EP0172416A1

EP0172416A1 - A self-protected sheathed resistor

Info

Publication number: EP0172416A1
Application number: EP85109056A
Authority: EP
Inventors: Duilio Capraro
Original assignee: IRCA SpA Industria Resistenze Corazzate e Affini
Current assignee: IRCA SpA Industria Resistenze Corazzate e Affini
Priority date: 1984-08-22
Filing date: 1985-07-19
Publication date: 1986-02-26
Anticipated expiration: 2005-07-19
Also published as: IT8422902V0; EP0172416B2; DE3564899D1; EP0172416B1; ATE37127T1

Abstract

This invention relates to a temperature limiter particularly useful with electric heater elements of the sheathed type.

A switch is supported on the flange of the heater by fulcrum means which allow for the oscillatory movement of the switch relatively to the flange.

The oscillation of the switch is actuated by a rod engaging both the sheath of the heater and the switch.

The switch comprises a movable control pushbutton engaging the flange.

Description

This invention relates to a temperature limiter particularly useful with electric heater elements of the sheathed type operating in air and/or liquid media.
As is known, temperature limiters currently known in the art generally operate either on a liquid expansion or thermal expansion differential principle between two metals or even on changes of electric quantities in electronic components.
In actual practice, prior limiters exhibit difficulties of application and thermal contact with the heater elements owing to the formation, over time, of oxide or lime or other heat-insulating materials deriving from the type of utilization, whereby the temperature limiter is either caused to be inoperative over time or to operate at different temperature values from the preset ones.
Known are, e.g. from British Patents No.s 1408388, 1223165, 1010375; 964355, and 591997, temperature limiters which comprise a rod to be used in combination with a sheathed resistor.
The expansion differential between the rod and sheath of the sheathed resistor results in the opening of the circuit of a switch which blocks the flow of electric power, thus preventing the resistor from overheating.
The solutions proposed to obviate such prior shortcomings, in particular as relates to application to sheathed electric heater elements for air and/or liquids, have, however, an excessive cost compared to the cost of the sheathed electric heater.element to which the temperature limiter is to be applied.
In the light of such problems, it is a primary object of this invention to obviate the above drawbacks affecting the prior art by providing a temperature limiter which operates on a principle of thermal expansion of the materials to which the limiter is to be applied, in particular for armored electric heater elements for air and/or liquids, that limiter being adapted to cut in always at the same temperature over time, without any problems of thermal contact.
A further object of this invention is to exploit that temperature limiter as a safety switch adapted to cut off the electric power supply to the heater element on exceeding a given temperature and /or as a temperature regulator or safety thermostat for the multiplicity of uses in which the electric heater elements are required to operate which can incorporate the temperature limiter of this invention.
Another object of this invention is to provide a temperature limiter with an extremely simple construction, which can be readily applied to existing electric heater elements.
,An important object of this invention is to provide a temperature limiter or safety thermostat which is highly reliable and inexpensive.
These and other objects, such as will be apparent hereinafter, are achieved by an electric self-protected sheathed resistor comprising a tubular metal sheath, a coil-like heater resistor enclosed in said sheath embedded in an insulating material also enclosed in said sheath, two electric terminals disposed at the end of said tubular metal sheath and connected to said coil-like heater resistor, a supporting flange connected rigidly to said tubular metal sheath close to said ends, a normally closed contact switch supported on said flange, a rigid rod connected at one end to said switch and at the other end to said tubular sheath, characterized in that said switch is supported on said flange through fulcrum means which allow for the oscillatory movement of said switch relatively to said flange, said switch comprising a movable control pushbutton. engaging with said flange, the movement of said rod resulting in the oscillation of said switch and actuation of said pushbutton.
Further features and advantages will be apparent from the following detailed descrption of a temperature limiter according to the invention, as illustrated, by way of example, in the accompanying drawing, where:

Figure 1 is a perspective view of the.self-protected armored electric resistor according to the invention as applied to an armored heater element of known design;
Figure 2 shows diagramatically a side view of the switch in the closed circuit position thereof and in normal operation;
Figure 3 is a side view of the switch of Figure 2 in the open circuit position and with the sheathed resistor in the overheated state thereof; and
Figure 4 is a partly sectioned front view of the switch of Figures 2 and 3, wherein the pushbutton drawn in full lines is indicative of normal operation and the one shown in dash lines is indicative of the resistor being overheated and the electric circuit opened.

