GB2160721A - Protection of electrical/electronic equipment - Google Patents

Abstract

A self-resetting protection unit for connection in series between electrical/electronic equipment and one or more transmission lines T1, T2 comprises a gas discharge tube GDT1 to divert large currents to earth during a surge voltage above a first level, current limiting thermistors TH1 to TH4, and a shunt circuit ZD1, ZD2, SCR1 to clamp the output voltage of the unit to a low level in response to live voltages less than the first level but greater than a second level. High amplitude surges cause the tube CDT1 to strike, zener diodes ZD1, ZD2 acting to link the output voltage during the period required for the tube to reach on arc condition. Surge currents of less than 10 amps will not cause the tubes to arc but cause thermistors TH1 to TH2 to heat up and limit the current. During such heating up, current flowing through the diode ZD2 turns on SCR1 to clamp the output voltage to a low level. Capacitor C1, and transistors TR11, TR12 provide a means to turn off thyristor SCR1. An alternative embodiment (Fig. 1) has only one transistor (TR1) and one zener diode (ZD). <IMAGE>

Description

SPECIFICATION Protection of electrical/electronic equipment This invention relates to the protection of electrical and/or electronic equipment from voltage surges or sustained electrical faults, for example from voltage surges due to lightning.

It is conventional to provide lightning protection units for the protection of radio, telemetry, computing and process control equipment when these are connected to transmission lines which are vulnerable to electrical interference. The protection unit functions to protect the equipment not only from lightning, but also from other induced voltage surges and continuous overloads, to which the transmission lines are vulnerable.

In normal operation, the protection unit passes dc or ac signals with negligible attenuation, but if presented with a high voltage surge it diverts the current safely to earth, open circuiting the lines if the overload is prolonged and resetting automatically after the surge has died away.

One example of such a lightning protection unit is described in our UK patent specification No. 1587681. In that specification there is disclosed a protection unit for connection in series between electronic equipment and one or more transmission lines, the unit comprising first means arranged to discharge to earth line voltages exceeding a first predetermined level, second means for regulating the voltage transmitted by the unit from the or each transmission line to the electronic equipment, and automatically resettable third means for protecting the second means against damage from voltages above a second predetermined level and below the first predetermined level.

Operation of this known protection unit can be considered in two stages. In the event of a voltage surge exceeding 250 volts for example, a three-electrode gas discharge tube fires, diverting the surge current to earth and clamping the voltage of the incoming lines to about 30 volts. While the surge is developing and until the gas discharge tube has fired, zener diodes limit the output voltage to a preset value which is safe for the equipment concerned. This working voltage can be preselected and changed by the user to any one of a number of different voltage levels. Problems can arise from prolonged surges or sustained electrical faults of 200 volts or less since these do not cause the gas discharge tube to fire and, if no action is taken, would destroy the zener diodes.It has been known to incorporate protective fuses which are replaced after each prolonged surge, but the protection unit described in our aforesaid patent specification overcomes this problem by using reed relays to protect each zener diode. The relays open during a long surge and close again when the surge has died away. Our aforesaid British patent specification 1587681 also describes an alternative arrangement where the reed relay which provides isolation protection is replaced by positive temperature coefficient thermistors inserted in the transmission lines.

The present invention is concerned with an improved form of self-resetting protection unit. In accordance with the present invention there is provided a self-resetting protection unit for connection in series between electrical and/or electronic equipment and one or more transmission lines, the unit comprising first means arranged to discharge to earth line voltages exceeding a first predetermined level, current-limiting means in the or each said transmission line, and a shunt circuit arranged in response to line voltages less than said first level but greater than a second predetermined level to clamp the output voltage from the unit to a low level.

The protection unit of the present invention in a preferred form has three main parts, namely a high surge capacity gas discharge tube, current-limiting positive temperature coefficient thermistors, and a shunt circuit. It is this shunt circuit which constitutes the significant feature of difference as compared with the protection unit described in our aforesaid British patent specification.

In order that the present invention may be more fully understood, two embodiments of protection unit in accordance with the present invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig.1 is a schematic circuit diagram of a first embodiment of self-resetting protection unit; Fig.2 illustrates the current/voltage characteristic of the shunt circuit of the protection unit shown in Fig.1; and, Fig.3 is aschematic circuit diagram of a second embodiment of protection unit.

As schematically shown in Fig.1, the protection unit embodying the invention is intended for series connection between a pair of transmission lines T1, T2 subject to voltage surges or electrical faults and associated electrical and/or electronic equipment which is connected to terminals 1 and 2.

Primary protection to divert a surge current to earth during a fault is obtained by a gas discharge tube GDT1 which has a high shunt resistance at normal signal levels and can divert large currents to earth during a surge.

