GB1585041A

GB1585041A - Over-voltage protection circuits

Info

Publication number: GB1585041A
Application number: GB902/78A
Authority: GB
Original assignee: Cerberus AG
Current assignee: Cerberus AG
Priority date: 1977-01-10
Filing date: 1978-01-10
Publication date: 1981-02-18
Also published as: ATA886377A; JPS5388138A; BE862700A; FR2377107B3; FI773876A; NO780076L; IT7812404A0; SE7800084L; FR2377107A1; CH607400A5; AT363542B; IT1102960B

Description

(54) IMPROVED OVER-VOLTAGE PROTECTION CIRCUITS (71) We, CERBERUS AG., of alte Landstrasse 411, CH-8708 MAENNE DORF, Switzerland., a Swiss company., do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention concerns improved overvoltage protection circuits.

Electrical and electronic apparatus and devices, including communicationstechnology arrangements, measurement, control and regulating apparatus and the apparatuses connected thereto are often subjected to over-voltages. These result from externally effective atmospheric fields and discharges (thunderstorms), electromagnetic fields of nuclear explosions at great heights (NEMP), inductive and capacitive effects of heavy-current apparatus, static discharges, arising from leads of high operating voltages and switching operations within the apparatus and devices themselves. Damage may be effected and persons endangered as a result of these overvoltages. In the worst case the whole operation of data and information transmission may become impossible and thus systems necessary to life may be made inoperative.

It is therefore usual to protect such arrangements with what are called overvoltage dischargers. For this purpose there may be used any arbitrary device, of which the resistance falls abruptly or collapses at a definite voltage, for example a gas-filled over-voltage discharger with a gas-discharge path formed between two electrodes, which fires at a definite voltage, or a semiconductor device exhibiting a sharp reduction in resistance with increase in applied voltage, for example Varistors, i.e. resistors with a high negative voltage exponent of resistance, such as for example zinc oxide discs with electrodes on each side, the semiconductor body lying therebetween forming the voltage limiting path, or non-polarised limiter diodes, known as 'clamping diodes', or other suitable components or combined circuits.

The relation between the present invention and the prior art and the advantages obtained by the use of the invention will now be described with reference to the accompanying drawings, of which: Figures 1, 2 and 3 illustrate known overvoltage dischargers; Figure 4 shows an equivalent circuit diagram of an over-voltage limiter; Figure 5 shows one embodiment of an over-voltage protection device suitable for use in performing the invention; Figure 6 shows another embodiment of an over-voltage protection device; and Figure 7 shows a circuit arrangement, including an over-voltage protection device, in accordance with the invention.

As is represented in Figures 1 and 2, over-voltage dischargers 1 or varistors are usually connected as two-pole elements between the connecting leads 3 and 4 of the apparatus portion or device 2 to be protected, for example soldered in parallel between the input terminals 5 and 6 of the device 2 to be protected. For this purpose known over-voltage dischargers 1 are provided with connecting leads 7 and 8, by means of which the electrodes 9 and 10 of the over-voltage discharger 1 are connected with the connecting points 5 and 6 of the connecting leads 3 and 4.

Such a known voltage-limiting arrangement is as a rule sufficient to exclude over-voltages with smaller flank steepness, or rate of voltage rise from the apparatus 2 to be protected.

For over-voltage of very great flank steepness, that is rapid voltage rise, such as are occasionally present during lightning strokes or in particular atomic explosions, it has however been found that the over-voltage appearing between the connecting leads 3 and 4 of the object 2 to be protected is substantially greater than the limiting voltage of the working element 1. This has been attributed to the leads 7 and 8 of the voltage limiter 1 still possessing an appreciable resistance, so that on firing of the discharger element 1, or on increase of current through the discharger element, a voltage drop builds up on these leads 7 and 8 which is proportional to the impedance and to the current flowing and which must be added to the limiting voltage of the discharger element.

In order to reduce this disturbing ohmic impedance it has been proposed, as is represented in Figure 3, greatly to increase the cross-section of the leads and to reduce their lengths. This is attained, for example, through a form of over-voltage discharger in which the over-voltage discharger device 1 is arranged between two massive metal bars 11 and 12, which are in contact with the electrodes of the discharger device. These bars 11 and 12 are mounted on an insulating mounting plate 15, the connecting leads for the electronic devices to be protected being connected to the connecting points 13 and 14. It is thus arranged that the resistances of the metal strips 11 and 12 serving as leads may be kept extremely small.

However, it is unexpectedly found that with extremely steep voltage transients there still arises at the connecting points 12 and 13 a voltage substantially higher than the limiting voltage of the discharger element 1. Even by eliminating the contact and transition resistance by soldering or welding the discharger electrodes and connecting leads to the bars 11, 12, this over-voltage cannot be eliminated, so that particularly with atomic explosions, which are characterized by extremely steeply rising overvoltages, damage to the object to be protected is to be reckoned on despite extremely low lead resistances.

The object of the invention is to abolish the disadvantage explained above and the provision of a voltage-limiting arrangement with which even over-voltages with extremely high flange steepnesses, that is, rates of voltages rise, may be limited to a non-destructive value.

