GB2039170A

GB2039170A - Overvoltage protection device

Info

Publication number: GB2039170A
Application number: GB8000216A
Authority: GB
Original assignee: Societe de Paris et du Rhone SA
Current assignee: Societe de Paris et du Rhone SA
Priority date: 1979-01-05
Filing date: 1980-01-03
Publication date: 1980-07-30
Also published as: FR2446026A1; GB2039170B; IT8019037A0; ES487447A1; IT1130175B; DE2952791A1; FR2446026B1; DE2952791C2

Abstract

A device intended to provide protection against the overvoltages which may occur at the terminals of a rectified alternating current generator 2, is of the type comprising two branches connected in parallel, one of which comprises a zener diode 7, the other having a thyristor 9 controlled by the energising of the zener diode. For automatic de- energising of the thyristor an electronic circuit 10 is provided, connected in parallel to zener diode and thyristor and having an avalanche device 18, one terminal of which is connected to the base of a transistor 20, the emitting-collecting circuit of which is connected in parallel to the thyristor, the other terminal being connected to means for locking, controlling and feeding the avalanche device. As shown, the avalanche device is a diac, but in alternative arrangements it is a double-base transistor or an assembly of two transistors equivalent to a double-base transistor. <IMAGE>

Description

SPECIFICATION An overvoltage protection device This invention relates to a device to provide protection against overvoltages and, more particularly, against overvoltages which may occur at the terminals of a rectified alternating current generator, in particular in the electrical circuit of an automotive vehicle.

More precisely, this invention obviates or mitigates the disadvantages of a well-known device for providing protection against overvoltages, comprising two circuit branches in parallel, one of which has a zener diode, the other having an element such as a thryristor, controlled by the energizing of the zener diode of the first branch (see French Patents No.

1494254 and No. 1 535 310). This type of protection circuit makes it possible to limit overvoltages by connecting the auxiliary "plus" terminal of the generator to earth through the thyristor, the control electrode of which is driven by the zener diode, which short-circuits the regulator controlling the excitation of the alternator. The usefullness of such a device on an automotive vehicle is also well known: when the alternator charge is suddenly cut out, an overvoltage occurs at its terminals which, if it is not remedied, risks destroying the electrical accessories located upstream of the circuit break; at present these risks are becoming all the greater since there is a trend towards multiplying, on automotive vehicles, the number of electrical and even electronic accessories, the latter of these being very sensitive to voltage surges.

In the protection devices under consideration here, one difficulty, following the disappearance of an over-voltage, consists of bringing the set of circuits back to the original normal condition automatically, by deenergising at the right moment the thyristor controlled by the zener diode.

A well-known solution, which is the solution described in French Patent No. 1 494 254, already mentioned above, consists of effecting automatic de-energising of the thyristor using a relay with coil and mobile contacts, inserted in the circuit branch through which the overvoltage discharges. This special solution, as well as similar solutions, is not fully satisfactory, as the relay is an expensive component, sensitive to vibrations and subject to rapid wear and deterioration, in particular as regards its contacts; moreover the relay controls de-energising at a moment when the shortcircuit current resulting from the overvoltages is still at a high level, which may constitute an additional cause of the deterioration of the relay.

This invention is intended to obviate or mitigate these disadvantages.

According to the present invention there is provided an overvoltage protection device for providing protection against overvoltages which may occur at the terminals of a rectified alternating current generator, of the type comprising two circuit branches in parallel, one of which includes a zener diode, the other including an element controlled by the energising of the said zener diode, an electronic circuit being provided for automatic de-energising of said element onnected in parallel to the zener diode and the element, and comprising an avalanche device, one terminal of which is connected to the base of a transistor, the emitter-collector of this transistor being connected in parallel to the element, the other terminal of which is connected to means of locking, controlling and feeding the said avalanche device.

These means preferably comprise an adjustable timer making it possible to de-energise the thyristor only after complete disappearance of the overvoltage, in this way avoiding any risk due to premature de-energising.

According to a preferred embodiment of the invention, said means of locking, controlling and feeding the avalanche device comprise: a first capacitor, constituting the store of energy connected in series to a diode, a second capacitor connected to the first capacitor through a load resistor and a transistor, the emitter-collector of which is connected in parallel to the second capacitor, one terminal of the avalanche device being connected to a point located between the second capacitory and the load resistor of the latter.

in normal operation the avalanche device is not activated or energised and the transistor, to which one terminal of this device is connected, remains blocked. Following the appearance of an overvoltage, the means defined above in a special embodiment produce, through the avalanche device, a pulse which controls the previously blocked transistor, so that it is suddenly saturated, which causes the de-energising of the thyristor mounted in parallel with this transistor.

