GB2258570A - Device for protecting high speed communications network from overvoltages - Google Patents

Abstract

The device has a gas tube 10, or a high voltage solid state element, to deal with mains voltage or lightning surges and a further arrangement for dealing with lesser overvoltages that arrangement being such that at least three diodes, including at least one voltage sensitive diode ZD1, are present in series across the network to provide a low impedance during overvoltages but a low capacitance circuit during normal conditions to allow use at high frequencies. A zener diode ZD1 (or a breakover of foldback diode) may be associated with a steering circuit formed by a diode bridge D1-D4 and diodes D5, D6 connected to ground 5, 6. In an alternative arrangement (Fig. 3), diode ZD1 is replaced by a short, diode D6 is omitted, diode D5 is replaced by inverse parallel diodes (D10, (D11) in series with a bidirectional foldback diode (D9) and additional foldback diodes (D7), (D8) are connected between points 3, 4 and the bridge D1-D4. The foldback diodes (D7), (D8), (D9) may be replaced by zener, breakover or varistor diodes. <IMAGE>

Description

PROTECTOR DEVICE FOR COMMUNICATIONS NETWORKS The invention relates to protection devices for communications networks.

In our earlier patent application 8802434, a system is described for use on the telephone network.

Whilst this known arrangement is suitable for normal telephone traffic, when higher rates are required, such as high speed digital data links and LANs, there is a need for a protector to have the ability to cope with data rates in excess of 2MHz. The earlier arrangements tend to cause a large degree of attenuation due to capacitive effects and so an alternative approach is required.

According to the invention there is provided a protecting device for a high speed communications network including: protector means for protecting the network from mains voltage and surges from lightning strikes; and, semiconductor protector means for dealing with lesser overvoltages, said semiconductor protector means including a diode bridge circuit for steering the overvoltage and voltage sensitive diode means connected to the bridge such that at least three diodes including at least one voltage sensitive diode is present across the network to provide a low capacitive circuit during normal voltage conditions and a low impedance circuit during overvoltage conditions.

means for protecting the network from mains voltage and surges from lightning strikes; and, low capacitance means for dealing with lesser overvoltages, said low capacitance means including a diode bridge circuit for steering the overvoltage and voltage sensitive diode means connected to the bridge such that at least three diodes including at least one voltage sensitive diode is present across the network to provide a low capacitive circuit during normal voltage conditions and a low impedance circuit during overvoltage conditions. The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows one embodiment of the present invention; Figure 2 shows the frequency response of the known arrangement compared to the Figure 1 arrangement; Figure 3 shows an alternative embodiment to the Figure 1 arrangement.

The arrangement for the protector device of Figure 1, includes inputs 1 and 2 connected to the data lines, and outputs 3 and 4 connected to the data equipment. Earthing points are provided at connections 5 and 6. Across the line connection points 1 and 2 is a gas discharge device 10 (eg BT21A). In series with the line connection points 1 and 3 and 2 and 4 respectively are resistors R1 and R2 (eg 10ohm). A diode bridge comprising diodes D1 to D4 are connected across the lines beyond the resistors R1 and R2.

The diodes will be normal low cost semiconductor devices.

A zener diode ZD1 is connected across the bridge to the node points between diodes D1 and D4 and diodes D2 and D3 respectively. Extending from these nodes are additional diodes D5 and D6 which are connected to the earth points 5 and 6 respectively. The central electrode of the gas tube 10 is also connected to the earth point 6. Failsafe contacts 10a and 10b are provided which typically are coated in meltable plastic so as to provide a failsafe mechanism in the event of a prolonged high voltage applied across the gas tube 10. This will short the lines to earth.

The zener diode ZD1 across the nodes is provided for steering the current down to earth in the circumstances where the line voltage exceeds the voltage of the zener.

The additional diodes D5, D6 give the advantage of helping to steer the voltage down to earth. So D5 or D6 will block the earth path in dependence on which direction the current is flowing. Additionally, D5 and D6 help reduce capacitance to earth and prevent the forward conduction to earth of D1-D4.

The impedance of R1, R2 provides a voltage drop so that gas tube 10 will trigger to take the bulk of the current so as to relieve the stress on the bridge circuit which will in practice take up the surplus, when a lightning surge, for example, is across the line.

The gas discharge device 10 will start to conduct typically in the region 200 volts and the bridge arrangement when the voltage across it is about 15 volts.

The expected line voltage will be less than 12 volts in this example.

So it can be seen that the bridge will cope normally with small excesses in line voltage but when a lightning surge of several KV is present this would cause the bridge to attempt to deal with it, but due to the impedance of the circuit (including R1 and R2) the gas tube in practice will conduct first to dissipate the excess voltage surge, in a non-destructive manner. If a high voltage (eg mains voltage) is continuously present the gas tube will operate as above but after a brief period (eg 10 seconds) the heating effect will cause the meltable sleeves in contact with it to melt and contacts 10, 10b will short circuit into a failsafe mode.

