GB2163307A - Fusible electrical resistor - Google Patents

Abstract

A fusible electrical resistor comprises a planar electrically insulating substrate 2 supporting a film electrical resistance element 1 provided with electrically conductive terminals 3, 5 and 4, 6, the resistor being arranged to be connected by the terminals in an electrical circuit and operative such that when a particular power level in the resistor is exceeded due to application of an excessively high current or voltage, thermal stress developed in the resistor results in fracture of the substrate 2 and consequent open-circuiting of the resistance element 1 and interruption of the circuit. The resistor is characterised in that a strip of material 7, e.g. a self- adhesive insulating tape, is adhesively secured to at least one major face of the substrate 2. The strip 7 is arranged to remain intact and adhered to the resistor when fracture of the substrate 2 occurs and prevents dispersion of fragments of the substrate material. <IMAGE>

Description

SPECIFICATION Fusible electrical resistor This invention relates to fusible electrical resistors of the type comprising a planar electrically insulating substrate supporting a film electrical resistance element provided with electrically conductive terminals. A resistor of this type is adapted whereby when it is connected in a circuit in which a fault develops such that a particular power level in the resistor is exceeded due to an excessively high current or voltage flowing through the resistor, thermal stresses develop in the resistor and result in fracture of the substrate and open-circuiting of the resistor. The circuit is thereby interrupted.

Resistors of this type have been known for a number of years. A problem with such resistors has been to ensure that the substrate, which is usually of a ceramic material, fractures cleanly and such that fragments of the fractured substrate material are not dispersed during the fusing process. A method which has been adopted to deal with this problem has involved the provision of one or more lines or zones of weakness in the substrate, such that the substrate fractures preferentially and cleanly along such lines or zones of weakness when the resistor is subjected to electrical overload.

We have devised a fusible electrical resistor which overcomes the above problem using a standard substrate without the provision of any lines or zones of weakness.

The present invention provides a fusible electrical resistor comprising a planar electrically insulating substrate supporting a film electrical resistance element provided with electrically conductive terminals, said resistor being arranged to be connected by said terminals in an electrical circuit and operative such that when a particular power level in said resistor is exceeded due to application thereto of an excessively high current or voltage, thermal stress developed in said resistor results in fracture of said substrate and consequent interruption of said circuit, said resistor being characterised in that there is adhesively secured to at least one major face of said substrate a strip of a material which remains intact and adhered to said resistor when said fracture of said substrate occurs, Preferably said resistance element is secured to one major face of said substrate and said strip of said material is adhesively secured to an opposite major face of said substrate.

Additionally or alternatively, a said strip of said material may be provided adhesively secured to that face of the substrate on which said resistance element is provided and overlying said resistance element.

Preferably the said strip of said material substantialiy entirely covers the said at least one major face of said substrate.

A variety of materials may be used to form the said strip. Electrically insulating materials are preferred although there may be applications where a strip of a metal, e.g. in the form of a foil, may be adhered to the substrate by means of an adhesive. Such a metal strip would, of course, have to be electrically insu -lated from the resistance element and terminals.

A number of flexible self-adhesive electrically insulating tapes are suitable for use as the said strip. Such tapes must be selected so as to withstand elevated temperatures encountered during operation of the resistor. Suitable tapes are commercially available, comprising for example, adhesive-coated paper, nylon, glass cloth or polyester materials. Adhesives used on such tapes may, for example, comprise silicone, acrylic, epoxy or rubber adhesive resin systems.

The resistor with the said strip is preferably provided with an electrically insulating protective coating. Such a coating should preferably be of a flame-resistant nature and serves to prevent or minimise electrical arcing in the resistor when fracture of the substrate occurs.

A silicone resin material may be applied as such protective coating.

Although in the resistor of the invention fragmentation of the substrate may occur when fracture takes place, the said strip of said material remains intact and adhered to the fragments, thus avoiding undesirable dispersing of the fragments.

The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 represents a front plan view of a fusible resistor according to the invention; Figure 2 represents a rear plan view of the fusible resistor of Fig. 1.

Referring to Fig. 1 a fusible resistor is prepared as follows.