It will be understood that since this circuit is entirely electronic, it will remain perfectly reliable; it is in addition made so as not to disturb the zener diode and thyristor circuit, so that the overvoltages are wiped out completely. The fact of the device in accordance with this invention being insensitive to vibrations is of special interest for applications in the automotive world.

In addition, where the means of locking, controlling and supplying the avalanche device are produced as described above, it is an advantage to arrange for the load resistor of the second capacitor to be adjustable. This ancillary device makes it possible to adjust the timing of the system, so as to de-energise the thyristor at the most appropriate moment.

The avalanche device may be produced according to various possibilities which are well-known in themseleves, examples of which shall be described hereinafter.

Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a circuit diagram showing the whole of the device in a first embodiment; Figures 2 and 3 are partial circuit diagrams, showing two variants of the device shown in Fig. 1.

Fig. 1 shows, in the form of a diagram, the electrial circuit of an automotive vehicle with battery 1, generator 2 and branch 3 towards the electrical accessories of the vehicle. Generator 2 consists of the connection of an alternator and a rectifier, which are not shown separately on the diagram. Regulator 4 is also shown, mounted between the auxiliary "plus" terminal of generator 2 and earth, and connected to the alternator excitation circuit.

In a well-known way a circuit designated in its entirity with the number 5, and intended to avoid over-voltages at the terminals of generator 2, is connected in parallel with regulator 4, therefore between the auxiliary "plus" terminal and earth. This circuit 5 has two branches in parallel. The first branch consists of the series connection of a resistor 6, a zener diode 7 and another resistor 8. The second branch comprises a thyristor 9, the control electrode of which is connected to a point on the first branch and located between zener diode 7 and resistor 8.

These is provided also an additional circuit, designated in its entirity with the number 10, and again connected between the auxiliary "plus" terminal and earth.

In the embodiment shown in Fig. 1 circuit 10 includes, between the "plus" terminal and earth, a diode 11 connected in series to a first capacitor 1 2. An adjustable resistor 1 3 is connected to a point located between diode 11 and capacitor 12, and to a second capacitor 14, itself connected to earth.

A first transistor NPN 1 5 has its emitter connected to earth and its collector connected to a point located between resistor 1 3 and capacitor 1 4. The base of this transistor 1 5 is connected by means of two resistors 1 6 and 1 7 respectively to the "plus" terminal and earth.

The collector of transistor 1 5 is also connected to a terminal of an avalanche device 18, here consisting of an element of the wellknown type and designated by "Diac". The other terminal of avalanche device 1 8 is connected to earth through a resistor 19, and to the base of a second transistor NPN 20. The emitter of this transistor 20 is connected to earth, and its collector is connected to the "plus" terminal. Between the collector of transistor 1 5 and avalanche device 1 8 again a resistor 21 is inserted making it possible to introduce a time constant intended to increase the conduction time of transistor 20, and this time must be greater than the thyristor deenergising time.

When an overvoltage occurs at the terminals of generator 2, due in particular to a sudden cut-out as shown symbolically at 22, circuit 5 intervenes, in a way which is wellknown in itself, to limit this overvoltage by short-circuiting momentarily the auxiliary "plus" terminal and earth. In fact, beyond a certain voltage, zener diode 7 is energised and makes thyristor 9, which in normal operation remains blocked, conduct; a short circuit is therefore produced through the circuit branch comprising thyristor 9.

The operation of circuit 10, which forms more particularly the object of the invention, is as follows: During the normal operation of generator 2, capacitor 1 2 is charged through diode 11.

Taking into account the relative values of resistors 1 6 and 17, the first transistor 1 5 is then saturated. The voltage at the terminal of avalanche device 1 8 connected to resistor 1 3 is less than the avalanche voltage, so that the second transistor 20 is blocked. The circuit is thus ' "locked".

When an overvoltage involves the conducting of thyristor 9 in circuit 5, it therefore causes, in circuit 10, a variation in the voltage such that transistor 1 5 is blocked. The second capacitor 14 is then charged through resistor 13, the first capacitor 1 2 playing the role of energy store.