The arrangement shown is capable of handling data at a higher rate because it presents a low capacitance across the line.

The series combination of diodes will keep this capacitance down. At any one time the diodes in circuit will be the following: D2 + D5 (via ZD1) D3 + D5 (via ZD1) D1 + D6 (via ZD1) D4 + D6 (via ZD1) also voltages developed across the line (transverse) will be clamped e.g. D2 + D4 via ZD1. Other combinations will follow.

So 3 diodes in series would give a capacitive effect of: 1/C = 1/cDl + 1/CD6 + 1/CZDl1 for example.

This give an improved value of capacitance, typically 30pF compared to the several thousand pF if the zener was employed on its own and merely strapped across the rail.

The effect of this improvement can be seen in Figure 2, where graph A shows a typical response against frequency for a known protector arrangement. Graph B shows the response of the Figure 1 embodiment where the much lower degree of attenuation is occurring even up to data rates of 20MHz or more.

The zener diode ZD1 could be replaced by other voltage sensitive diodes such as foldback or breakover diodes, for example. The gas discharge device could be replaced with a high voltage solid state device if desired.

An alternative arrangement is shown in Figure 3. As before the gas discharge device 10 and the diode bridge circuit D1 to D4 is provided. However 3 pairs of foldback diodes D7 to D9 are now included, two between the rails and one between the bridge and the earth connection point 5.

In addition, an optional pair of diodes D10 and 11 are connected between the foldback diode D9 and the earth point 6. At any one time the conduction in an overvoltage situation will be as follows: D7 + D9 + D1 or D2 D7 + D8 + D1 + D3 D7 + D8 + D2 + D4 D9 + D8 + D4 or D3 As seen, in some circumstances, only three diodes will be conductive whilst in other circumstances four diodes will be conductive this will cause some capacitive changes.

To overcome this the optional pair of diodes D10 and Dll have been inserted so that when diode 9 conducts four diodes will be conducting and their inherent capacitance will be as in other conductive scenarios. This will remain constant at about 30/40pF rather than increasing to 70pF without the aid of diodes D10/Dl1 conducting as discussed above.

Thus the Figure 3 arrangement is an improvement on Figure 1 in that the zener diode of Figure 1 would typically have to handle the whole surplus power passing through the diode bridge during excess voltage and have to dissipate all this heat. Such circumstances could be when a high voltage is on the line and the portion not being handled by the gas tube is conveyed through the zener via the bridge. Also, when medium voltage is applied (below the gas tube threshold) this will all be passed through the zener. The modified circuit however reduces the circuit stress experienced by the zener. This is removed and a link shorting the bridge is provided and the three bidirectional diodes put into its place also.

The diode arrangement shown in Figure 3 tends to be more reliable in the long term yet minimising the capacitance seen across the line, thus making it ideal for high frequency data circuitry.

As before the foldback diodes could be replaced by zener diodes, breakover diodes or even possibly a varistor diode.

The device could be modular in form with all the circuit components being mounted in a small housing and connecting pins or sockets 1-6 arranged so that the device could plug into the system and be replaced when faulty.

The gas tube itself could also be modular so that this could be replaced in circumstances where continuous over voltage had caused the meltable sleeve to short the gas tube into the failsafe mode.

Claims (9)

1. A protector device for a high speed communications network including: protector means for protecting the network from mains voltage and surges from lightning strikes; and, semiconductor protector means for dealing with lesser overvoltages, said semiconductor protector means including a diode bridge circuit for steering the overvoltage and voltage sensitive diode means connected to the bridge such that at least three diodes including at least one voltage sensitive diode is present across the network to provide a low capacitive circuit during normal voltage conditions and a low impedance circuit during overvoltage conditions.

2. A device as claimed in claim 1 wherein the protector means and the semiconductor protector means are contained within a plug-in housing and wherein impedance means are provided is series with the diode bridge within the housing.

3. A device as claimed in claim 1 or 2 wherein the protector means includes failsafe contacts for actuation in the event of prolonged high overvoltage conditions.

4. A device as claimed in claim 1, 2 or 3 wherein the voltage sensitive diode means includes at least three voltage sensitive semiconductor diodes extending from the diode bridge.

5. A device as claimed in any one of claims 1 to 4 wherein additional semiconductor diode means are provided connected between the diode bridge and the protector means.

6. A device as claimed in claim 5 wherein the additional semiconductor diode means include a pair of diodes connected in parallel.

7. A device as claimed in any one of claims 1 to 6 wherein the protector device comprises a gas discharge device.

8. A device as claimed in any one of claims 1 to 7 wherein the voltage sensitive diode means include pairs of foldback diodes.

9. A protector device for a high speed communications network substantially as described herein with reference to Figure 1 or 3 of the accompanying drawings.