With reference to the cited drawing figures, this armored electric resistor 10 comprises a tubular metal sheath 11, a coil-like resistance heater 12 enclosed in the sheath 11 and embedded in an insulating material 14. The two electric terminals 15 and 16 are connected to the ends of the sheath 11.
Close to the terminals 15 and 16, the ends of the sheath 11 are welded to the flange 17 to provide a rigid form of connection. A normally closed contact microswitch 18 is provided supported on the flange 17, a rigid rod 19 connected at its end 20 to the microswitch 18 and at its end 21 to the point 22 on the sheath.
A fulcrum means is provided to support the switch 18 on the flange 17, this fulcrum means permitting the oscillatory movement of the switch 18 relatively to the flange 17. The switch 18 comprises a control pushbutton 23 for the switch itself. The pushbutton 23 is movable relatively to the body of the switch 18 and effects, when depressed, opening of the normally closed circuit of the switch 18. The movement of the rod 19 caused by the thermal expansion of the sheath 11 causes the switch 18 to oscillate toward the flange 17 and the actuation of the pushbutton 23 which, under the push from the flange 17 and rod 19, is inserted back into the body of the switch 18 and causes the circuit to be opened.
The fulcrum means which connects the flange 17 to the switch 18 comprises, for example, two supporting members 31, in the drawing the second supporting member 31 is not visible, it being arranged symmetrically. The supporting members 31 may be formed from a small plate bent to a "U", with the base of that "U" attached to the flange 17. The supporting members 31 support a pin 30 which engages pivotally-with the body of the switch 18. The pin 30 is the fulcrum around which the oscillation of the switch 18 takes place. The rod 19 is connected to the body of the switch 18 through a hinge connection which enables the switch 18 to rotate around the rod 19. The end of the rod 19 not shown in the drawing is configured T-like. The two wings of the "T" engage in two seats provided in the body of the switch 18.
The point 32 of the switch 18 whereto the rod 19 is journalled lies preferably between the pin 30 and pushbutton 23.
Electrically conductive elastic means are provided which comprise, for example, a wire 40 bent to a U-like shape, in order to effect the electric connection of the electric terminal 41 of the switch 18 and terminal 16 of the resistance heater 12. The elasticity of the wire 40 bent to a U-like shape enables the switch 18 to oscillate relatively to the flange 17 and fulcrum 30. Resetting of the circuit of the switch 18 to the closed condition may take place either manually or automatically. Where resetting is provided automatically, the known elastic means not forming a part of this invention, which normally resist actuation of the pushbutton 23, automatically determine, on release of the urge from the rod 19, resetting of the closed circuit conditions. Where resetting occurs manually, according to the preferred embodiment of the invention, the pushbutton 23 comprises elastic locking means adapted to lock the pushbutton 23 in the open circuit position, resisting the action of the cited known elastic means which would tend to close the circuit on removal of the urge from the rod 19. The elastic locking means according to this invention comprise a pin 50 movable crosswise to the direction of movement of the pushbutton 23. The pin 50 engages, under the urge of elastic means of its own, comprising for example the spring 51, in at least one seat 52 correspondingly provided in the body of the switch 18. The seat 52 is provided at a relative position between the pushbutton 23 and switch 18 which causes the electric circuit to be opened.
Of preference, a second seat 53 for the pin 50 is provided at the relative position between the pushbutton 23 and switch 18 which effects the electric circuit closing.

Claims

1. A self-protected sheathed electric resistor comprising a tubular metal sheath, a coil-like heater resistor enclosed in said sheath embedded in an insulating material also enclosed in said sheath, two electric terminals disposed at the end of said tubular metal sheath and connected to said coil-like heater resistor, a supporting flange connected rigidly to said tubular metal sheath close to said ends, a normally closed contact switch supported on said flange, a rigid rod connected at one end to said switch and at the other end to said tubular sheath, characterized in that said switch is supported on said flange through fulcrum means which allow for the oscillatory movement of said switch relatively to said flange, said switch comprising a movable control pushbutton engaging with said flange, the movement of said rod resulting in the oscillation of said switch and actuation of said pushbutton.

2. A resistor according to Claim 1, characterized in that said fulcrum means comprises two supporting members extending from said flange, said supporting members supporting a pin, said pin engaging pivotally with the body of said switch.

3. A resistor according to Claim 1, characterized in that said rod is hingedly connected to said switch at a point on said switch located between said fulcrum and said pushbutton.

4. A resistor according to Claim 1, characterized in that an electric terminal of said switch and an electric terminal of said resistance heater are connected electrically through electrically conductive elastic means allowing said oscillation of said switch relatively to said flange and said fulcrum.

5. A resistor according to Claim 4, characterized in that said electrically conductive elastic means comprises a wire bent to a U-like shape.

6. A resistor according to Claim 1, characterized in that re-closing of the circuit controlled by said switch occurs automatically, said pushbutton being returned automatically by the action of known elastic means to the starting position on cessation of the overheating.

7. A resistor according to Claim 1, characterized in that the circuit controlled by said switch is re-closed manually, said pushbutton comprising elastic locking means adapted to lock said pushbutton in the open circuit position.

8. A resistor according to Claim 7, characterized in that said elastic locking means comprises a pin movable crosswise to the direction of movement of said pushbutton, said pin engaging, under the urge of spring means of its own, in at least one seat correspondingly provided in the body of said switch.

9. A resistor according to Claim 8, characterized in that said movable pin engages alternately in two seats provided in the body of said switch, alternately on opening and closing the electric circuit controlled by said switch .