GB2172453A

GB2172453A - Overvoltage protection arrangements

Info

Publication number: GB2172453A
Application number: GB08605571A
Authority: GB
Inventors: John Peter Phillips
Original assignee: DUBILIER BESWICK DIV
Current assignee: DUBILIER BESWICK DIV
Priority date: 1985-03-11
Filing date: 1986-03-06
Publication date: 1986-09-17
Also published as: GB8506270D0; GB8605571D0; GB2172453B

Abstract

An overvoltage protection arrangement for protecting an electrical line and presenting a high shunt impedance and low shunt capacitance to signals normally carried by the line, comprises a gas discharge tube G connected in shunt with the lines L1, L2 to be protected and a non-linear solid state clamping device D1, D2 connected across the gas discharge device. Both the clamp voltage of the solid state device and the D.C. sparkover voltage of the gas discharge tube are chosen to be greater than the peak signal voltage to be normally carried by the line and the clamp voltage is greater than said D.C. sparkover voltage. A heat responsive protection device F1, F2 which is responsive to overheating of the gas discharge tube is connected so as to provide a short circuit path across the gas discharge tube in the event of it becoming overheated. The clamping device may be a voltage dependent resistor, a zener diode, or a foldback diode (DFB), (Figure 5b). A resistor (R) may be connected in series with such a foldback diode at least partially to compensate for the negative resistance characteristic of the diode. The heat responsive device may be a fusible link or a shorting wire covered with a fusible insulating material. <IMAGE>

Description

SPECIFICATION Overvoltage protection arrangements The present invention relates to overvoltage protection arrangements for protecting an electrical line against hazardous voltages such as high voltage transients and voltage surges.

It is known to connect a gas discharge tube to a line to act as an overvoltage protection device and because the response time of gas discharge tubes is relatively slow compared to the rise time of some hazardous voltages, such as voltage spikes and transients, it is also known to include a solid state device having a faster response time in the circuit with the gas discharge tube.

However, known circuit arrangements frequently require the inclusion of a third component, such as a resistor or inductor, connected in series in the line between the gas discharge tube and solid state device since it has been believed that the presence of such a component is necessary to enhance the operation of the protection arrangement. It has also been proposed to connect a form of back-up semiconductor diode across a gas discharge tube, the arrangement being such that the diode does not conduct unless the gas discharge tube fails to operate and moreover the diode then becomes a permanent short circuit once it has operated.

It is an object of the present invention to provide an improved overvoltage protection arrangement which presents a high shunt impedance and a low shunt capacitance with respect to a line to which it is connected and which is suitable for use on lines carrying digital signals without causing unacceptable attenuation or distortion of the line signals.

Accordingly, the present invention provides an overvoltage protection arrangement for protecting an electrical line and presenting a high shunt impedance and low shunt capacitance to signals normally carried by the line, said arrangement comprising a gas discharge tube connected in shunt with the line to be protected and a non-linear solid state clamping device connected directly across the gas discharge device, wherein both the clamp voltage of the solid state device and the D.C.

sparkover voltage of the gas discharge tube are chosen to be greater than the peak signal voltage to be normally carried by the line and said clamp voltage is greater than said D.C. sparkover voltage, and wherein a heat responsive protection device which is responsive to overheating of the gas discharge tube is connected so as to provide a short circuit path across said tube in the event of it becoming overheated.

The non-linear solid state device may be a voltage dependent resistor or a semiconductor diode, for example a zener diode or a fold back diode.

The overload protection arrangement according to the invention may be applied to unbalanced lines, balanced lines or floating lines and both the gas discharge tube and the non-linear solid state device may be either two electrode or multi-electrode devices according to the type of line with which they are to be used and their mode of connection. It will be appreciated that in the case of balanced lines the arrangement may comprise two separate gas discharge tubes, one connected to each line, and each shunted by a clamping device. However in all cases the non-linear solid state device is connected directly to the gas discharge tube. According to one embodiment of the invention the overvoltage protection arrangement may take the form of a unitary composite component or module.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a circuit diagram of one embodiment of overvoltage protection arrangement according to the invention, Figure2 is an explanatory graph, Figures 3a and 3b are respectively a side view and an end view of an unitary composite component or module forming the circuit arrangement of Figure 1.

