, the series resistor is arranged in series between the input and the second load circuit. Preferably, the series resistor is arranged in parallel with the switch means such that the switch means effectively bypasses the series resistor.

Preferably, the demand sensing means senses demand on the second load circuit by sensing current through the series resistor. Preferably, the demand sensing means comprises a current sensing means for sensing current drawn by the second load circuit in use. Preferably, the current sensing means senses current flowing through the series resistor.

Preferably, the demand sensing means compares the current sensed by the current sensing means against a predetermined changeover level. Preferably, said demand sensing means is coupled to said switch means to determine the state of the switch means. Preferably, the demand sensing means controls said switch means to supply power through said changeover switch according to said first position when current drawn by said second load circuit is below a predetermined level and in said second position when said current drawn by said second load is above a predetermined level. Preferably, said predetermined level corresponds to a changeover point determined with reference to the current sensed as flowing through said series resistor.

Preferably, said current sensing means comprises a current transformer. Preferably, a primary circuit of the current transformer is coupled in series with said second load circuit. Preferably, a secondary circuit of said current transformer is coupled to a level detecting circuit.

Preferably, the level detecting circuit is coupled to a control input of the switch means. Preferably, the level detecting circuit resets the switch means to power the first load circuit when load is removed from the second load circuit.

Preferably, said changeover switch comprises manual override means for manually selecting power supply to either said first load circuit or said second load circuit.

Preferably, said level detecting means comprises said manual override means.

Preferably, the switch means is a solid state switch.

Preferably, the current sensing means is a solid state circuit.

The changeover switch is preferably suitable for use in caravans and chalets located on sites with a limited power supply available. Preferably, the first load circuit is used to power a low priority appliance such as a heating or cooling arrangement. Preferably, the second load circuit is used to power higher priority appliances such as domestic electrical equipment including a kettle or microwave oven.

Conveniently, a third load circuit is provided to permanently power lighting and other low consumption devices such as TV, radio, vacuum cleaner and the like. Preferably, the switch circuit is arranged to maintain total power consumption from a mains power supply below a preselected limit. Where, for example, this limit is 16 amps then, for example, 3 amps might be available for the third circuit and 13 amps selectively distributed between the first and second load circuits.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: Figure 1 is a block diagram of an automatic changeover switch according to a first preferred embodiment; Figure 2 is a simplified circuit diagram relating to the first preferred embodiment; Figure 3 is a detailed circuit diagram relating to the first preferred embodiment; Figure 4 shows a changeover switch according-to a second embodiment; and Figure 5 shows a changeover switch according to a third embodiment.

The preferred embodiments of the present invention will be described with particular reference to the example of a domestic 240V mains electrical power supply or equivalent, by way of illustration. In particular, the preferred embodiments relate to a changeover switch suitable for a mains power supply having a current limiting trip switch.

Specific details of preferred circuit arrangements will be given. However, it will be appreciated that the present invention can be applied in a variety of situations with appropriate modifications as will be familiar to the skilled person.

With reference to figure 1, a schematic circuit diagram is shown incorporating the automatic changeover switch according to a first preferred embodiment. The circuit comprises an input 101 couplable to a power supply and outputs 102 and 103 couplable to load circuits load 2 and load 1, respectively.

The changeover switch operates in two preferred states.

In a first state up to full power is supplied from input 101 to the first load circuit 103 and limited power is also supplied to the second load circuit 102. In the secbnd state, full power is supplied only to the second load circuit 102.

To this end, a demand sensing circuit 105 and a series resistor R1 are coupled between input 101 and output 102.

Resistor R1 limits current to the second load circuit 102.

However, the current reaching the second load circuit 102 is sufficient to power, for example, a low-demand appliance such as a clock.

When the demand sensing circuit 105 senses that a demand is being placed upon the second load circuit 102 above a predetermined changeover limit, the demand sensing circuit 105 drives the switching circuit 104 to disconnect the first load circuit 103 from the power supply input 101 and connects the second load circuit 102 to the power supply input 101.

Therefore, for higher current loads on the second load circuit, the full power supply from input 101 is available.

Also, it will be seen that the series resistor R1 is switched out of circuit and effectively bypassed by the switching circuit 104. When the demand sensing circuit 105 determines that the demand from the second load circuit 102 is below a predetermined change-back level (which may or may not be the same as the changeover level), the switch 104 is driven to reconnect the first load circuit 103.

Referring now to figure 2, the level detecting circuit preferably comprises a current sensing means such as a current transformer 106 suitably arranged with respect to the second load circuit 102. In this example, the current sensing transformer 106 is provided in series with the second load circuit 102 and resistor R1 is in parallel with the switch 104. The secondary winding of current sensing transformer 106 is coupled to detecting circuit 107 which drives the switch 104 which in this example is shown as a relay. When the current drawn by the second load circuit 102 is below the predetermined limit, relay 104 is not energised and the changeover contacts are connected to the first load circuit 103. Therefore, in this normal operating state the changeover circuit itself draws minimal operating current.