The tube is also inherently self-resetting after a surge. The tube is preferably a three-electrode tube, as shown. This gas discharge tube operates at a nominal 200 volts to discharge voltages in excess of this level to earth. During a discharge of the tube the voltage across the tube drops to approximately 30 volts.

Before the gas discharge tube GDT1 fires, the transient voltage is clipped by a zener diode ZD1 and a plurality of steering diodes indicated at D1 to D10.

Connected in parallel with the zener diode ZD1 is a silicon controlled rectifier SCR1. Also connected in parallel with the zener diode ZD1 is an NPN transistor TR1. A resistor R1 is connected in series with the zener diode ZD1, a resistor R2 is connected to the base of transistor TR1, and a resistor R3 is connected to the emitter of transistor TR1. Two positive temperature coefficient thermistors TH 1 and TH3 are connected in series into the first transmission line T1, and two equivalent thermistors TH2 and TH4 are connected in series into the second transmission line T2. The connections between the shunt circuit and the transmission lines are made between the respective pairs of thermistors TH 1 ,TH3 and TH2, TH4.

If there occurs a low surge current, insufficient to cause the gas discharge tube GDT1 to arc, but with sufficient voltage to cause the zener diode ZD1 to conduct, then the silicon controlled rectifier SCR1 will fire, clamping the voltage at the protected equipment terminals 1 and 2 to a low level. If this surge is prolonged, then the thermistors TH1 and TH2 will eventually enter a hrgh resistance stage, thereby reducing the current in the rectifier SCR1 to a low level. The transistor TR1 and zener diode ZD1 then operate like a high power zener diode, allowing the thermistors TH1 and TH2 to cool and to return to their low resistance state when the surge has passed.

The second thermistors TH3 and TH4 in each transmission line serve to prevent a low impedance source connected to the protected equipment terminals 1 and 2 from keeping the rectifier SCR1 turned on.

The current/voltage characteristic of the shunt circuit of Fig.l is shown in Fig.2. The clamping voltage is indicated by the broken line V, and the transmission current from zener to clamp mode of the shunt element SCR1 is indicated at It. This transition current should be above the switching current of the thermistors TH1 and TH2, in order to prevent them from latching on under trip conditions.

With this self-resetting unit overvoltage surges are handled in a number of different ways depending on the amplitude and duration of the surge. Each kind of surge and its effect on the unit is described separately below: 1) High amplitude surges ( > 250 volts and > 50 amps) High amplitude surges of this magnitude will cause the gas discharge tube to strike and clamp the voltage across the lines to a very low level. The gas discharge tube takes up to 2 microseconds before reaching an arc condition. During this time ZD1, D4 to D7 and the appropriate bridge diodes limit the output transient voltage to a few volts above the zener voltage.

2) Medium amplitude surges ( < 10 am and > 0.5 amps) Surge currents of less than 10 amps will not cause the discharge tube to arc to a low voltage.

The current in TH1 or TH2 will be sufficient to cause then to heat and eventually reach a high resistance state. During this period while TH1 and TH2 are heating, current flowing through ZD1 and R1 will trigger SCR1 and hence clamp the output voltage to a low level.

The increase in resistance of the thermistors will soon cause the current in SCR1 to drop below its holding value (this action is assisted by TR 1) and hence turn off.

At this point the output voltage will rise to just above the zener voltage of ZD 1. When the surge eventually decays, TR1 will no longer conduct, allowing the thermistors to cool and return to their low resistance state.

3) Low amplitude surges ( < 0.2 amps) Low surge currents will not trigger SCR 1 into conduction because transistor TR 1 will carry all the surge current. In this case the output voltage will be clamped to just above the zener voltage of ZD1.

Reference is now made to the modified embodiment shown in Fig.3.

Before the gas discharge tube GDT1 fires, the transient voltage is clipped by a zener diode ZD1, and a plurality of steering diodes D11,D12,D13 and D17, D18, D19.

Connected in parallel with the zener diode ZD1 is a silicon controlled rectifier SCR1. Also connected in parallel with the zener diode ZD1 is an NPN transistor TR12. Three voltage dropprng diodes D14, D15, D16 are connected in series with the cathode of the silicon controlled rectifier SCR1. In series with zener diode ZD2 is a programmable unijunction transistor TR11. A resistor R13 connects the anode of zener diode ZD2 to the gate of the silicon controlled rectified SCR 1. A resistor R 1 4 connects the gate of the silicon controlled rectifier SCR 1 to the anode of the programmable unijunction transistor TR 11. A resistor R12 connects the cathode of the diode D14 to the gate of the programmable unijunction transistor TRi 1. A capacitor C1 and a resistor Rl6 are connected in series, between the anode of the programmable unijunction transistor TR1 1 and the emitter of the transistor TRi 2. A resistor R1 5 is connected in series with the cathode of the programmable unijunction transistor TR 11, and a resistor R11 is connected between the gate and the cathode of the silicon controlled rectifier SCR 1. Two positive temperature coefficient thermistors TH1 and TH3 are connected in series into the first transmission line T1, and two equivalent thermistors TH2 and TH4 are connected in series into the second transmission line T2. The connections between the shunt circuit and the transmission lines are made between the respective pairs of thermistors THi, TH3 and TH2, TH4.

