GB2284938A

GB2284938A - Current-limiting device

Info

Publication number: GB2284938A
Application number: GB9423656A
Authority: GB
Inventors: Dierk Boy
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1993-12-14
Filing date: 1994-11-23
Publication date: 1995-06-21
Anticipated expiration: 2014-11-23
Also published as: GB9423656D0; DE4342592C1; SE9404345L; SE9404345D0; GB2284938B

Abstract

A current limiting device which is supplied with energy from a voltage source is proposed for electrical loads. The current limiting device has a series-connected resistor (bias resistor) (7) which is arranged between the electrical load (8) and the voltage source (6). The series-connected resistor (7) is fitted on a film (foil), and the film itself is fitted to the housing wall (4, 5) of a housing which encloses the electrical load (8). This results in it being possible to emit the heat from the series-connected resistor to the environment over a large area in the event of an excessively high current. … <IMAGE> …

Description

1.

1 2284938 Current-limiting device Prior art

The invention concerns a current-limiting device of the type according to the main claim. In many electrical devices, it is expedient to limit the power consumption of the electrical load in order to preclude hazards presented by the electrical device in the case of a short-circuit or in the event of failure of the electrical device. Thus, for example, explosion-protected designs of electrical devices such as transmitters and/or receivers must be constructed so that, even in case of malfunction within the device, the power consumption of the electrical device and the maximum current flow do not exceed a defined value. A further requirement for many devices is that defined surface temperatures are not exceeded, both on and within the device. To meet these requirements, currentlimiting circuits are used, the electrical device being either positively switched off by electronic means or the current being limited by a dropping resistor. In cases where the current is limited by a dropping resistor, sheet or wire resistors are used for this purpose. These resistors, however, present the problem of heat dissipation, which is solved by increasing the surface area of the dropping resistor, with heat sinks or similar devices being mounted on the dropping resistor. This measure, however, increases the weight of the electrical device and also requires a relatively large amount of space within the electrical device.

2 Advantages of the invention In comparison, the current-limiting device according to the invention, having the characteristics according to the main claim, offers the advantage of boch large-area heat dissipation and minimal space requirement. A current-limiting device is achieved which has both a large surface area and a small volume. In addition, there is a low heat transfer to the device housing, so that the heat is instead dissipated very rapidly into the environment. The flexibility of the film means that it can be made to match the contours of the housing and cut in virtually any shape, so that the currentlimiting device can be adapted to the various shapes of housing. A further advantage is that the currentlimiting device according to the invention has a small mass and is largely insensitive to mechanical influences.

Advantageous further developments and improvements of the currentlimiting device according to the main claim are stated in the sub-claims. Accordingly, it is particularly advantageous for the dropping resistor to be applied on the film in meander form. This enables very long conduction paths to be accommodated on a given surface area of film and, by variation of the spacings between the conductors, the dropping resistor can be varied so that the heat discharge per unit area does not exceed predefined values, even in the least favourable case. It is further advantageous to stick the film to the housing wall. This produces a very close contact between the housing wall and the film so that a good heat transfer is achieved between the film and the housing, particularly when a thermally conductive adhesive is used. This provides for a defined and reliable dissipation of the heat produced in the case of the current--- limiting device being tripped. Also to be considered as a further advantage is the fact that, in the case of the housing wall being composed of plastic, the conductor applied on the film can serve simultaneously as an antenna for a radio transmitter and/or 3 receiver. This avoids the necessity of elaborate antenna structures for the radio transmitter and/or receiver and, furthermore, it enables the current-limiting device to act as a necessary antenna in any case, eliminating the need for the antenna which would otherwise be required. It is also advantageous that the film and the housing wall be formed as a single unit. This enables the resistor to be vapour-deposited or printed directly on to a non-conductive housing wall without the necessity of inserting the film. The currentlimiting device is advantageously applicable in radio transmitters and/or receivers, in which it acts as an explosionprotection device.

Drawing Embodiment examples of the invention are illustrated in the drawing and explained in greater detail in the following description. Figure 1 shows a current-limiting device on a film, Figures 2 and 3 show the arrangement of the film on the housing wall, Figure 4 shows the basic circuit for an electrical load with a current-limiting device and Figure 5 shows the electrical circuit of a radio transmitter/receiver in which the current- limiting device acts simultaneously as an antenna.

Description of the embodiment examples

Figure 1 shows a film 1, on which are applied electrical conductors 2 in meander form. Applied at two corners of the film 1 are conductive surfaces 3, which act as terminals for the connectors. A plastic film, preferably polyimide, is suitable for use as a film 1. The electrical conductors 2 are applied on this film in meander form. The thickness of 4 the conductors is determined by the maximum current which is to flow through the conductor 2. The conductor density is determined by the quantity of heat discharged through the conductors 2 in the case of a short-circuit. For example, if the current flow through the conductor 2 is high in such a case, then the spacing between the conductors 2 is greater than when only a relatively low current flows, even in the event of a short-circuit. Finally, the spacing between the conductors is also determined by the maximum amount of heat per unit area that can be discharged through the film or the amount of heat that can be discharged through the housing. If the housing wall is of a material which has a high thermal conductivity, then clearly the conductors 2 can be arranged more densely than in the case of a plastic housing with a lesser heat dissipation capability. The conductors are connected to the terminals 3, and to both the battery and the load, as explained more fully by Figure 4.

