WO1995019659A1

WO1995019659A1 - A switching circuit

Info

Publication number: WO1995019659A1
Application number: PCT/AU1995/000022
Authority: WO
Inventors: Kanti Naran
Original assignee: Premlex Pty. Ltd.
Priority date: 1994-01-18
Filing date: 1995-01-18
Publication date: 1995-07-20
Also published as: AU5382494A

Abstract

A switching circuit in the form of a hybrid power relay comprising a semiconductor switch (11) and a relay (12) connected in parallel. Both the semiconductor switch (11) and relay contact (12) are actuated by respective transistor switching circuits having a common switching control input (19). However, the closing of the relay is delayed momentarily relative to the closing of the semiconductor switch (11), and the switching off of the semiconductor switch (11) is delayed momentarily after the opening of the relay (12). The staggered switching ensures that the relay does not switch high currents at high voltages, thereby eliminating contact flashing and burning.

Description

"A SWITCHING CIRCUIT" THIS INVENTION relates to a switching circuit suitable for high current switching. In particular, the invention is directed to a hybrid power relay having a semiconductor switch connected in parallel with a relay for zero cross switching of high (AC) currents, although the invention is not so limited.

BACKGROUND ART

Solid state switches are unsuitable for high current applications due to their limited ability to dissipate heat, and other limiting factors. Mechanical relays are therefore still used to switch high currents.

However, such relays have limited operating life due to excessive wear caused by flashing between the relay contacts when they make or break.

It is an object of the present invention to provide a switching circuit which overcomes or ameliorates the disadvantages of known hig^"" current switches, or which at least provides the consumer with a useful choice.

SUMMARY OF THE INVENTION In one broad form, the present invention provides a switching circuit comprising mechanical switch means; a first actuator circuit for the mechanical switch means; solid state switch means connected in parallel with the mechanical switch means; a second actuator circuit for the solid state switch means, the first and second actuator circuits having a common input; and delay means for delaying operation of the mechanical switch means relative to the solid state switch means such that the mechanical switch means closes a short time after the solid state switch means.

The mechanical switch means suitably comprises a relay contact, and the first actuator circuit is suitably a . transistor switching circuit for controlling the energising of the relay coil. In this embodiment, the delay means utilises the inherent inductance of the relay coil to delay the closing of the mechanical relay relative to the solid state switch. The solid state switch may be any semiconductor based switch. Typically, the solid state switch is a silicon-controlled rectifier (SCR) (or thyristor (TRIAC)), and the second actuator circuit comprises a transistor switching circuit. The SCR is electrically isolated from the transistor switching circuit, but optically coupled to it. In the preferred embodiment, the SCR is triggered by a zero cross SCR which, in turn, is optically controlled by a light emitting device in the transistor TR2 switching circuit. The switching circuit also comprises delay means for delaying the opening of the solid state switch momentarily after the contacts of the relay open. Such delay can suitably be effected by the use of an RC circuit in the transistor switching circuit. In another broad form, the present invention provides a hybrid power relay comprising a semiconductor switch and a relay connected in parallel; and respective switching circuits for controlling the operation of the semiconductor switch and the relay; characterised in that power relay also comprises delay means for delaying the closing of the relay relative to the semiconductor switch, and the opening of the semiconductor switch relative to the relay.

In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described by way of example, with reference to the accompanying drawings. Advantages of the invention will also be apparent from the following description. BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, Fig. 1 is a schematic block diagram of a hybrid power relay according to a preferred embodiment of the invention; and Fig. 2 is a circuit diagram of a specific form of the hybrid power relay of Fig 1.

As shown in Fig. 1, the switching circuit of the preferred embodiment is in the form of a hybrid power relay comprising a solid-state or semiconductor switch 11 connected in parallel with a mechanical relay contact switch 12. In the illustrated embodiment, the hybrid power relay is used to connect a load 13 between a line 14 carrying phase voltage and neutral connection 15. However, the illustrated embodiment is given by way of example only and the hybrid power relay can suitably be used for other switching purposes.

The relay contact 12 is controlled (i.e. opened or closed) by its actuator circuit, namely relay switch 16. The semiconductor switch 11 is controlled by another actuator circuit in the form of an electronic switch 17 which is optically coupled to semiconductor 11 via optocoupler circuit 18. Both actuator circuits 16 and 17 are connected to a common input 19 to which a switching control voltage is applied.

A more detailed circuit diagram of one form of the hybrid power relay of Fig. 1 is shown in Fig. 2. The relay switch 16 comprises a relay coil for relay contact 12, the relay coil being connected to a transistor switch comprising transistor TR1 and current limiting resistor Rl, as shown.

The actuator circuit for the semiconductor switch 11 is also a transistor switch comprising TR2, current limiting resistor R2 and resistor R4. A light emitting diode 11 is connected in the load circuit of

TR2.

The semiconductor switch or thyristor 11 is suitably a silicon-controlled rectifier (SCR) which is triggered by a zero cross SCR 21. The zero cross SCR 21 is, in turn, triggered optically by light emitted from the light emitting diode 20.