If there occurs a low surge current, insufficient to cause the gas discharge tube GDT1 to arc, but with sufficient voltage to cause zener diode ZD2 to conduct, then the silicon controlled rectifier SCR1 will fire, clamping the voltage at the protected equipment terminals 1 and 2 to a low level. If this surge is prolonged, then the thermistors TH1 and TH2 will eventually enter a high resistance state, thereby reducing the current in the rectifier SCR1 to a low level.

Under conditions of a fast rising surge, zener diode ZD1 serves to clamp the output voltage during the period of time which the rectifier SCR1 takes to turn on.

Turning off the rectifier SCR1 is effected in the following manner. The capacitor C1 is charged through resistor R 14 until the pro grammable unijunction transistor TR 11 fires.

The charge stored by capacitor C1 flows through the programmable unijunction transistor TRi 1 and into the base of transistor TR 12. Resistor R16 limits the base current, and resistor R15 limits the turnoff time of transistor TRi 2. The transistor TRi 2 diverts current away from the rectifier SCR 1. If the surge current has fallen to a sufficiently low level, then all the current is diverted away from the rectifier SCR1, and the rectifier SCRi is turned off. When capacitor C1 has discharged sufficiently, the programmable unijunction transistor TR11 turns off, and the transistor TR 1 2 turns off.If an overload is still present, the rectifier SCR1 will fire again, and the process will repeat itself.

If this surge is prolonged, then the thermistors TH 1 and TH2 will eventually enter a high resistance state, thereby reducing the current in the rectifier SCR 1 to a low level. When the surge has passed, the thermistor TH 1 and TH2 will cool and return to their low resistance state.

In this embodiment of the present invention overvoltage surges are handled in the following ways: 1) High Amplitude surges ( > 250 volts and > 50 amps) High amplitude surges of this magnitude will cause the gas discharge tube to strike and clamp the voltage across the lines to a very low level. The gas discharge tube takes up to 2 microseconds before reaching an arc condition. During this time zener diodes ZD1, ZD2 and the appropriate bridge diodes limit the output transient voltage to a few volts above the zener voltage.

2) Medium Amplitude surges ( < 10 amps) Surge currents of less than 10 amps will not cause the discharge tube to arc to a low voltage. The current in thermistor TH1 or TH2 will be sufficient to cause them to heat and eventually reach a high resistance state.

During this period, current flowing through zener diode ZD2 and resistor Ri 3 will trigger rectifier SCR1 and hence clamp the output voltage to a low level. As the current is reduced, the action of the transistor TR ii will allow the output voltage to rise to the zener voltage in a series of spikes. As the surge decays, insufficient voltage will be applied to trigger rectifier SCR1, and the thermistors will cool and return to their low resistance state.

Claims (9)

1. A self-resetting protection unit for connection in series between electrical and/or electronic equipment and one or more transmission lines, the unit comprising first means arranged to discharge to earth line voltages exceeding a first predetermined level, current-limiting means in the or each said transmission line, and a shunt circuit arranged in response to line voltages less than said first level but greater than a second predetermined level to clamp the output voltage from the unit to a low level.

2. A protection unit as claimed in claim 1, in which two thermistors are connected in series in the or each transmission line, and the shunt circuit is connected to the line or lines between the pair of thermistors or between the respective pairs of thermistors.

3. A protection unit as claimed in claim 2, in which the shunt circuit switches between a zener mode and a clamp mode at a predetermined transition current, said transition current being at a level higher than the switching current of the thermistors.

4. A protection unit as claimed in claim 2 or 3, in which the thermistors are positive temperature coefficient thermistors.

5. A protection unit as claimed in any preceding claim, in which the shunt circuit includes a silicon controlled rectifier.

6. A protection unit as claimed in any preceding claim, in which the shunt circuit includes a zener diode connected in parallel with a silicon controlled rectifier with transistor means connected in parallel with said zener diode.

7. A protection unit as claimed in claim 6, in which a programmable unijunction transistor is connected in series with said zener diode.

8. A protection unit as claimed in any preceding claim, in which said first means comprises a gas discharge tube.

9. A self-resetting protection unit substantially as hereinbefore decribed with reference to Figs. 1 and 2 or Figs. 2 and 3 of the accompanying drawings.