According to the present invention there is provided a circuit arrangement to protect an eletrical or electronic apparatus against over-voltages arising between supply lines for said apparatus, said circuit arrangement comprising an over-voltage protection device connected in shunt with said supply lines and including two electrodes having a voltage-limiting path there-between and provided with connecting leads, at least one of the two electrodes being provided with two connecting leads extending separately from the electrode, one said connecting lead being connected to the apparatus and the other to a respective said supply line.

The invention stems from the recognition that it is not the resistance only of the leads that it is responsible for the defective voltage limiting of prior art over-voltage discharger arrangements, but in addition to this the self-inductance of the leads. Figure 4 shows an equivalent circuit diagram of a voltage-limiting arrangement including the leads. Here these leads connected to the actual discharger element A are represented by the series connection of resistances R, R' and self-inductances L, L'. When a current I flows through the device the voltage drop on the resistance amounts to UR = R.I and the voltage built up across the self-inductance to UL = L. dI/dt.The total voltage appearing between the connecting leads is then the sum of the limiting voltage UA of the discharger element A and the voltages caused by the resistances and selfinductances of the leads: U = UA + UR + U!R + UL + UIL. While with slower rates of current increase the increase in voltage caused by the self-inductances of the leads is negligible for discharger elements, this component obviously predominates for discharger elements subjected to extraordinarily rapid response performance, which because of this characteristic are regarded as particularly suitable for reliable over-voltage protection.That in practice these often do not provide the expected result, must obviously have its basis therein, that owing to the rapid rise in current in such discharger elements even the lowest self-inductances in the leads become significant. Although over-voltage protective circuits have already been known for some tens of years and are employed in large numbers, this fact has obviously not formerly been given any attention.

In Figures 5 and 6 there are represented two examples of voltage-limiting devices in accordance with the invention, which take this newly realized factor into account.

Figure 5 shows a gas-filled over-voltage discharger in a housing 16, here of hollow cylindrical form, of for example ceramic or glass, which is closed in a gas-tight manner at each end by metallic electrode-members 17, 18 of thin metal. The mid-portions 19 and 20 of electrodes 17, 18 are spaced slightly apart and form the actual electrodes and the interspace 21 the voltage limiting path. The firing or limiting voltage of the discharger element is determined by the choice of the gas filling or by firing aids such as emission-promoting layers, radioactive substances or conductive coatings on the inner walls of the housing. On the backs of each of the two electrodes 19 and 20 there are now secured, e.g. welded, two conducting wires 22, 23 and 24, 25 respectively, which are led away separately from the corresponding electrode 19 or 20.If one of the electrodes is connected to earth, then alternatively only the other electrode may be provided with two separately led-away leads.

Figure 6 shows a semi-conductor element with voltage-limiting characteristics. The voltage-limiting path is formed by a semiconductor body 26 with a high negative voltage coefficient of resistance. Such elements are known as Varistors. A body with a basis of zinc oxide has proved particularly suitable, for example. Metallic electrode layers 27 are placed on each side of this disc-shaped element 26, the semi-conductor body lying there-between serving as the voltage-limiting path. From each of these electrodes 27 two leads 28, 29 and 30, 31 respectively are again led away separately.

In this example the two lead ends are for this purpose connected into a through-loop which may be curved-or straight. It is merely essential that the two lead ends 28 and 29 shall be led away separately from one another from the point of fastening to the electrode 27.

Figure 7 shows the use of an over-voltage discharger element in accordance with the invention, in accordance with Figure 5 or Figure 6, for example, for the protection of an electronic device 32 against overvoltages, which may appear on the connecting leads 33 and 34. For this purpose one of the leads 35, 36 of each of the two electrodes is connected with the connecting leads 33 and 34, while the other leads 37, 38 are connected directly to the device 32 to be protected.Since on firing of the overvoltage discharger element 39 as a result of an over-voltage arriving by way of the connecting leads 33, 34 an increased current with rapid rise flows only in the leads 35 and 36, while in the other leads 37 and 38 only the current consumed by the device 32 flows unaltered, an unavoidable inductive overvoltage builds up only in the leads 35 and 36, but not in the other leads 37 and 38.

The effect of this is that the device 32 to be protected receives at its connection terminals only the voltage drop across the discharger element 39, without inductive over-voltage from the connections of this element. It is thus achieved that the voltage on the protected device 32 is limited to a harmless value even for extremely steep voltage alterations and an extremely rapidly switching discharger element is obtained.

It is further mentioned that the separately led-away leads 37 and 38 may consist of a different material from the other leads 35 and 36. For example the leads connected to the apparatus to be protected may be constructed of highly conductive material, e.g. copper, while the leads connected with the connecting leads consist of resistance wire. This provides the advantage that a part of the energy in the event of an over-voltage peak is dissipated in the resistance wires and the discharger element is protected from damage.

A further advantage of the employment of a voltage limiting element in accordance with the invention as shown in Figure 7 is that in the event of an interruption of one of the leads 35 - 38 the supply current to the device 32 is automatically interrupted, so that the readiness for operation of the limiting lead may be supervised, which is not possible with the arrangement of Figure 2.

