GB2046539A

GB2046539A - Overvoltage protection circuits

Info

Publication number: GB2046539A
Application number: GB8010535A
Authority: GB
Original assignee: STACO WERNER STAUBER GmbH
Current assignee: STACO WERNER STAUBER GmbH
Priority date: 1979-03-29
Filing date: 1980-03-28
Publication date: 1980-11-12
Also published as: FR2452813A1; IT8009393A0; DE2912415A1; IT1153793B

Abstract

A circuit for connection between an AC voltage supply source and a load, for protecting the load from over-voltages, comprises a pair of power supply lines between which are connected a selenium transient voltage suppressor diode stack 6 and a metal oxide varistor 8, in parallel. The circuit may also include a choke 5 connected in one of the power supply lines; a spark gap 7 connected in parallel with the voltage suppressor; and a capacitor 9 connected in parallel with the varistor. The circuit may be formed on a printed circuit board, the choke being formed in one of the printed circuit connectors. <IMAGE>

Description

SPECIFICATION Overvoltage protection circuits The invention concerns circuits for protecting electrical equipment against overvoltages.

Protection circuits of the kind using controllable semi-conductor arrangements with diodes in their control circuit are known. In these the semiconductor arrangement may be brought to a conducting state controllably above the threshold voltage of the diodes. A circuit arrangement of this kind certainly responds rapidly but has only a limited energy absorption capacity (see for example West German published specification DE-OS 26 54 419).

Circuit arrangements are also known in which a thyristor or triac is coupled in series with at least one diode, the control voltage for the thyristor or triac being obtained from a branch of the circuit comprising resistors and a zener diode. Such a device certainly ensures that the equipment to be protected is not switched off but carries on being supplied without interruption when transient overvoltages occur, but with this type of circuit the energy absorption capacity is again too limited.

Selenium transient voltage suppressors, so-called suppressor diodes, are also known as means for suppressing overvoltages, however, these have the disadvantage that they do not produce a sharp suppression effect.

Lastly, use is made of metal oxide varistors for voltage suppression; these are certainly characterised by sharp onset of suppression but can only absorb a relatively low amount of energy without being permanently changed or destroyed.

Aims of the invention include the provision of a circuit for protecting electrical equipment against overvoltages alleviating or overcoming the disadvantages of the known arrangements.

According to the invention there is provided a circuit for protecting electrical equipment against overvoltages, in which circuit a selenium transient voltage suppressor and a metal oxide varistor are connected in parallel with one another across power supply lines included in the circuit.

Preferably the rating of the metal oxide varistor is such that its power consumption at the nominal working voltage is less than 1 0It1 times its continuous load rating.

It is advantageous if use is made of a selenium transient voltage suppressor the continuous load rating of which is at least 50 times the continuous load rating of the metal oxide varistor.

The rating of the selenium transient voltage suppressor should be such that its power consumption at the nominal working voltage is not more than 3% of its continuous LOAD RATING.

One particularly advantageous embodiment provides that the selenium transient voltage suppressor bridges the connections of the circuit to a voltage supply and the metal oxide varistor bridges the connections of the circuit to the equipment, and that an inductor is coupled into at least one of the conductors joining the voltage supply and load connections, between the selenium transient voltage suppressor and the metal oxide varistor.

This inductor preferably is in the form of a printed circuit.

A capacitor may, with advantage, be connected in parallel with the metal oxide varistor.

It may also be advantageous to connect a spark gap, preferably in the form of a gap in the printed conductor and the printed circuit board, in parallel with the selenium transient voltage suppressor.

An embodiment of the invention will now be described in more detail with reference to accompanying drawings, in which: Fig. lisa circuit diagram of a circuit embodying the invention; Figure 2 is a plan view of a device embodying the circuit shown in Figure 1.

The protection circuit shown in Figure 1 is designed to be connected between an A.C. mains supply, (via connections 1 and 2) and a load (via connections 3 and 4). Connections 1 and 4 are directly connected together and an inductor, 5, is connected between connections 2 and 3 as shown.

Connections 1 and 2 are bridged by a selenium transient voltage suppressor 6 and a spark gap 7 as shown. The selenium transient voltage suppressor 6 comprises nine unpoled, so-called suppressor diodes connected in series.

Connections 3 and 4 are bridged by a metal oxide varistor 8, as shown, with which a capacitor 9 may be connected in parallel.

Figure 2 is a plan view of a practical embodiment of the protection device, which comprises a circuit board 10 carrying two mutually insulated printed conductors 11 and 12. One end of conductor 11 includes a drilling 13 corresponding to the connection 2; conductor runs straight for a substantial part of the length of the circuit board 10 until it passes into a spiral 14 shape forming an inductor.