Typical component values are as follows. Rl 10K ohm R2 10K ohm

R3 47K ohm

R4 IK ohm

R5 100 ohm CI 0.22μF

The bases of TR1 and TR2 are connected to the common input 19 via their respective current limiting resistors. The input 19 may be connected to the output of a manual switch, a programmable logic controller, or some other suitable switching control signal.

When a voltage is applied to input 19, both transistors TR1 and TR2 are turned on. The switching on of TR2 causes current to flow through LED 20, thereby triggering the zero cross SCR 21 (at the zero crossing point of the AC phase current). The triggering of the zero cross SCR 21, in turn, triggers the semi conductor switch SCR, thereby connecting the load 13 to the phase voltage.

When the input voltage is applied to the base of TR1, it also switches on. However, since TR1 has an inductive load (the relay coil), the opening of relay contact 12 is delayed relative to the switching on of the semiconductor switch. Typically, the SCR is switched in about 4ms, while the closing of the relay contact 12 is delayed by 75ms.

Thus, when the switching control voltage is applied to input 19, both the mechanical relay switch 12 and the semi conductor switch 11 close, but the closing of the relay switch is delayed relative to the semiconductor switch. When the SCR is triggered (at the zero cross point), it carries the full load current as the relay contacts have not yet closed. Some milliseconds later, the relay contacts close and, as the mechanical relay provides a path of negligible resistance, it takes over virtually all of the load current.

At the moment when the relay contacts close, there is little voltage across the relay and hence there is no flashing. The life of the relay is thereby prolonged significantly. As the SCR carries the full load current only for a very short period, heat dissipation is not a problem. The abovedescribed procedure is reversed to disconnect the load. Namely, when the voltage at input 19 is removed, TR1 switches off, causing relay contacts 12 to open. Transistor TR2 switches off at the same time, but the time constant of R3.C1 delays TR2 from switching off for a short period. Thus, the semiconductor switch SCR 11 remains conducting for a short time after the relay contacts open, and carries the full load current.

Since the opening of the relay contacts does not result in a large instantaneous voltage across the contacts, arcing is eliminated. Further, although the SCR will carry the full load current after relay conta^*~*s open, it does so for only a short time and dissipation is controlled to a minimum. The abovedescribed power relay has a numbe f significant advantages. Large AC and DC currents can De switched without the problems of contact flashing, burning encountered by conventional relay switches, r heating in semiconductor relays. This results in lon r operational life for the relay.

Secondly, the switching control circuit is optically isolated from the high current circuit.

Thirdly, the use of zero cross switching reduces mechanical and radio frequency noise. Fourthly, the hybrid power relay can be switched by a variety of voltages from different sources.

The foregoing describes only one embodiment of the invention, and modifications which are obvious to those skilled in the art may be made thereto without departing from the scope of the invention as defined in the following claims.

For example, although an optocoupler is preferred between the electronic switch and the SCR, the SCR could, if desired, be switched directly by the TR2 transistor switching circuit.

Further, although the illustrated embodiment shows the load being switched on a single phase circuit, the hybrid power relay can be used, with simple modification, for three phase switching.

Claims

CLAIMS:

1. A switching circuit comprising mechanical switch means; a first actuator circuit for the mechanical switch means; solid state switch means connected in parallel with the mechanical switch means; a second actuator circuit for the solid state switch means, the first and second actuator circuits having a common input; and delay means for delaying operation of the mechanical switch means relative to the solid state switch means such that the mechanical switch means closes a short time after the solid state switch means.

2. A switching circuit as claimed in claim 1, wherein the mechanical switch means comprises a relay and the first actuator circuit comprises a transistor circuit for controlling the energisation of a coil for the relay.

3. A switching circuit as claimed in claim 2, wherein the delay means comprises an inductance associated with the relay coil.

4. A switching circuit as claimed in claim 1, further comprising second delay means for delaying operation of the solid state switch means relative to the mechanical switch means such that the solid state switch means opens a short time after the mechanical switch means opens.

5. A switching circuit as claimed in claim 4 wherein the second actuator circuit comprises a transistor switching circuit, and the second delay means comprises a capacitance connected to the transistor switching circuit.

6. A switching circuit as claimed in claim 1, wherein the solid state^* switch means is electrically isolated from the second actuator circuit, but optically coupled thereto.

7. A switching circuit as claimed in claim 6, wherein the solid state switch means comprises a silicon- controlled rectifier adapted to be triggered by the second actuator circuit.

8. A switching circuit as claimed in claim 7, wherein the silicon-controlled rectifier is triggered by a zero cross SCR which, in turn, is optically controlled by a light emitting device in the second actuator circuit.

9. A hybrid power relay comprising a semiconductor switch and a relay connected in parallel; and respective switching circuits for controlling the operation of the semiconductor switch and the relay; characterised in that power relay also comprises delay means for delaying the closing of the relay relative to the semiconductor switch, and the opening of the semiconductor switch relative to the relay.