WHAT WE CLAIM IS: 1. A circuit arrangement to protect an electrical or electronic apparatus against over-voltages arising between supply lines for said apparatus, said circuit arrangement comprising an over-voltage protection device connected in shunt with said supply lines and including two electrodes having a voltage-limiting path therebetween and provided with connecting leads, at least one of the two electrodes being provided with two connecting leads extending separately from the electrode, one said connecting lead being connected to the apparatus and the other to a respective said supply line.

2. A circuit arrangement in accordance with claim 1 wherein each of said electrodes is provided with two connecting leads extending separately therefrom, one lead from each said electrode being connected to the apparatus and the other lead from each said electrode being connected to a respective said supply line.

3. A circuit arrangement in accordance with claim 1 or 2, wherein the two connecting leads of the or each of the two electrodes form a loop which is secured to the respective electrode at at least one point and of which the ends are lead away separately from one another.

4. A circuit arrangement in accordance with claim 2 wherein the two leads of each respective electrode are of different resistances, the lead of lower resistance being connected with the apparatus to be protected and the lead of higher resistance being connected to the respective said supply line.

5. A circuit arrangement in accordance with any one of claims 1 to 4 wherein the voltage-limiting path consists of a gas discharge between the two electrodes.

6. A circuit arrangement in accordance with any one of claims 1 to 4, wherein the voltage-limiting path consists of a semicon ductor material having a high negative voltage exponent of resistance.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. are now secured, e.g. welded, two conducting wires 22, 23 and 24, 25 respectively, which are led away separately from the corresponding electrode 19 or 20. If one of the electrodes is connected to earth, then alternatively only the other electrode may be provided with two separately led-away leads. Figure 6 shows a semi-conductor element with voltage-limiting characteristics. The voltage-limiting path is formed by a semiconductor body 26 with a high negative voltage coefficient of resistance. Such elements are known as Varistors. A body with a basis of zinc oxide has proved particularly suitable, for example. Metallic electrode layers 27 are placed on each side of this disc-shaped element 26, the semi-conductor body lying there-between serving as the voltage-limiting path. From each of these electrodes 27 two leads 28, 29 and 30, 31 respectively are again led away separately. In this example the two lead ends are for this purpose connected into a through-loop which may be curved-or straight. It is merely essential that the two lead ends 28 and 29 shall be led away separately from one another from the point of fastening to the electrode 27. Figure 7 shows the use of an over-voltage discharger element in accordance with the invention, in accordance with Figure 5 or Figure 6, for example, for the protection of an electronic device 32 against overvoltages, which may appear on the connecting leads 33 and 34. For this purpose one of the leads 35, 36 of each of the two electrodes is connected with the connecting leads 33 and 34, while the other leads 37, 38 are connected directly to the device 32 to be protected.Since on firing of the overvoltage discharger element 39 as a result of an over-voltage arriving by way of the connecting leads 33, 34 an increased current with rapid rise flows only in the leads 35 and 36, while in the other leads 37 and 38 only the current consumed by the device 32 flows unaltered, an unavoidable inductive overvoltage builds up only in the leads 35 and 36, but not in the other leads 37 and 38. The effect of this is that the device 32 to be protected receives at its connection terminals only the voltage drop across the discharger element 39, without inductive over-voltage from the connections of this element. It is thus achieved that the voltage on the protected device 32 is limited to a harmless value even for extremely steep voltage alterations and an extremely rapidly switching discharger element is obtained. It is further mentioned that the separately led-away leads 37 and 38 may consist of a different material from the other leads 35 and 36. For example the leads connected to the apparatus to be protected may be constructed of highly conductive material, e.g. copper, while the leads connected with the connecting leads consist of resistance wire. This provides the advantage that a part of the energy in the event of an over-voltage peak is dissipated in the resistance wires and the discharger element is protected from damage. A further advantage of the employment of a voltage limiting element in accordance with the invention as shown in Figure 7 is that in the event of an interruption of one of the leads 35 - 38 the supply current to the device 32 is automatically interrupted, so that the readiness for operation of the limiting lead may be supervised, which is not possible with the arrangement of Figure 2. WHAT WE CLAIM IS:

1. A circuit arrangement to protect an electrical or electronic apparatus against over-voltages arising between supply lines for said apparatus, said circuit arrangement comprising an over-voltage protection device connected in shunt with said supply lines and including two electrodes having a voltage-limiting path therebetween and provided with connecting leads, at least one of the two electrodes being provided with two connecting leads extending separately from the electrode, one said connecting lead being connected to the apparatus and the other to a respective said supply line.

7. A circuit arrangement in accordance

with claim 6, wherein the semiconductor material constitutes a metal oxide varistor.

8. A circuit arrangement in accordance with claim 6 or 7, wherein the semiconductor material contains zinc oxide as a basic material.

9. A circuit arrangement in accordance with any one of claims 1 to 4 and including an over-voltage protection device substantially as herein described with reference to Figure 5, or Figure 6 of the accompanying drawings.

10. A circuit arrangement arranged to protect an electronic apparatus against overvoltage arising between supply lines for said apparatus, substantially as described with reference to Figure 7 of the accompanying drawings.