One end of the other conductor 12 also includes a drilling 15 corresponding to the connection 1; conductor 12 runs straight, as shown, to the other end of the circuit board where it includes a drilling 16 corresponding to the connection 4.

The inner end of the spiral section 14 of conductor 11, and a point on conductor 12 adjacent thereto, are provided with small drillings 17 and 18 respectively, through which the connecting wires of the metal oxide varistors 8 are passed enabling the varistor to be soldered to the conductors 11 and 12 at these points. On the other side of the circuit board 10 there is a printed conductor 19 (shown in dotted outline) which runs from drilling 17 to a drilling 20 corresponding to the connection 3.

Connecting tags 21 and 22 of the selenium transient voltage suppressor 6 are soldered to the straight sections, as shown, of conductors 11 and 12 respectively. Between the solder points and the drillings 13 and 15 thetwo conductors 11 and 12 are provided with arms 23 and 24 which extend, as shown, toward one another and terminate at a dumbell-shaped aperture 25 in the circuit board 10, forming a spark gap bridged when a certain voltage is exceeded.

In Figure 2 the selenium transient voltage suppressor 6 is shown to have five suppression diodescoupled in series however, generally nine unpoled suppression diodes are required for working voltages of the order of 220 volts A.C.

Metal oxide varistor 8 is chosen so that a current of 1 mA is passed with a D-C voltage of 430 volts and to have a continuous load rating of 0.6 watts.

The selenium transient voltage suppressor 6 contains suppression diodes with a plate size of 23 x 23 mm in which pass a current of 600 mA at 430 volts.

When overvoltages occur, the selenium transient voltage suppressor becomes active first and can dissipate energy at a level of 100 W of electrical overvoltage pulses without its characteristics changing and without damage.

The spark gap section 23, 24, 25 is bridged when the voltage at the drillings 13 and 15 corresponding to connections 1 and 2 exceeds 1000 volts.

When an overvoltage occurs, the rise in voltage at the metal oxide varistor 8 is suppressed by the inductor 5 in combination with the self-capacitance of the varistor (which is of the order of 1000 pF). This avoids the metal oxide varistor 8 being overloaded when very steep and high voltage peaks occur. Thus, the major part of the energy of an overvoltage pulse is dissipated in the selenium transient voltage suppressor 6-and possibly in the spark gap 7, while the metal oxide varistor8 provides very sharp suppression in conjunction with the inductor 5 (and the, possible, capacitor 9 connected in parallel with the metal oxide varistor) so that reliable protection is obtained for equipment connected to the load side.

The protection circuit shown combines high energy absorption capacity with sharp voltage suppression and is relatively robust and simple to produce.

The ratings of the individual components may naturally be adapted for particular applications without departing from the scope of the invention which is defined by the appended claims.

Claims

1. A circuit for protecting electrical equipment against overvoltages, in which circuit a selenium transient voltage suppressor and a metal oxide varistor are connected in parallel with one another across power supply lines included in the circuit.

2. A protection circuit as claimed in Claim 1, wherein the rating of the metal oxide varistor is such that its power consumption at the nominal working voltage is less than 10-11 times its continuous load rating.

3. A protection circuit as claimed in Claim 1 or Claim 2, wherein the selenium transient voltage suppressor has a continuous load rating which is at least 50 times the continuous load rating of the metal oxide varistor.

4. A protection circuit as claimed in Claim 3, wherein the rating of the selenium transient voltage suppressor is such that its power consumption at the nominal working voltage is not more than 3% of its continuous load rating.

5. A protection circuit as claimed in any one of Claims 1 to 4, wherein the selenium transient voltage suppressor bridges the connections of the circuit voltage to a supply and the metal oxide varistor bridges the connections of the circuit to a load, and wherein a choke is coupled into at least one of the conductors joining the voltage supply and a load connections, between the selenium transient voltage suppressor and the metal oxide variston

6. A protection circuit as claimed in Claim 5, wherein the choke has the form of a printed circuit.

7. A protection circuit as claimed in Claim 5 or Claim 6, wherein a capactor is connected in parallel with the metal oxide varistor.

8. A protection circuit as claimed in any one of Claims 5 to 7, wherein a spark gap is provided in parallel with the selenium transient voltage suppress sor.

9. A protection circuit claimed in Claim 8, wherein the spark gap is formed by a gap in a printed conductor and the board of a printed circuit board.

10. A circuit for protecting electrical equipment against overvoltages substantially as herein described with reference to the accompanying drawings.