Figure 4 is a graph showing the characteristic of a foldback diode, Figure 5a is an explanatory graph; and Figure 5b illustrates a further circuit arrangement.

Referring to Figure 1,the overvoltage protection arrangement shown is connected to a pair of balanced lines L1, L2 and comprises a three electrode gas discharge tube G having electrodes el, e2, e3, in combination with a pair of bi-directional clamping zener diodes D1, D2 respectively directly connected between electrodes e1, e2 and electrodes e2, e3. The electrodes el and e3 are respectively connected to lines L1 and L2 and electrode e2, together with the common point between the two diodes D1, D2, is connected to ground.

As is known, the gas discharge tubes are low capacitance devices having a relatively slow response and a high energy handling capability. On the other hand, the zener diodes have a relatively high capacitance, a fast response and a low energe handling capacity. In order to be able to handle digital signals, e.g. up to 64 kilobits and more; the components are also chosen so that the overvoltage protection arrangement avoids high series impedances and low shunt impedance which would attenuate or distort the line signals to an unacceptable degree. For this reason it is not acceptable to have any resistor or inductor in series in the lines between the diodes and the gas discharge tube.

If the knee or clamp voltage of each zener diode section isV2, and in each gas discharge tube section: Vd = DC Sparkover Voltage V = Impulse Sparkover Voltage VA = Arc Mode Voltage and V3 (max) = Peak line signal voltage the arrangement is designed such that; Vd > Vs (max) and Vz > Vs (max) also Vz > Vd. In order that the gas discharge tube will ionize within the constraints of these inequalities Vz is chosen as low as possible while Vd is low enough to ensure operation at mains (power line) voltage (peak).

It is characteristic of the gas discharge tubes employed that under these conditions: V1V2 and VA < VZ As explained above, the gas discharge tube or tubes are chosen for their high current handling capacity and the zener diodes for their fast switch-on characteristics. Moreover, the impulse response characteristics of the gas discharge tube ortubes and the zener diodes are matched to each other so as to minimise the pulse energy dissipated by the zener diodes thereby enabling their junction area to be reduced and hence their capacitance to be minimised.All the components are mounted in close proximity with short connections for minimum capacitance and inductance to ensure that the characteristic of a rapid response to high voltage transients obtained by the matching of the gas discharge tube and the zener diodes is not substantially degraded.

The action of the arrangement described above is as follows and the voltage/time relations are shown in the graph of Figure 2.

(a) In the case of a surge voltage having a slowly rising edge since Vz > Vd, the gas discharge tube ionizes and its glow-to-arc transition reduced the surge voltage to VA (=30V). Under these conditions, the zener diodes have no function and do not conduct.

(b) When the leading edge of the surge voltage rises rapidly, the gas discharge tube breakdown voltage becomes Vl and in consequence it does not ionize. Meanwhile the surge wave voltage reaches Vz and the zener diode conducts within a few nS, thus limiting the surge voltage to the clamp voltage Vz. However since Vz > Vd, after a formative delay tL of the order of a few S the gas discharge tube then ionizes, and strikes at instant1. Its conduction passes into the arc mode, further reducing the surge level down to VA. Since VA < VZ the zener diode then turns off.Hence, the time sequence in which the surge current is initially carried by the zener diode and subsequently switched to the gas discharge tube is achieved by means of a relationship between voltages, and since the pulse energy required to be dissipated by the zener diodes before the gas discharge tube ionizes and the diodes turn off is only small, the zener diodes need only have a small junction area and therefore have a small capacitance. Heat responsive protection devices F1, F2 are connected across each gap of the gas discharge tube G and are operated to provide a short circuit across the tube, and hence across the lines L1, L2 in the event of overheating of the gas discharge tube due to severe overloading on the lines.This is preferable to prior proposals in which the diodes may fail to short circuit on overload since such an arrangement can be unreliable and may leave a substantial resistance across the lines.

Figures 3a and 3b show how the gas discharge tube G and zener diodes Z1 and Z2 may be formed into a unitary composite component or module with the zener diodes directly connected to and located between the electrodes el, e2 and e3 of the gas discharge tube. The overheating protection devices, for the gas discharge tube are shown at F1, F2 and may take any known form. For example they may be fusible links or shorting wires covered with a fusible insulating material. The device is encapsulated and enclosed in a case as indicated at C.