Referring now to figure 3, a preferred circuit arrangement will be described in detail.

When no load is applied to the second load circuit 102, all the available power from the power supply input 101 is applied to the first load circuit 103.

When a small load is applied to the second load circuit 102, current will flow through resistor R1 and through the primary winding of current sensing transformer 106. Resistor R1 limits the current drawn by the second load circuit 102.

When the current through the current transformer 106 is sufficient, the detecting circuit 107 operates to switch over a relay 104 to supply full power to the second load circuit 102.

As shown in figure 3, detector circuit 107 comprises power transformer TX1 and rectifier D1 and C1. A bias arrangement R2 and R3 acts to bias one side of the secondary winding of the current sensing transformer 106. When the current induced in the secondary winding of transformer 106 is sufficient to turn on diode D2, transistor T1 is biased on to energise the switching coil 104a of relay 104.

When the load on a second load circuit 102 is switched off, no current flows through the current sensing transformer 106 and the transistor T1 switches off, thereby de-energising the relay 104 whose contacts change back to supply power from input 101 to the first load circuit 103.

A manual changeover switch S2 is provided which biases the input to transistor T1 thereby switching over relay 104 even if there is no load on the second load circuit 102.

Conveniently, the override switch S2 could be a room thermostat or a timer clock for a heating or cooling appliance.

The preferred changeover switch itself uses only a very small maximum current to effect the changeover in response to a priority load being switched on in the second load circuit 102, and which switches back to the secondary load on circuit 103 when the priority load is switched off.

The circuit shown in figure 3 will change over when a load of 200 ohms is connected to the second load circuit 102 and lock over. If a load of 180 ohms is connected, the series resistor Rl which is suitably a 330R 25W resistor will dissipate 21 watts. In order to prevent R1 overheating a thermostatic cut out switch S1 is provided. When a load, such as an electric kettle of say 3 kilowatts, is connected to the second load circuit, the relay 104 switches over to supply full power. For an instant during switching, current flows through resistor R1 but is still small, in this example being of the order of 670 milli amps.

To further illustrate the present invention, alternative embodiments of the circuit will be described with reference to figures 4 and 5.

The automatic electrical circuit changeover device of figure 4 is adapted to switch mains power from a line-in 1 between two circuits 2 and 3. To this end it comprises five relays. Of these, relay 4 detects the use of the second lesser used circuit 3. Relay 5 acts to switch power between circuit 2 and circuit 3. Relay 6 resets the circuit when the demand for power in circuit 3 ceases. Relay 7 acts as a protecting circuit for relay 6. Relay 8 allows the circuit 2 to be switched on and off under external control.

Typically, circuit 2 will provide power to a heater and a thermostat 9 is provided in the power supply to the coil 10 of relay 8.

Line-in 1 is also connected to the second lesser used circuit 3 through relay 4 via a resistor 20 and the primary winding 21 of a current transformer 22. A control circuit comprises a bridge rectifier 26, a smoothing capacitor 27 and a Zener diode 28.

When there is no demand for power from the circuit 3, the voltage at the junction 22 will be determined by the resistor 20, but is sufficient to power the relay 4.

If an electrical appliance is connected to the circuit 3, the voltage at the junction 22 will collapse, causing the contacts 14 of relay 4 to go open circuit. This in turn disconnects line-in 1 from the coil 13 of relay 5 and results in power being disconnected from circuit 2.

As long as the circuit 3 continues to draw power the relays 4 and 5 remained switched. In order to detect when circuit 3 ceases to draw power and reset relays 4 and 5, relay 6 is provided.

Relay 6 is connected to the secondary winding 32 of transformer 22 via a bridge rectifier 33. The transformer 22 detects when current is being drawn by circuit 3 and, when it is, maintains relay 6 open circuit. When circuit 3 ceases to draw current the relay 6 switches off, thereby connecting a resistor 40 connected to the bridge rectifier 26 associated with relay 4 to line-in 1. The reconnection of power to relay 4 causes it to switch. Power is now re-applied to relay 5, the contacts of which switch to connect line-in 1 to circuit 2.

Referring now to figure 5 there is shown a preferred solid state version of an automatic electrical circuit changeover device. The device switches line-in 40 between a first predominantly used circuit 41 and a second lesser used circuit 42. Switching between the circuits 41 and 42 is effected by means of a relay 43, the operation of which is under the control of a high gain power transistor 44 operating as a switch.

