GB2302621A - Electrical surge protector for signal lines - Google Patents

Abstract

Signal transmission lines operating at frequencies from 1MHz to several hundred MHz are protected against surges, eg. due to lightning, by means of a surge protector incorporating one or more chokes 16 having a saturating magnetic core, particularly a ferrite core. The choke presents a high shunt impedance to the high frequency signals. The frequency components of a surge are at lower frequencies at which the choke 16 will saturate at relatively low currents (tens of amps) to present a low impedance to the surge. Such a choke (30, Fig.9) may be used by itself as a surge protector, eg. connected between the inner conductor and shield of a coaxial cable. Alternatively, one or more saturating core chokes 16 may be connected in series with a surge protection device 22 which would otherwise divert wanted signal current due to low capacitive reactance at the signal frequencies. Such a surge protector may be in the form of a transient suppression diode 22 together with steering diodes. A gas discharge tube 20 may be provided as an additional surge protector.

Description

ELECTRICAL SURGE PROTECTORS This invention relates to electrical surge protectors, and in particular to surge protectors for protecting high frequency circuits, e.g. in the range of 1MHz to several hundred MHz.

Where signals are conveyed between a transmitter and a receiver via a transmission line one can draw a simple electrical equivalent circuit where the transmitter is represented by a voltage source and an associated source resistance, and the receiver is represented by a load resistance. The transmission line has an associated "characteristic impedance". For optimum signal transmission, the source resistance, the load resistance and the characteristic impedance of the transmission line are made as near equal as possible.

A problem arises when surge protection devices are added to the transmission line. Surge protectors operate by switching from a relatively high impedance, non-conducting state to a relatively low impedance, conducting state when a threshold voltage is exceeded by a surge. At frequencies up to as much as several hundred megahertz, the surge protector in its nonconducting state appears predominantly as a capacitive reactance. It diverts signal current away from the load and results in a degraded "return loss, which is a measure of the quality of power transfer between source and load.

It is an object of the present invention to provide an improved surge protector, especially for protecting high frequency circuits.

In accordance with the present invention this is achieved by using as the surge protector or incorporating into the surge protector one or more magnetically-cored chokes. Ferrite chokes are particularly suitable, consisting of a plurality of turns of wire wound on a ferrite core.

The choke or chokes are used to isolate the protection circuit at high frequencies, e.g. in the range given above. The magnetically-cored choke appears as a combination of series inductance and resistance, depending upon frequency. The provision of one or more chokes can considerably reduce the high frequency current flowing through the surge protector, thereby improving the transfer of signal power between source and load and giving an improved, i.e. higher, return loss.

In accordance with a preferred feature of the invention, ferrite cores are used which saturate at low currents, in practical terms of the order of tens of amps.

In one practical embodiment of the invention the surge protector comprises the combination of one or more magnetically-cored chokes with a diode network.

In order that the invention may be more fully understood, reference is now made to the following more detailed description which is given by way of example and with reference to the accompanying drawings. In the drawings: Fig. 1 is a block schematic diagram showing the simple electrical equivalent of a conventional surge protector added to a transmission line; Fig. 2 shows the circuit of Fig. 1 with the addition of a ferrite choke in accordance with the invention; Fig. 3 illustrates the ideal current path for the Fig. 2 arrangement with a surge source; Fig. 4 is an electrical circuit diagram of a hybrid protector using a diode network; Fig. 5 is the equivalent circuit of the arrangement shown in Fig. 4; Fig. 6 shows the addition of ferrite chokes to the diode network of Fig. 4; Fig. 7 shows the equivalent circuit of Fig. 6;; Fig. 8 is an illustration of one example of a ferrite choke suitable for use with the invention; and, Fig. 9 shows the use of a magnetically-cored choke as a surge protector in its own right.

As mentioned above, Fig. 1 shows a simple electrical equivalent circuit of a surge protector 10 added to a transmission line 12. The current i flowing in the transmission line is shown as current ip flowing through the surge protector 10 and current il flowing through the load 14.

Fig. 2 shows the addition to the circuit of a magnetically-cored choke 16, here a ferrite choke, in series with the surge protection device 10. This is to reduce the unwanted signal current flowing through the surge protection device 10. However, the introduction of the ferrite choke 16 gives rise to another potential problem in that it may present a high impedance to the flow of surge current induced in the transmission line, for example by lightning activity. This is indicated in Fig. 3 by the addition of a surge generator equivalent source 18. This would reduce the effectiveness of the surge protector by isolating it from the circuit in a similar way to the case for the signals.However, advantage can be taken of two factors: a) for high frequency transmission, e.g. from 1MHz to several hundred MHz, the frequency components of the wanted signal can occupy a higher frequency range than the frequency components of the surge energy. This means that the ferrite choke can be made to offer a relatively low impedance to the surge, as compared to the wanted signal, by using a sufficiently low inductance.

b) a ferrite core can be used which saturates at quite low currents, in practice of the order of tens of amps. This means that soon after the onset of the surge current, the effect of the ferrite core in producing inductance and resistance disappears, leaving just the relatively low impedance of the wire.