It is advantageous for the film shown in Figure 1 to be fixed to the housing wall 4, as illustrated in Figure 2. In this case, the terminals 3 and the conductors 2 are located, advantageously, on the side facing away from the housing wall 4. The film itself is advantageously fixed to the housing wall 4 by adhesive, so as to produce a good thermally conductive contact between the housing wall 4 and the film 1.

Figure 3 shows another embodiment example, in which the film 1 is attached to a curved housing wall 5. This illustrates that the current- limiting device is easily adaptable to a variety of housing designs and occupies hardly any space within the housing.

Figure 4 shows the current-limiting device connected in circuit to a battery 6 and a load 8. One terminal connects the battery 6 directly to the load 8, while the other terminal is connected to the load 8 across the dropping resistor 7. The film resistor according to Figure 1 is used as the dropping resistor 7. If there is an increase in current. within the load 8 caused by a fault then, according to ohm's law, the current that can be drawn from the battery 6 is limited by the resistor 7. The energy produced is converted into heat by the resistor 7 and discharged through the film 1 to the housing wall 4, 5. Since the housing wall normally has a large surface area, the heat is then distributed evenly within the space, thereby preventing an increase in temperature above a defined limiting value. A device protected in this way can be operated, in particular, in explosion-hazardous areas, due to the fact that danger to persons and objects or jeopardy of the device in the event of malfunction are prevented. In particular, the battery is not damaged by excessive current consumption, so that battery explosions, discharged batteries and build-up of detonating gas in lead batteries can be reliably prevented.

Figure 5 shows the circuit arrangement for a radio transmitter and/or receiver, serving in this case as the load 8. The battery 6 is on the one hand directly connected to the radio transmitter and/or receiver, while the other battery terminal leads to a coil 9, after which it is connected the dropping resistor 7. Between the coil 9 and the dropping resistor 7 there is a branch to which is connected a capacitor 11. The dropping resistor 7 is followed by a further coil 10, the further end of which is connected to the additionally supplied supply voltage input of the radio transmitter and/or receiver. Located between the dropping resistor 7 and the coil 10 there is an additional tap which is again connected to a capacitor 11. Wires 12, by means of which the high frequency can be tapped or injected, can be connected to further terminals of the capacitors 11.

6 The circuit shown in Figure 5 operates as follows. The coil 9 and the coil 10 act as a very low resistance to the direct current from the battery 6, so that the current for the radio transmitter and/or receiver is limited only by the dropping resistor 7. On the other hand, the coils 9 and 10 act as very high resistances to the high frequency, so that the HF energy taken up or emitted by the dropping resistor can be tapped or injected through the capacitors 11. The capacitors 11 serve to prevent the direct voltage reaching the HF output or HF input 12. The dropping resistor 7 can be formed on the film in such a way that it is highly capable of taking up and emitting the high frequency. For this purpose, the meanderform structure shown in Figure 1 can be altered so that it constitutes an antenna for the transmitter or receiver which is adjusted to the frequency of the latter. The dropping resistor 7 can therefore be used as both a transmitting and receiving antenna, its foil structure rendering it possible for the antenna to be designed so that there is no need for any greater adaptation to the network.

If the housing wall 4 and 5 is a non-conductive surface then it is also possible, by variation of the invention, to print the dropping resistor directly on to the housing, so that there is no need for the film. The resistance conductors are applied to the film or housing in the known manner, by vapourdeposition or sputtering.

7

Claims

Claims

1. Current-limiting device for an electrical load (8) which is supplied with power from a voltage source (6), with a dropping resistor (7) located between the load (8) and the voltage source (6), characterized in that the dropping resistor (7) is applied on a film (1) and that the film (1) is attached to the housing wall (4, 5) of a housing which contains the electrical load (8).

2. Current-limiting device according to Claim 1, characterized in that the dropping resistor (7) is applied on the film (1) in meander form.

3. Current-limiting device according to either of Claims 1 or 2, characterized in that the film (1) is attached to the housing wall (4, 5) by adhesive.

4. Current-limiting device according to any one of the preceding Claims, characterized in that the film (1) is attached to the inside of the housing wall (4, 5).

5. Current-limiting device according to any one of the preceding Claims, characterized in that the housing wall (4, 5) is composed of plastic and that the conductor (2), which acts as a resistor, applied on the film (1) serves as an antenna for a radio transmitter and/or receiver.

6. Current-limiting device according to any one of the preceding Claims, characterized in that the film (1) and the housing wall (4, 5) form a unit.

8 7. Current-limiting device according to any one of the preceding Claims, characterized in that it serves as an explosion-protection device for a radio transmitter and/or receiver.

8. Any of the current-limiting devices substantially as herein described with reference to the accompanying drawings.