GB2104323A - Electric circuit incorporating safety means - Google Patents

Abstract

A relay, such as a reed relay, having a solenoid (17) and a mechanical switch (15) permits electricity from a point (20) to flow through a load (10). Safety means is provided to prevent trouble if the switch (15) should fail to open. The safety means comprises an electronic switch in the form of a thyristor (30) in series with the mechanical switch (15), the load (10) being connected to a terminal (11) between the switches. A control transistor (22) renders the thyristor (30) non-conductive when a current passes through the solenoid (17), and renders it conductive when no current passes through the solenoid. If the mechanical switch (15) fails to open when the current through the solenoid ceases, a current passes through the thyristor (30) causing a fuse (16) to blow. In the absence of a fuse the potential across the load (10) may be reduced to an acceptable level. <IMAGE>

Description

SPECIFICATION Electric circuit incorporating safety means This invention relates to an electric circuit incorporating safety means.

The invention aims to overcome or reduce a problem that can arise when an electric relay fails to operate correctly.

According to the present invention there is provided an electric circuit incorporating safety means for obviating at least some of the effects consequent upon the malfunctioning of a relay, which relay constitutes part of the circuit and comprises a mechanical switch and electrically controlled operating means therefor, the arrangement being such that in normal operation of the relay a signal of a first kind received by the operating means causes or allows the mechanical switch of the relay to close, while a signal of a second kind causes or allows the mechanical switch of the relay to open, the mechanical switch being operative in use to control the supply of electricity to a location or terminal connected to or suitable for connection to a load, and the safety means comprising an electronic switch in series with the mechanical switch, the location or terminal being situated between the two switches, and control means operative to cause the electronic switch to open in response to the operating means receiving a signal of the first kind, and to cause the electronic switch to close in response to the operating means receiving a signal of the second kind, and protective means operative when both said switches -are closed, in the event of the me mechanical switch malfunctioning and remaining closed when the operating means receives a signal of the second kind, to cut off or reduce the supply of electricity to the load, or the location or terminal.

In use, when a load is connected to the location or terminal, the circuit operates in the following manner. Signals of the first and second kinds are fed alternately to the operating means. In normal operation these cause the mechanical switch to close and open respectively. Each time the mechanical switch closes the electronic switch opens, and each time the mechanical switch opens the electronic switch closes. Thus in normal operation, the circuit tends to assume states in which one or the other of the switches is closed, but not to assume a state in which both switches are closed. Therefore, no electrical path is normally completed through both switches. Should the mechanical switch fail to open, however, when the operating means receives a signal of the second kind, both switches will be closed at the same time and a current can be passed through them both.

When that occurs the protective means cuts off or reduces the supply of electricity to the load.

The consequence of this is that in spite of the malfunctioning of the relay, the feeding of a signal of the second kind to the operating means does have the effect of cutting off the supply of current to the load or at least of reducing it to any desired value.

It is to be understood that a signal of one kind or the other may be constituted by the absence of an electric current. In many forms of relay, in fact, the arrangement is such that the mechanical switch assumes one state in response to the supply of current to its solenoid or other operating means and assumes the other state when the supply of current ceases.

It is often necessary or at least desirable to control the supply of electricity to a load by way of a mechanical switch incorporated in a relay, particularly where relatively high voltages and currents are being controlled. While relays are generally quite reliable, they do occasionally fail. For example it does occasionally happen that the points or terminals of the switch become welded together so that the switch can no longer open. It will be appreciated that the failure of a relay switch to open is likely to be more serious than the failure of a relay switch to close, and the present invention provides means enabling the effects of such a failure to be overcome or at least reduced.In order to reduce the likelihood of failure it has often been the practice in the past for the designers of control equipment to employ relays of a more robust nature than are strictly necessary to switch the current controlled by the relay. This has led to the equipment being more bulky than it need be. The present invention, by reducing the possible risks associated with small relays, makes the use of small relays reasonable in circuits where their use might previously have been considered inadvisable.

While the invention is applicable to relays of many kinds, it is of particular value with reed relays, as they tend to be rather more liable to this kind of malfunction that are some others.

The protective means may comprise a fuse or electro-mechanical cut-out in series with the switches and operative to cut off the supply of electricity to the location or terminal in response to the simultaneous closure of the two switches. Alternatively the protective means may be constituted by design features of the circuit such that in use, if both switches are closed simultaneously the potential across the switches falls to an extent sufficient to vary the potential of the location or terminal so as to reduce the flow of current to the load to a desired level.