By suitable choice of the ferrite core and winding it is possible to achieve a surge protector which is isolated from the signal path under normal conditions, but which is effectively "switched" into the circuit in the presence of a surge which is induced by lightning or some other phenomenon.

Fig. 3 shows in idealised form the surge current path via the conducting surge protector 10 and the saturated ferrite core 16. In this ideal case, all the surge current is diverted through the protector and none flows through the signal transmitter or receiver.

A typical circuit diagram of a so-called "hybrid" protector, i.e. a surge protector employing more than one type of voltage limiting element, separated by a series impedance element, is shown in Fig. 4. The first voltage limiting element is a gas-discharge tube 20 which has a very low capacitance and therefore a very small effect upon the signal. In the case of a surge protector for use with high signal frequencies the series impedance element is preferably a resistor having a low associated series inductance, indicated as Z, in order to minimise high frequency loss. The second voltage limiting element comprises a diode network which, as shown, consists of a voltage-limiting transient suppression diode 22, together with a set of "steering" diodes which route the surge current through the transient suppression diode 22.The steering diodes have a considerably lower capacitance than the transient suppression diode and are therefore used in order to reduce the effect on the signal.

Although the circuit shown in Fig. 4 represents one practical implementation for a hybrid protector, there are a number of alternative possibilities. These include for example the use of two-terminal gasdischarge tubes, or alternatively a diode-only protector which has no gas discharge tube or series impedance element.

The crucial point, which is illustrated by Fig. 5, is that in each of these cases the diode network appears to the signal as a capacitance, which, at high frequencies, tends to shunt the signal.

Fig. 6 shows an embodiment in accordance with the invention in which two ferrite chokes 16 are added to the diode network. Although two chokes are shown, one may be sufficient. As before, the ferrite choke may typically consist of a few turns of wire wound on a ferrite core. Again, it is desirable that the core is designed so that magnetic saturation will occur at relatively low current.

Fig. 7 shows the effect of the ferrite chokes in Fig. 6. They have the effect of introducing an impedance in series with the shunt capacitance of the diode network. This reduces the loss imposed by the diode network, and improves the impedance match. When surge current flows, the diode network conducts, and the ferrite choke or chokes saturate, greatly reducing the impedance of the ferrite choke or chokes, and allowing the surge protector to appear as a low overall impedance to the surge current, which is thus conducted safely away from the load, i.e. away from the device which is being protected.

Fig. 8 shows a typical ferrite choke which can be used in the embodiments of the invention. This is a 2aperture core 24 with a winding 26. The particular core 24 is manufactured by Siemens as a "SIFERRIT" core, part number B62152 A8X30.

A choke based on a saturating magnetic core can be used in accordance with the invention as a surge protector in its own right at frequency ranges of the order of 1MHz to several hundred MHz. This is illustrated in Fig. 9. The choke 30 is connected between two signal lines 31, 32. For system impedances of the order of tens of ohms, and over the frequency range of the invention, the choke 30 has a sufficiently high impedance to present, for many purposes, a negligible loss of wanted signals. Should a transient over-voltage occur between the signal lines, with a rise time of the order of microseconds to tens of microseconds, howver, the choke will conduct current.

When this current reaches perhaps tens of amperes, the core will saturate, and the impedance presented by the choke will drop, and the choke's current carrying capacity will be limited only by the wire with which it is wound.

Although Fig. 9 shows the choke 30 connected between signal lines, which could be the inner conductor and shield of a coaxial cable, its use would be equally applicable between either signal line and a connection to earth.

Claims (9)

CLAIMS:

1. A surge protector for signal lines, utilisable over a frequency range of 1MHz to several hundred MHz, comprising a choke based on a saturating magnetic core.

2. A surge protector for signal lines, comprising a surge protection device and at least one magnetically-cored choke.

3. A surge protector according to claim 1 or 2, in which the core is a ferrite core.

4. A surge protector according to claim 2, in which the surge protection device and said at least one choke are connected in series between a pair of signal lines.

5. A surge protector according to claim 2 or 4, comprising more than one voltage limiting element connected between a pair of signal lines and separated by an impedance element connected in series in the signal line or lines.

6. A surge protector according to claim 2, 4, or 5, which includes a diode network connected in series with said at least one choke between a pair of signal lines.

7. A surge protector according to claim 6, in which the diode network comprises a voltage-limiting transient suppress ion diode and steering diodes to route any surge through the suppression diode.

8. A surge protector according to any preceding claim, connected between a pair of signal lines which are the inner conductor and shield respectively of a coaxial cable.

9. A surge protector substantially as hereinbefore described with reference to Fig. 2, Fig 3, Figs. 6 and 7, or Fig. 9 of the accompanying drawings.