However, where zener diodes are used, especially when passing a high current, the clamp voltage Vz will tend to rise with increase in time t, i.e. create a drive-up voltage, thereby departing from the desired matched characteristics between the gas discharge tube and the zener diode. The drive-up voltage VDR is illustrated in chain lines in Figure 2. This VDR effect has to be accommodated by closer tolerances in the characteristics of the gas discharge tube.

In order to overcome this, the zener diode may be replaced by a foldback diode which has in effect a negative drive-up voltage. This is illustrated by the graph of Figure 4where: Vso = the diode breakovervoltage VBO - VFB = the foldback voltage and t1 = the instant at which the gas discharge tube strikes.

Although such an arrangement reduces the effect of drive-up voltage thereby enabling wider tolerance gas discharge tubes to be used, it does in fact increase the ionisation time of the gas discharge tube which is clearly a disadvantage. However, this in turn can be overcome by inserting a resistor in series with the foldback diode, which resistor is chosen to have such a value that it at least partially compensates for the negative resistance of the downward sloping region ofthefoldbaclcdiode characteristics. This characteristic is shown in Figure 5a, whilst Figure Sb shows the equivalent basic circuit of a gas discharge tube G arranged in parallel with a bidirectional foldback diode DFB in series with a resistor R.As will be noted, since the resistor R is only in series with the diode DFB the arrangement is still effectively a two terminal device which can be connected directly to a pair of lines L1, L2. It therefore does not require to be inserted in the line nor does it provide any undesirable impedance in series with the signal path.

Referring back to the graph of Figure 5a; V50 = the diode breakover voltage which, without the inclusion of a resistor R, causes the gas discharge tube to strike at instant t1. The effect of adding the resistor R in series with the foldback diode is shown in chain lines. As can be seen, the negative resistance slope of the foldback diode is flattened and the response of the gas discharge tube is faster thereby reducing dissipation in the diode. The amount of flattening of the negative resistance slope and the precise instant t2 of striking of the gas discharge tube depends on the value of the resistor R. Even when design considerations do not allow the optimum value of resistor to be used, the inclusion of a resistor of a lower value is still beneficial.

Clearly various modifications can be made to the arrangements specifically described, in particular so as to suit any form of balanced, unbalanced or floating signal path configuration.

Claims

1. An overvoltage protection arrangement for protecting an electrical line and presenting a high shunt impedance and a low shunt capacitance to signals normally carried by the line, comprising a gas discharge tube connected in shunt with the line to be protected and a non-linear solid state clamping device connected directly across the gas discharge tube, wherein both the clamp voltage of the solid state device and the D.C. sparkover voltage of the gas discharge tube are chosen to be greater than the peak signal voltage to be normally carried by the line and said clamp voltage is greater than said D.C.

sparkover voltage, and wherein a heat responsive protection device which is responsive to overheating of the gas discharge tube is connected so as to provide a short circuit path across said tube in the event of it becoming overheated.

2. An arrangement as claimed in claim 1, in which the non-linear solid state clamping device is a zener diode.

3. An arrangement as claimed in claim 2, in which the impulse response characteristics of the gas discharge tube and the zener diode are matched to each other so as to minimise the pulse energy dissipated by the zener diode thereby enabling its junction area to be reduced and hence its capacitance also to be reduced.

4. An arrangement as claimed in claim 1, in which the non-linear solid state clamping device is a fold back diode.

5. An arrangement as claimed in claim 4, in which a resistor is connected in series with the foldback diode in order to at least partiallycompen- sate for the negative resistance region of the foldback diode characteristic.

6. An arrangement as claimed in any preceding claim, in which the heat responsive protection device is a fusible link or a shorting wire covered with a fusible insulating material.

7. An arrangement as claimed in any preceding claim, in which a pair of balanced lines are to be protected and a gas discharge gap and a non-linear solid state clamping device is associated with each line.

8. An arrangement as claimed in claim 7, in which the two gas discharge gaps are provided in a single multielectrode gas discharge tube.

9. An arrangement as claimed in claim 1 or 4, in which the diodes are bidirectional diodes.

10. An arrangement as claimed in any preceding claim, in which said arrangement is in the form of a unitary composite component or module.

11. An overvoltage protection arrangement substantially as hereinbefore described with reference to Figures 1 and 2, or Figures 3a and 3b, or Figure 4 or Figures 5a and 5b of the accompanying drawings.