A film electrical resistance element 1 is provided of well-known form and by wellknown techniques on a major face of an electrically insulating substrate 2, e.g. of ceramic or glass. Film conductors 3, 4 are provided in electrical contact with the resistance element 1 and terminal leads 5, 6 connected to the conductors 3, 4 enable the resistor to be connected to an electrical circuit (not shown).

Referring now to Fig. 2, a strip 7 of adhesive-coated crepe paper material is applied to cover completely that major face of the substrate 2 opposite to the face of the substrate 2 to which the resistance element 1 is adhered.

A suitable material for the strip 7 is Type 214 Masking Tape manufactured by the 3M Company. Other adhesive-coated tapes manufactured by that company which may also be suitable for this application are: Type 853 paper tape; Type 855 Nylon tape; Types 361 and 365 Glass Cloth tape; and Type 851 Polyester tape. Substantially the entire surface of the resistor, apart from the leads 5, 6 is then provided with a protective coating of a silicone resin (not shown), which is subse quently hardened in known manner. Other organic or inorganic protective coatings well known in the art may be employed, if desired, as an alternative.

When the resulting resistor is connected by its terminal leads 5, 6 in an electrical circuit (not shown), it will operate in the manner of a normal electrical resistor until a fault condition occurs in the circuit which causes an exces sively high current or voltage to be applied to the resistor. A rapid rise in temperature occurs in the resistor and when the rated power of the resistor is exceeded thermal stresses de velop in the resistor with the result that the substrate 2 fractures. When this occurs, the resistance element 1 is ruptured and open circuited and as a consequence the malfunc tioning electrical circuit is broken. When the substrate 2 fractures, although it may break up into a number of fragments of various sizes and shapes, the strip 7 of adhesive tape remains intact and adhered to the fragments of the substrate. The fragments of the sub strate are thereby prevented from dispersing.

In the absence of the strip 7, the somewhat explosive nature of the resistor fusing process when the substrate fractures would result in fragments of substrate material being dis persed, possibly over quite a wide area, with self-evident consequences.

The aforementioned protective coating, e.g.

of hardened silicone resin, prevents arcing from occurring during fusing of the resistor.

Claims (11)

1. A fusible electrical resistor comprising a planar electrically insulating substrate support ing a film electrical resistance element pro vided with electrically conductive terminals, said resistor being arranged to be connected by said terminals in an electrical circuit and operative such that when a particular power level in said resistor is exceeded due to appli cation thereto of an excessively high current or voltage, thermal stress developed in said resistor results in fracture of said substrate and consequent interruption of said circuit, said resistor being characterised in that there is adhesively secured to at least one major face of said substrate a strip of a material which remains intact and adhered to said resistor when said fracture of said substrate occurs.

2. A resistor according to Claim 1 in which said resistance element is secured to one major face of said substrate and said strip of said material is adhesively secured to an opposite major face of said substrate.

3. A resistor according to Claim 1 or 2 in which additionally or alternatively, a said strip of said material is provided adhesively secured to that face of the substrate on which said resistance element is provided and overlying said resistance element.

4. A resistor according to Claim 1, 2 or 3 in which the said strip of said material substantially entirely covers the said at least one major face of said substrate.

5. A resistor according to any preceding claim in which said strip comprises an electrically insulating material.

6. A resistor according to Claim 5 in which said strip of electrically insulating material comprises a flexible self-adhesive tape.

7. A resistor according to Claim 6 in which said tape is selected from the group comprising adhesive-coated paper, nylon, glass cloth and polyester materials.

8. A resistor according to Claim 7 in which said adhesive is selected from the group comprising silicone, acrylic, epoxy and rubber adhesive systems.

9. A resistor according to any of Claims 1 to 4 in which said strip comprises a metal, electrically insulated from said resistance element and terminals.

10. A resistor according to Claim 9 in which the metal strip comprises a foil secured by means of an adhesive.

11. A resistor according to any preceding Claim further provided with an electrically insulating protective coating.

1 2. A fusible electrical resistor constructed and arranged substantially as herein described with reference to the accompanying drawing.