Appliances such as microwave ovens with programmes and timers which have to be kept on between normal heavy current use are maintained by the current passing through resistor 52. This current is not sufficient to switch on the transistor 44. Should the quiescent current be so high as to overheat resistor 52 a thermostat 60 on its surface allows the current to be turned off.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings) and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment (s) . The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. An automatic changeover switch having a first position for coupling a power supply input to a first load circuit and for coupling said input via a current limiting means to a second load circuit; and a second position for decoupling said first load circuit from said input and coupling said input to said second load circuit other than via said current limiting means.

2. A switch as claimed in claim 1, comprising switch means for coupling said power supply input to either said first load circuit or said second load circuit.

3. A switch as claimed in claim 2, wherein said switch means provides a low resistance path from said input to either said first or said second load circuit.

4. A switch as claimed in claim 2 or 3, wherein said switch means provides full power from the input to a selected one of said first and second load circuits.

5. A switch as claimed in claim 2, 3 or 4, wherein said switch means comprises a relay.

6. A switch as claimed in claim 2, 3 or 4, wherein said switch means is a solid state switch.

7. A switch as claimed in any preceding claim, wherein, in said second position, said first load circuit is decoupled from said input before said second load circuit is coupled to said input.

8. A switch as claimed in any preceding claim wherein said current limiting means comprises a series resistor for coupling said input to said second load circuit.

9. A switch as claimed in claim 8, wherein said series resistor provides a relatively high resistance path between the input and the second load, by comparison with said switch means.

10. A switch as claimed in claim 8 or 9, wherein, in use, said series resistor provides a permanent low current supply to said second load circuit.

11. A switch as claimed in claim 8, 9 or 10, wherein said series resistor has a value of the order of 200 ohms to 5K ohms.

12. A switch as claimed in claim 8, 9, 10 or 11, wherein said series resistor is arranged in series between said input and said second load circuit.

13. A switch as claimed in claim 8, 9, 10, 11 or 12, wherein said series resistor is arranged in parallel with said switch means such that the switch means selectively bypasses said series resistor.

14. A switch according to any preceding claim, further comprising demand sensing means for sensing demand from the second load circuit.

15. A switch according to claim 14, where said demand sensing means is coupled to control said switch means.

16. A switch according to claim 14 or 15, wherein said demand sensing means senses demand on the second load circuit by sensing current through the series resistor.

17. A switch as claimed in any of claims 14 to 16, wherein said demand sensing means comprises a current sensing means for sensing current drawn by the second load circuit in use.

18. A switch as claimed in claim 17, wherein said current sensing means senses current flowing through the series resistor.

19. A switch as claimed in claim 17 or 18, wherein said the demand sensing means compares the current sensed by the current sensing means against a predetermined changeover level.

20. A switch according to any of claims 14 to 19, wherein said demand sensing means controls said switch means to supply power through said changeover switch according to said first position when current drawn by said second load circuit is below a predetermined level and in said second position when said current drawn by said second load is above a predetermined level.

21. A switch according to claim 20, wherein said predetermined level corresponds to a changeover point determined with reference to the current sensed as flowing through said series resistor.

22. A switch according to any of claims 14 to 21, wherein said current sensing means comprises a current transformer.

23. A switch according to claim 22, wherein a primary circuit of said current transformer is coupled in series with said second load circuit.

24. A switch according to claim 23, wherein a secondary circuit of said current transformer is coupled to a level detecting circuit.

25. A switch according to any of claims 14 to 24, wherein said demand sensing means is coupled to a control input of said switch means.

26. A switch according to any of claims 14 to 25, wherein said demand sensing means resets the switch means to power the first load circuit when load is removed from the second load circuit.

27. A switch according to any preceding claims, comprising manual override means for manually selecting power supply to either said first load circuit or said second load circuit.

28. A switch according to claim 27 when dependent upon any of claims 14 to 26, wherein said demand sensing means comprises said manual override means.

29. A switch according to any of claims 17 to 21, wherein said current sensing means is a solid state circuit.

30. A switch according to any preceding claim, suitable for use in caravans and chalets located on sites with a limited power supply available.

31. A switch according to any preceding claim, wherein said first load circuit coupled to power a low priority appliance.

32. A switch according to claim 31, wherein said low priority appliance is a heating or cooling arrangement.

33. A switch according to any preceding claim, wherein said second load circuit is coupled to power a higher priority appliances.

34. A switch according to claim 33, wherein said higher priority appliance comprises domestic electrical equipment including a kettle or a microwave oven.

35. A switch according to any preceding claim, wherein a third load circuit is provided to permanently power lighting and other low consumption devices such as TV, radio, vacuum cleaner and the like.

36. A switch according to any preceding claim, arranged to maintain total power consumption from a mains power supply below a preselected limit.

37. A changeover switch substantially as hereinbefore described with reference to figures 1, 2 and 3 of the accompanying drawings.

38. An automatic circuit changeover device substantially as hereinbefore described with reference to figures 4 or 5 of the accompanying drawings.