The circuit may operate in such a manner that when a signal of one kind is succeeded by a signal of the other kind both the mechanical switch and the electronic switch are for a short period closed simultaneously; in that case, however, the arrangement is made such that the protective means does not operate or has no significant effect. In general, however, such an arrangement is considered to be unsatisfactory, and in a preferred arrangement there is no occasion in normal use of the circuit when both the mechanical and electronic switches are closed.In order to ensure that the circuit operates in this latter manner, the arrangement is preferably such that when a signal of the first kind is received there is a delay before the mechanical switch closes, so as to allow the electronic switch to open first, and that when a signal of the second kind is received there is a delay before the electronic switch closes, so as to allow the mechanical switch to open first. This is preferably done by providing appropriate circuitry for both switches.

An embodiment of the invention will now be described in more detail, by way of example only, with reference to the accompanying drawing, which constitutes a circuit diagram.

The diagram illustrates part of a circuit intended for use in controlling the supply of electricity to a solenoid-operated gas-valve for controlling the supply of combusti52ia gas to a gas-burning appliance. The valve solenoid 10 constitutes the load referred to above and is connected between a terminal 11 and a line 1 2 which is connected to the neutral side of a conventional single-phase domestic mains supply. Also connected to the same neutral side of the supply is a line 1 3. The live side of the supply is connected to a terminal 14 which is connected to the terminal 11 by way of a mechanical switch 1 5 and a fuse 1 6 which are in series with each other.The fuse 1 6 constitutes protective means while the mechanical switch 1 5 constitutes part of a reed relay with an operating coil 1 7 which constitutes the operating means of the relay.

The operating coil has an inbuilt rectifying diode 1 8 in parallel with it, to prevent oscillating currents occuring in use. The diode is indicated in broken lines. The operating coil 1 7 is connected between the neutral line 1 3 and a line 1 9 which is connected to a terminal 20 which is arranged to receive, at different times, two kinds of signals described in more detail below. A capacitor 21 is also connected between the lines 13 and 19.

Signals received at the terminal 20 also pass to the base of transistor 22 by way of the line 19, a line 23 and a resistor 24. The emitter of the transistor is connected to the neutral line 12, while of the collector thereof is connected to a junction 25. A resistor 26 is connected between junction 25 and a terminal 27 which in use is connected to the positive side of a low-voltage source of electric current, the negative side being connected to the line 1 2. A capacitor 28 is connected between junction 25 and line 1 2. Also connected to junction 25 is a resistor 29 which is in turn connected to the gate of a thyristor 30 which constitutes an electronic switch.A resistor 31 is connected between that gate and the line 1 2. The transistor 22 and its associated circuit elements constitute control means for the thyristor 30. The thyristor is connected between the line 1 2 and a resistor 32 which is in turn connected to terminal 11. It will thus be seen that the mechanical switch 1 5 of the relay and the electronic switch constituted by the thyristor 30 are connected in series between the neutral side 1 2 of the mains supply and the terminal 14 which is connected to the live side of that mains supply.

The terminal 11 connected to the load 10 is between the two switches.

In use, signals of two kinds are fed alternately to the terminal 20 from a gate circuit (not shown). A signal of a first kind is constituted by the application to the terminal 20 of a positive potential relative to the line 12, while a signal of the second kind is constituted by the reduction of the potential of the terminal 20 to the same value as that of the neutral line 1 2. When it is desired to operate the solenoid 10, a signal of the first kind is applied to terminal 20. When this occurs, the capacitor 21 becomes charged and the potential across the operating coil rises to a value such that the relay operates and the mechanical switch 1 5 is closed. The presence of the capacitor serves to delay closure of the switch for a short period, typically 20 milliseconds.

When the switch 1 5 is closed, the solenoid 10 is connected to the live terminal 14 and the neutral line 12, by way of the fuse 16 and switch 15.

Immediately the signal of the first kind is applied to the terminal 20, the transistor 22 is rendered conductive, thus causing the potential of the junction 25 to be reduced to the potential of the line 1 2. This in turn reduces the potential of the gate of the thyristor to the same value, thus rendering the thyristor nonconductive, which is the equivalent of stating that the the electronic switch constituted by the thyristor is open. This opening of the electronic switch is effected more rapidly than the closure of the mechanical switch (any charge on the capacitor 28 being rapidly discharged through the transistor).

