GB2260230A

GB2260230A - Electrical surge protector

Info

Publication number: GB2260230A
Application number: GB9120124A
Authority: GB
Inventors: Paul Turner; Alistair Mark Hunt
Original assignee: Semitron Industries Ltd
Current assignee: Semitron Industries Ltd
Priority date: 1991-09-20
Filing date: 1991-09-20
Publication date: 1993-04-07
Also published as: GB9120124D0

Abstract

The protector comprises, for protecting against excess voltage. a transient suppression device 1 and, for protecting against excess current in the absence of excess voltage, a positive temperature coefficient resistor 2 that is mechanically fast with the transient suppression device 1 by an interference or pressure fit, or by being moulded onto the transient suppression device, thereby avoiding utilising a more intimate form of bonding such as soldering or welding and giving good thermal contact between the two so that heat generated by the transient suppression device upon operation is transferred to the positive temperature coefficient resistor and speed of operation of the protector is enhanced. If resistor 2 is of ceramic form it is moulded in a configuration to give a press fit onto device 1, whereas a plastics resistor 2 may be heat crimped around the device 1. The device 1 may be a gas tube or a thyristor. <IMAGE>

Description

ELECTRICAL SURGE PROTECTORS This invention relates to protectors for use in telecommunication and data transmission systems for protecting against excessive currents and voltages.

In telecommunications and data transmission systems excessive currents and voltages can arise, for example because of strikes by lightning, from which it is necessary to protect equipment in the systems. To this end there can be incorporated in such a system an electrical surge protector that utilises for protection against excess voltage a gas discharge tube that is only conductive if a relatively high voltage is applied across it, and for protection against excess current in the absence of excess voltage a resistance wire and associated switch. The gas discharge tube is connected between input and earth. Normal voltage at the input is not sufficient to pass across the gas discharge tube whereas if the voltage at the input becomes excessive the gas discharge tube becomes conductive and connects the input to earth to prevent the excessive voltage being passed to output.The contacts of the switch are sprung biased together but normally held open by a member formed of a low melting point alloy. If an excessive current flows (in the absence of an excess voltage triggering the gas discharge tube), heat is generated in the resistance wire sufficient to melt the alloy and allow the contacts of the switch to close and thereby connect the input to earth and prevent the excessive current passing to output. Constructions as Just outlined are disclosed in US-4 215 381 and US-4 796 150. A very significant disadvantage is that after the low melting point alloy has melted, allowing the switch to close, the protector cannot easily be reset without replacing the switch.

Clearly this is inconvenient and expensive. Furthermore the overall construction of such a protector is quite complex and in this regard there are protectors that utilise solid-state components, for example thyristors, in place of the gas discharge tube but there still remains the problem of mounting this component in association with a mechanically operating device protecting against excess current in the absence of excess voltage.

According to one aspect of the present invention there is provided an electrical surge protector comprising, for protecting against excess voltage, a transient suppression device, and, for protecting against excess current in the absence of excess voltage, a positive temperature coefficient resistor that is mechanically fast with the transient suppression device by an interference or pressure fit.

According to another aspect of the present invention there is provided an electrical surge protector comprising, for protecting against excess voltage, a transient suppression device, and, for protecting against excess current in the absence of excess voltage, a positive temperature coefficient resistor that is moulded onto the transient suppression device so as to be mechanically fast with the transient suppression device.

In use of a positive temperature coefficient resistor, that is a resistor made from conductive polymer compositions (conductive particles such as metal or carbon dispersed in a polymer matrix such as polyolefins or fluoropolymers), in the presence of sufficiently low currents the resistive Joule heating is inadequate to disturb the low resistance state equilibrium of the resistor. On the other hand, an overcurrent causes the resistor to self heat at a rate faster than its thermal dissipation rate. Thermal equilibrium is re-established when the resistor resistance reaches a value very much greater than its initial value, thus reducing device power generation to a level equal to its dissipation rate. In this way circuit current is reduced to a trickle, thereby protecting circuit components from over current damage.Upon removal of the excess current the resistor cools and returns to its normal low resistance value, returning the circuit to normal operating conditions. In other words, the positive temperature coefficient resistor is self-resetting upon removal of the excess current. The transient suppression device for protecting against excess voltage can be a gas discharge tube or a solid-state component such as a thyristor as outlined above.The mechanical interconnection by interference or pressure fit between the positive temperature coefficient resistor and the transient suppression device, or by moulding the positive temperature coefficient resistor onto the transient suppression device, avoids utilising a more intimate form of bonding such as soldering or welding and gives good thermal contact between the two so that heat generated by the transient suppression device upon operation is transferred to the positive temperature coefficient resistor and speed of operation of the protector is enhanced.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing, in which: Figure 1 is a diagrammatic illustration of an electrical surge protector, Figure 2 illustrates how the surge protector of Figure 1 is connected in an electrical circuit, and Figure 3 is a diagramatic sectional view illustrating a moulding technique.

The electrical surge protector of Figure 1 comprises, for protecting against excess voltage, a transient suppression device 1, and, for protecting against excess current in the absence of excess voltage, a positive temperature coefficient resistor 2. The transient suppression device 1 can be a gas tube or a solid-state component such as a thyristor.

The positive temperature coefficient resistor 2 is mechanically fast with the transient suppression device 1 by an interference or pressure fit. For example, if the resistor 2 is of ceramic form it is moulded in a configuration to give a press fit onto the device 1. If the resistor 2 is of plastics material it is heat crimped around the resistor 2. The mechanical connection is such as to give good ohmic contact between the resistor 2 and the device 1.

Connections are as shown in Figure 2 with the resistor 2 connected in series in a circuit between input L1 and output L2, input L1 also being connected via the resistor 2 and the device 1 in series to earth at GND.

Figure 3 illustrates moulding the positive temperature coefficient resistor onto the transient suppression device. The positive temperature coefficient resistor 2 is held in a closed mould 3 so as to project into the mould cavity 4. The material to form the positive temperature coefficient resistor is injected into the mould cavity 4 via an inlet 5 so as to be moulded onto the resistor 2 and onto the input L1, also held in the mould 3 so as to project into the cavity 4. At the conclusion of the moulding process, the mould is opened and the completed surge protector is removed.

Claims

1. An electrical surge protector comprising, for protecting against excess voltage, a transient suppression device, and, for protecting against excess current in the absence of excess voltage, a positive temperature coefficient resistor that is mechanically fast with the transient suppression device by an interference or pressure fit.

2. An electrical surge protector as claimed in 1, where the positive temperature coefficient resistor is moulded in a configuration to give a press fit onto the transient suppression device.

3. An electrical surge protector as claimed in 1, where the positive temperature coefficient resistor is heat crimped onto the transient suppression device.

4. An electrical surge protector comprising, for protecting against excess voltage, a transient suppression device, and, for protecting against excess current in the absence of excess voltage, a positive temperature coefficient resistor that is moulded onto the transient suppression device so as to be mechanically fast with the transient suppression device.

5. An electrical surge protector substantially as hereinbefore described with reference to Figure 2 and either Figure 1 or Figure 3 of the accompanying drawing.