Capacitor 14 charges in this way until the voltage at its terminals reaches the avalanche voltage of device 1 8. Through this avalanche device 1 8 sudden discharging of capacitor 14 then occurs through transistor 20; the latter is saturated by this pulse or surge and blocks thyristor 9.

In this way automatic de-energising of thyristor 9 is brought about, therefore the return to normal operating conditions, at the end of a period of time which is determined by the time constant of the circuit formed by resistor 1 3 and capacitor 14, this period being adjustable since resistor 1 3 is adjustable. It should be noted further that if thyristor 9 is not blocked, the cycle previously described commences again, as long as capacitor 1 2 contains energy in reserve, and until thyristor 9 is de-energised.

It should be noted that according to an ancillary characteristic, a diode 23, mounted in series with thyristor 9, makes it possible in every case to have a "diode plus thyristor" voltage drop which is greater than the voltage drop between the emitter and collector of transistor 20, in this way ensuring reliable deenergising of thyrsitor 9.

Fig. 2 shows only circuit 10, in a variant of the embodiment in which avalanche device 1 8 consists of a double-base transistor. The circuit then has an additional branch 24, which connects the double-base transistor to a point located between diode 11 and the first capacitor 1 2. The other elements, not modified, are designated with the same reference as in Fig. 1.

Fig. 3 again shows another variant in which avalanche device 1 8 consists of an arrangement equivalent to the double-base transistor in Fig. 2, this arrangement being produced with two transistors 25, and 26 forming an assembly which is well-known in itself.

The device in accordance with the invention to provide protection against overvoltages, intended more particularly for electrical circuits of automotive vehicles, may in practice be built into the regulator, or placed in a separate box, located near the regulator.

Obviously the invention is not limited simply to the embodiments of this overvoltage protection device which have been described above, merely as examples. On the contrary it also embraces any variants comprising equivalent means. The spirit of the invention will therefore still be adhered to even if other elements or circuits are selected which are equivalent to those described, to produce avalanche device 18, or again in replacing transistors NPN 1 5 and 20 with transistors PNP, with corresponding modifications in circuit 10.

Claims

1. An overvoltage protection device for providing protection agains overvoltages which may occur at the terminals of a rectified alternating current generator, of the type comprising two circuit branches in parallel, one of which includes a zener diode, the other including an element controlled by the energies; ing of the said zener diode, an electronic circuit being provided for automatic de-energising of said element connected in parallel to the zener diode and the element, and comprising an avalanche device, one terminal of which is connected to the base of a transistor, the emitter collector of this transistor being connected in parallel to the element, the other terminal of which is connected to means of locking, controlling and feeding the said avalanche device.

2. A device in accordance with claim 1, in which said element is a thyristor.

3. A device in accordance with claim 1 or claim 2 in which said means of locking, controlling and feeding the avalanche device comprises a timing unit for de-energising the element only after the complete disappearance of the overvoltage.

4. A device in accordance with claim 3, in which said timing unit is adjustable.

5. A device in accordance with claim 3 or claim 4, in which said means of locking, controlling and feeding the avalanche device comprises: a first capacitor acting as an energy reservoir connected in series to a diode, a second capacitor connected to the first through a load resistor, and a transistor, the emitter-collector of which is connected in parallel to the second capacitor, one terminal of the avalanche device being connected to a point located between the second capacitor and the load resistor of the latter.

6. A device in accordance with claim 5 in which the load resistance of the second capacitor is adjustable.

7. A device in accordance with any one of the preceding claims in which the avalanche device is of the "Diac" type.

8. A device in accordance with any one of claims 1 to 7 in which the avalanche device consists of a double-base transistor.

9. A device in accordance with any one of claims 1 to 7 in which the avalanche device includes an assembly equivalent to a doublebase transistor, produced by two transistors.

1 0. A device in accordance with claim 5 and claims 8 and 9, in which a circuit branch connects the double-base transistor, or its equivalent assembly, at a point located between the first capacitor and the diode, in series to this capacitor.

11. A device in accordance with any one of claims 2 to 10, in which a diode is mounted in series with the thyristor, this diode making it possible to have under all circumstances a "diode plus thyristor" voltage drop which is greater than the voltage drop between the emitter and collector of the transistor, the emitting-collecting circuit of which is connected in parallel to the thyristor.

1 2. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.