When a signal of the second kind is applied to the terminal 20, the mechanical switch 1 5 of the relay opens after a short delay resulting from the discharge of the capacitor 21. The transistor 22 becomes non-conducting so that current from terminal 27 can charge the capacitor 28 by way of resistor 26 and thus cause the potential of the junction 25 to rise. This in turn causes the potential applied to the gate of the thyristor 30 to rise so that the thyristor becomes conductive.The presence of the capacitor 28, however, does give rise to a short delay before the thyristor becomes conductive, the delay typically being about one third of a second or even less, but greater than the delay in the opening of the relay switch 1 5. In this way the closure of the electronic switch constituted by the thyristor 30 is delayed until after the opening of the mechanical switch 1 5.

Should the reed relay malfunction and the switch 1 5 fail to open when a signal of the second kind is applied to the terminal 20, a heavy current would pass through the two closed switches 1 5 and 30. The arrangement is such that the fuse 1 6 would quickly blow.

Thus the supply of electricity to the valve solenoid 10 would be cut off.

Typical values of components of the circuit are as follows:fuse 16 rated to blow at 125 ma capacitor 21 22yF resistor 24 100is} resistor 26 1 0K capacitor 28 22yF resistor 29 3.3K resistor 31 1Kdl resistor 32 390s2 relay 15, 1 7 rated to operate at 1 2v 10 ma The resistor 24 limits the current to the transistor 22. The resistor 26 limits the current for charging the capacitor 28. Resistor 31 limits the sensitivity of the system, so preventing spurious operation of the thyristor in response to stray currents; and resistor 32 limits the maximum fusing current to about 300 ma.

It will be appreciated that numerous modifications can be made to the circuit within the scope of the present invention, and that the circuit or a similar circuit can be used to control the supply electricity to loads other than solenoid-operated gas valves.

In one modified form of circuit, using a source of electricity having a relatively high impedance, the fuse is omitted, and the circuit is so arranged that in the event of the relay switch failing to open, the potential across the load falls to a level such that the load fails to carry out its function or carries it out at an acceptably reduced rate or power.

Claims (8)

1. An electric circuit incorporating safety means for obviating at least some of the effects consequent upon the malfunctioning of a relay, which relay constitutes part of the circuit and comprises a mechanical switch and electrically controlled operating means therefor, the arrangement being such that in normal operation of the relay a signal of a first kind received by the operating means causes or allows the mechanical switch of the relay to close, while a signal of a second kind causes or allows the mechanical switch of the relay to open, the mechanical switch being operative in use to control the supply of electicity to a location or terminal connected to or suitable for connection to a load, and the safety means comprising an electronic switch in series with the mechanical switch, the location or terminal being situated between the two switches, and control means operative to cause the electronic switch to open in response to the operating means receiving a signal of the first kind, and to cause the electronic switch to close in response to the operating means receiving a signal of the second kind, and protective means operative when both said switches are closed, in the event of the mechanical switch malfunctioning and remaining closed when the operating means receives a signal of the second kind, to cut off or reduce the supply of electricity to the load, or the location or terminal.

2. An electric circuit according to claim 1 in which the relay is a reed relay.

3. An electric circuit according to either of claims 1 and 2 in which the protective means comprises a fuse or electro-mechanical cut-out in series with the switches and operative to cut off the supply of electricity to the location or terminal in response to the simultaneous closure of the two switches.

4. An electric circuit according to either of claims 1 and 2 in which the protective means is constituted by design features of the circuit such that in use, if both switches are closed at the same time the potential across the switches falls to an extent sufficient to vary the potential of the location or terminal so as to reduce the flow of current to the load to a desired level.

5. An electric circuit according to any one of the preceding claims in which the arrangement is such that when a signal of the first kind is received there is a delay before the mechanical switch closes, so as to allow the electronic switch to open first.

6. An electric circuit according any one of the preceding claims in which the arrangement is such that when a signal of the second kind is received there is a delay before the electronic switch closes, so as to allow the mechanical switch to open first.

7. An electric circuit according to any one of the preceding claims in which the electronic switch comprises a thyristor.

8. An electric circuit incorporating a relay and safety means therefor, substantially as hereinbefore described with reference to the accompanying drawing.