GB1582054A

GB1582054A - Fusible power resistors

Info

Publication number: GB1582054A
Application number: GB2260777A
Authority: GB
Original assignee: Kenneth E Beswick Ltd
Current assignee: Kenneth E Beswick Ltd
Priority date: 1978-05-26
Filing date: 1978-05-26
Publication date: 1980-12-31

Description

(54) IMPROVEMENTS IN OR RELATING TO FUSIBLE POWER RESISTORS (71) We, KENNETH E. BESWICZC LIMITED, a British Company, of Alert Works, Frome, Somerset BA11 lPP, do hereby declare the invention, for which we -pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention concerns fusible power resistors. Power resistors, as the name implies, are intended to sustain relatively high power loadings which could not be sustained by most of the ordinary ranges of carbon or metal film resistors.

Owing to their high rating, in the event of accidental overload the heat generated will cause damage to the resistor and also constitute a fire hazard.

To prevent both of these effects, a so called "fusible" power resistor is used, in which, when the resistor reaches a dangerously high temperature, the current is interrupted.

In most of the known fusible resistors the connection to the resistor element is by way of a spring lead or spring wire terminal which lies along one side of the resistor casing and which is tensioned into contact with, and is soldered to, the element termination lead, with solder of known melting point.

The heat generated by an overload is transferred to the solder by way of the element termination lead, which causes the said solder to melt and the tensioned leaf or wire spring to revert to its pre-tension position, thus breaking the contact.

It is obvious that in so doing, the lead or wire spring will then stand away from the resistor casing and might conceivably interfere with other parts of the circuit and cause subsequent damage.

An object of the present invention accord ingly is to provide a fusible resistor giving a similar clean breakage of contact, with similar satisfactory clearances, but with the operation of contact breaking taking place within an area similar to that taken up by the resistor casing, thus avoiding any danger of interference with any other portion of, or component in, the circuit.

Accordingly the present invention consists in a fusible power resistor having a single resistor element within a ceramic casing, the resistor element having a terminal lead projecting beyond one end of the casing and soldered to a further lead under tension so that when the solder melts the leads separate into positions which lie within a projection of the cross-sectional area of the casing.

In order that the present invention may be more readily understood two embodiments thereof will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is an end view of a fusible power resistor constructed in accordance with the present invention, Figure 2 is a side elevation of the resistor shown at Figure 1, Figure 3 is a side elevation, in the opposite plane, of the resistor shown at Figure 1, and Figure 4 is a perspective view of a second embodiment.

Referring to Figure 1 there is shown a fusible power resistor 1 having an element terminal lead 2 which extends through a ceramic casing 3 having four cover holes 31 and two opposed longitudinal "T" slots 32, The terminal leads 2 of the fuse element project from opposite ends of the casing 3, and the resistor is designed to be vertically mounted. The lower terminal lead 21 is left at its assembled length for mounting and connection purposes, while the upper terminal lead 21l is cut short, cranked at 4, flattened and positioned to act as a solder post 5.

A further lead 6 of spring-like material is passed through the corner hole 31 which is approximately diagonally opposed to the said solder post 5, and is so formed that a bent portion 61 locates within one of the aforementioned "T" slots, whilst a projection in the form of a square cornered arch is so disposed as to be radially deformable to meet the said solder post 5.

This latter, arch shaped, portion 61 is tensioned against its own torsional spring action, into contact with the said solder post and attached thereto by means of solder of a known melting point.

In operation, should heat generated by the resistor being overloaded cause the temperature to rise and reach the critical level, the heat conducted along the terminal lead/ solder post 5 will cause this solder to melt and the spring lead 6 to return to its free position as shown by the broken line 101, thus separating from the solder post 5.

This arrangement results in a broken contact with adequate clearance to ensure that no arcing or bridging takes place, whilst it will be appreciated that movement of the spring lead is contained within an area which is the same as the area occupied by the cross-section of the resistor casing and which is a projection of this cross-section. Thus no leads project beyond the perimeter of the casing to interfere with other circuit elements which may be adjacent.

A slightly simpler embodiment is shown in Figure 4. In this Figure the casing 3 is shown as being identical to the casing in Figure 1. However it will be appreciated from the following description that the corner holes 31 and one of the opposed "T" slots 32 may be omitted.

The main lead 2 extends through the centre of the casing and, as in the first embodiment, the lower terminal lead 21 is left at its assembled length for mounting and connection purposes. The upper terminal lead 21sl is cut short and bent through approximately 900 to provide a solder post 10. A further lead of spring temper, for example of phosphor bronze, is located in one of the "T" slots so that a section 61 of said lead wire extends beyond the ceramic casing and lies parallel to the part of the upper terminal lead 21 forming the solder part 10. The central portion 6" of said lead is formed into a series of shallow bends so that when this section is inserted into the "T" slot in the ceramic body 3 it is firmly and positively retained by the spring action of these bands. The upper portion 6111 of the said lead wire is angularly offset, the last section 12 being further angled and bent at right angles to the plane of the bends in the leads so that it is spring biassed away from the centre line of the casing 3.

