GB2374218A

GB2374218A - Switch & switching circuit

Info

Publication number: GB2374218A
Application number: GB0108742A
Authority: GB
Inventors: John Russell Fielden
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-04-06
Filing date: 2001-04-06
Publication date: 2002-10-09
Also published as: GB0108742D0; WO2002082485A1

Abstract

A switching circuit comprises a switching element, preferably a triac, connected in series with a drive coil of a switch, the switch being operable in response to a pulse of current through the drive coil, the switching element being operable to allow current to flow in either direction in response to a control pulse, the drive coil and switching element being connectable, in use, to an alternating current source, and the circuit further comprising means for supplying a control pulse to the switching element. The control pulse may be supplied by a microprocessor or RC network, and allows current to be supplied to the drive coil in either direction depending on the point in the alternating current cycle at which the control pulse is supplied. The circuit can be used with a mains supply to readily provide a pulse of current in either direction through the drive coil, of a sufficiently high level to overcome the contact force provided in the switch by a strong permanent magnet.

Description

Switch and Switching Circuit

The present invention relates to a switch and a switching circuit, in particular of ZD the kind typically used for load switching operations. t : l In order to switch loads in an electrical circuit, it is common to have a set of contacts made from silver/cadmium or more recently silver/tin alloys, which are held in tight connection with each other to enable the full load current to be carried, without melting the contact material. Contact loads needed are in the order of 1 kg.

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Fig. 1 shows a typical bi-stable switch which needs no operating current to maintain the contacts while carrying a large current. The contact pressure is z : l provided by means of a magnet 11, typically a rare earth magnet, which produces sufficient force to maintain the required 1 kg pressure at the contact buttons 12.

This type of switch is widely used in load control operations for the electrical supply industry, for example to switch on night storage heaters at the correct times controlled by an electronic clock or a radio operated receiver called a teleswitch.

Fig. 1 shows the switch in the condition where the contacts are made and the current is switched on. To reverse the switch, a pulse of current in the right direction must be passed through the drive coil 13, which overrides the magnetism of the permanent magnet, and the toggle 14 rocks over, lifting the contacts apart.

The contacts are then held apart by the strength of the permanent magnet acting on the toggle 14, and again no current is needed to keep the contacts apart. A similar pulse in the opposite direction will once again cause the toggle to reverse and then remain stable in the new direction.

Commonly the coil is wound as two separate coils wound in opposite directions, so that current from a single common supply can be switched to each coil to achieve the desired direction of switching.

Because the magnet needs to be very strong to provide the correct contact pressure and to prevent accidental reversal from an external magnet, it also needs a very large pulse of current through the coil to overcome the magnet and make sure that proper reversal takes place. For optimum operation, the current should be sufficient to saturate the iron pole pieces, so that the operation of the switch becomes independent of temperature or tolerances in the value of the drive

voltage. In a sample switch designed to these principles, the typical operating t : l Z-1 voltage and current during switching were 35 volts and 3 amps for 5 ms.

4 1 C) This level of current is not simple to generate or steer in either direction. In most applications there will be only one dc supply voltage available with one polarity.

In these circumstances it is usual to split the coil into two windings so that the'on' pulse and'off'pulse can both be derived from the same supply. This means that the coil winding for each direction is halved, and therefore even more drive voltage is needed. Furthermore, this arrangement does not make optimum use of the copper of the coil, which is a relatively expensive material, and this is another reason why it is desirable to provide a switch which requires only a single drive coil.

The result of all these demands for drive current is that nearly all latched relays are designed with a number of compromises from optimum performance, simply to make the drive requirements fit in with the available drive power in a typical circuit. Thus the holding magnet is typically a weak ferrite magnet and the contact force needed will be derived from the physical design of the current circuit, so that the current itself produces the extra force when the current is large and the extra contact force is needed. This leads to a much lower switching power, but the need for careful screening of the switch to avoid the influence of an external magnet.

This is hard to make effective and adds considerable cost and insulation penalties. Also, the switching time becomes dependent on the magnitude of the current flowing which makes zero cross switching difficult to organise.

It is therefore desirable to use a single drive coil, which makes optimum use of the relatively expensive copper, and to use a circuit with no current fold back, so that the switching time remains approximately constant, typically at around 4 ms, and no careful screening of the switch is required because of the use of a powerful rare earth magnet. However, as described above, this arrangement leads to the problem of providing a source of around 30 volts at a current of about 3 amps, which may be easily switched in the appropriate direction.