On assembly, said formed lead wire 6 is inserted into the "T" shaped slot opposed to the slot across which the formed upper resistor lead portion 10 lies. The section 61 of the lead wire is then forced into contact with the solder post 10 of the resistor and soldered thereto with solder of a known melting point as in the previous embodiment.

In use this embodiment performs in a manner identical to the embodiment of Fig.

1. Thus excess heat will cause the solder to melt allowing the upper portion of the wire 6 to flex away from the portion 10, thus ensuring adequate clearance. Furthermore it is also arranged that the portion 6111 does not project beyond an area which is a projection of the cross-sectional area of the casing 3.

WHAT WE CLAIM IS:- 1. A fusible power resistor having a single resistor element with a ceramic casing, the resistor element having a terminal lead projecting beyond one end of the casing and soldered to a further lead under tension so that when the solder melts the leads separate into positions which lie within a projection of the cross-sectional area of the casing.

2. A resistor as claimed in claim 1, wherein the resistor element extends through a central bore in said casing.

3. A resistor as claimed in claim 1 or claim 2, wherein the further lead extends through the casing in a bore lying parallel to the resistor element.

4. A resistor as claimed in claim 3, wherein the terminal to which said further lead is soldered, is bent in a direction away from the longitudinal axis of said casing, cranked and flattened to provide a solder post.

5. A resistor as claimed in claim 4, wherein said further lead has a section formed into a loop, the end of the loop being held in a slot provided in one side of said casing to tension said further lead away from said solder post.

6. A resistor as claimed in claim 1 or claim 2, wherein said casing has a longitudinal slot, said further lead lying within and extending along said slot and being formed into a plurality of bends which engage the walls of the slot to hold said further lead within said slot and to tension said one end away from the terminal lead to which it is soldered.

7. A resistor as claimed in claim 6, wherein said further lead is of phosphor bronze.

8. A fusible power resistor substantially as hereinbefore described with reference to either Figures 1 to 3 or Figure 4 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. This latter, arch shaped, portion 61 is tensioned against its own torsional spring action, into contact with the said solder post and attached thereto by means of solder of a known melting point. In operation, should heat generated by the resistor being overloaded cause the temperature to rise and reach the critical level, the heat conducted along the terminal lead/ solder post 5 will cause this solder to melt and the spring lead 6 to return to its free position as shown by the broken line 101, thus separating from the solder post 5. This arrangement results in a broken contact with adequate clearance to ensure that no arcing or bridging takes place, whilst it will be appreciated that movement of the spring lead is contained within an area which is the same as the area occupied by the cross-section of the resistor casing and which is a projection of this cross-section. Thus no leads project beyond the perimeter of the casing to interfere with other circuit elements which may be adjacent. A slightly simpler embodiment is shown in Figure 4. In this Figure the casing 3 is shown as being identical to the casing in Figure 1. However it will be appreciated from the following description that the corner holes 31 and one of the opposed "T" slots 32 may be omitted. The main lead 2 extends through the centre of the casing and, as in the first embodiment, the lower terminal lead 21 is left at its assembled length for mounting and connection purposes. The upper terminal lead 21sl is cut short and bent through approximately 900 to provide a solder post 10. A further lead of spring temper, for example of phosphor bronze, is located in one of the "T" slots so that a section 61 of said lead wire extends beyond the ceramic casing and lies parallel to the part of the upper terminal lead 21 forming the solder part 10. The central portion 6" of said lead is formed into a series of shallow bends so that when this section is inserted into the "T" slot in the ceramic body 3 it is firmly and positively retained by the spring action of these bands. The upper portion 6111 of the said lead wire is angularly offset, the last section 12 being further angled and bent at right angles to the plane of the bends in the leads so that it is spring biassed away from the centre line of the casing 3. On assembly, said formed lead wire 6 is inserted into the "T" shaped slot opposed to the slot across which the formed upper resistor lead portion 10 lies. The section 61 of the lead wire is then forced into contact with the solder post 10 of the resistor and soldered thereto with solder of a known melting point as in the previous embodiment. In use this embodiment performs in a manner identical to the embodiment of Fig. 1. Thus excess heat will cause the solder to melt allowing the upper portion of the wire 6 to flex away from the portion 10, thus ensuring adequate clearance. Furthermore it is also arranged that the portion 6111 does not project beyond an area which is a projection of the cross-sectional area of the casing 3. WHAT WE CLAIM IS:-

1. A fusible power resistor having a single resistor element with a ceramic casing, the resistor element having a terminal lead projecting beyond one end of the casing and soldered to a further lead under tension so that when the solder melts the leads separate into positions which lie within a projection of the cross-sectional area of the casing.