In the usual microprocessor controlled circuits commonly used with this kind of switch, this sort of power is simply never available. It is therefore typical for switches to be designed to be operated by currents and voltages which are practical in the circuits where they are to be used. This has resulted in the almost universal use of heavily compromised latched relays, including for example a current fold back or the use of a split coil, which have the corresponding disadvantages described above.

One aspect of the present invention provides a switching circuit comprising a switching element connected in series with a drive coil of a switch, the switch being operable in response to a pulse of current through the drive coil, the switching element being operable to allow current to flow in either direction in response to a control pulse, the drive coil and switching element being connectable, in use, to an alternating current source, and the circuit further

comprising means for supplying a control pulse to the switching element. The z tzl switching element preferably comprises a triac, in which case the control pulse can be supplied to the gate of the triac.

The circuit according to the invention can be used to provide the two required opposite voltages with the necessary current to overcome the contact force of a strong permanent magnet, using a simple circuit which derives the power needed directly from the mains voltages being switched.

A large driving current, typically of several amps, can therefore be provided in both directions in a reliable and consistent manner, without the need to provide a dc source or any other processing in hardware.

Another aspect of the invention provides a switch comprising a drive coil, switching contacts and means for moving the switching contacts between an open and a closed position in response to a pulse of current through the drive coil, the switch further comprising a switching element connected in series with the drive coil, the switching element being operable to allow a pulse of current to flow through the drive coil in either direction in response to a control pulse supplied to the switching element, when the drive coil and switching element are connected, in use, to an alternating current supply.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 shows schematically a known switching arrangement and load circuit; Fig. 2 shows a switch operating circuit according to the invention; and Fig. 3 illustrates the switching current in the switch coil shown in Fig. 2, relative to the timing of the mains cycle.

In the load circuit of Fig. 1, a load 15 is supplied with a current typically of up to 100 Amps by the mains supply 10. A switch 16, having contacts 12 of a silver/tin alloy, is provided in the load circuit. The required contact pressure, corresponding to a force of around 10 N or more, is maintained on the contacts by a mechanical connection 20 to a pivotable toggle 14 which is provided with a strong permanent magnet 11. As described above, Fig. 1 shows the switch in the condition where the contacts are made and the current is switched on. The toggle 14 is provided with two end plates 18, which correspond to the north and south poles of the magnet 11. The toggle 14 is arranged to be pivotable between two positions, such that in each position the end plates 18 contact respective contact members 19, which are connected to respective ends of the drive coil 13. Each end plate is arranged so that it makes contact with one of the contact members 19 in one of the positions of the toggle 14, and with the other contact member in the other position. In this way, the end plates rest against respective contact members when the toggle is in a first position, and when the toggle is pivoted into the second position, the contact members with which the end plates are in contact are reversed.

The magnetic force between end plates 18 and contact members 19 provides a force on the toggle 14, which operates on the switch contacts 12 via the mechanical connection 20. In the closed position shown in Fig. l, this magnetic force maintains the required contact force between the contacts 12, whereas in the second (open) position, the force holds the contacts apart.

In order to move the switch between the illustrated closed position and the open position, a pulse of current in the appropriate direction must be passed through the drive coil 13, which overrides the magnetism of the permanent magnet and the toggle 14 rocks over, lifting the contacts apart. The contacts are then held apart by the strength of the permanent magnet 11 acting on the toggle 14. A similar pulse in the opposite direction will once again cause the toggle to reverse and then remain stable in the new position.

No current is needed to provide the required contact force between contacts 12, once the switch is in the closed position, due to the force provided by the permanent magnet. Similarly, no current is needed to keep the contacts apart in the open position.

Fig. 2 illustrates the switching circuit according to the invention, which is used to supply the drive coil of the switch with the required pulse of current in either direction, to overcome the force of the permanent magnet in the arrangement of Fig. 1, and switch the toggle between its two operating positions. The circuit comprises a mains input 21, to which is connected a switch coil 22 in series with a triac 23. A limiting resistor 24 is also provided between the switch coil and the mains input. The same mains input 21 may also be used to supply power to further components which may be connected in parallel with the switch coil and triac, as shown in Fig. 2. Such further components are shown in Fig. 2 as a diode and a capacitor, and are merely intended to be representative of a load circuit 26 of a kind suitable for any particular application.

These further components may therefore make use of the same limiting resistor 24, in order to avoid the need to duplicate this component. In this way, the same

mains voltages being switched may be used to operate the switch itself.

Z-1 The switch coil 22 may equate to the drive coil 13 of Fig. 1, and is used to move a switch (not shown in Fig. 2) between its operating positions, typically by overriding the magnetic force provided by a permanent magnet as described in connection with Fig. 1.

When the triac 23 is provided with a control pulse at its trigger input 25, the triac will go into conduction mode, and allow the mains supply to provide the necessary current through the switch coil 22 to effect the required change in position of the switch. By using the mains supply to provide the current in this way, a positive or a negative pulse can be provided through the switch coil, depending on the timing of the control pulse relative to the timing of the mains cycle.

Because the circuit would usually be used in conjunction with a microprocessor (pop), in a preferred embodiment the necessary switching logic may be derived from the program of the microprocessor. Preferably, the control pulses are

similarly derived from the uP and make use of a phase locked loop in the software so that the relative phase of the supply voltage is known. A positive pulse of 5 volts at a few milliamps can therefore be applied at the trigger input of the triac which will therefore go into conduction mode until the current goes to zero. A

single mains cycle at 50 Hz takes 20 milliseconds ; 10 milliseconds positive and 10 ZD

milliseconds negative. As can be seen from Fig. 3, if the uP provides a pulse 6 m I z : l milliseconds into a mains cycle 31, the voltage will be positive for a further 4 milliseconds, providing a positive pulse 32 through the switch coil 22, typically with a peak current of around 2 Amps. If the uP gives a pulse after 16

milliseconds then the voltage will be negative for a further 4 milliseconds, mi t) providing a negative pulse 33. Since the switching time of a typical switch is t t : l around 4 milliseconds, this arrangement can be used to provide the necessary pulse through the switch coil 22 to move the switch in either direction, by selecting the appropriate timing of the control pulse. In this example, therefore, the trigger point for switching the switch on is at 6 ms into a mains cycle, and at

16 ms into a cycle for switching it off.

0 The control signal may be supplied by means other than a microprocessor, for example using a resistance-capacitance (RC) network. An RC circuit using steering diodes is suitable for this application. However, it is preferred to use means which easily allow switching to be achieved with a zero current, or close to zero current break, in order to preserve the contact material of the switch. This can be more easily achieved using a microprocessor with a phase locked loop in the software than when using an RC network.

An example of an application for which the invention is particularly suitable, due to its reliable and consistent operation, is as a disconnect switch in a dispensing electricity meter, where it is required that the electricity supply is able to be disconnected under certain circumstances, possibly under computer control, in a repeatable manner without significant life limitation of the switch.

Claims

Claims :

1. A switching circuit comprising a switching element connected in series with a drive coil of a switch, the switch being operable in response to a pulse of current through the drive coil, the switching element being operable to allow current to flow in either direction in response to a control pulse, the drive coil and switching element being connectable, in use, to an alternating current source, and the circuit further comprising means for supplying a control pulse to the switching element.

2. A switching circuit as claimed claim 1, wherein the control pulse supplying t, t : l means are operable to supply a control pulse to the switching element at a selected one of at least two predetermined points in the cycle of the alternating current source corresponding to a positive and a negative portion of the cycle respectively, whereby to provide either a positive or negative pulse of current through the drive coil.

3. A switching circuit as claimed in claim 1 or 2, wherein the switching element comprises a triac.

4. A switching circuit as claimed in claim 1,2 or 3, wherein the control pulse supplying means comprise a microprocessor.

5. A switching circuit as claimed in any preceding claim, wherein the control pulse supplying means comprise a resistance-capacitance (RC) network.

6. A switching circuit as claimed in any preceding claim, further comprising a limiting resistor connected between the drive coil and the alternating current source.

7. A switching circuit as claimed in any preceding claim, wherein the alternating current source is a mains electricity supply.

8. A switching circuit as claimed in any preceding claim, wherein the switch is operable to switch a power supply to a load.

9. A switching circuit as claimed in claim 8, wherein the alternating current supply supplies current to both the drive coil and the load.

10. A switch comprising a drive coil, switching contacts and means for moving 1 : 1 zn C) the switching contacts between an open and a closed position in response to a pulse of current through the drive coil, the switch further comprising a switching element connected in series with the drive coil, the switching element being operable to allow a pulse of current to flow through the drive coil in either direction in response to a control pulse supplied to the switching element, when the drive coil and switching element are connected, in use, to an alternating current supply.

11. A switch as claimed in claim 10, wherein the switching element comprises t : l a triac.

12. A switch as claimed in claim 10 or 11, wherein the switching contacts are held in the open or closed position by means of a permanent magnet.

13. A switching circuit substantially as hereinbefore described with reference to Figs. 2 and 3 of the accompanying drawings.

14. A switch substantially as hereinbefore described with reference to the accompanying drawings.