Positive-temperature-coefficient-resistor package

Abstract

A PTCR package, according to the present invention, has one or a plurality of stacked PTCR elements having at least two electrical film terminals on opposite faces of the stack. At least one metal heat conductor contacts one of the terminal faces. An insulating housing contains the PTCR elements and the heat conductors. The heat conductors are each in broad thermal contact with a portion of the inner wall of the housing such that thermal coupling is realized between the package environment and each PTCR element that is contacted by a heat conductor. A heat sinking metal part may contact the outer wall of the housing, opposite to the inner heat conductors so as to further enhance the thermal coupling to the environment.

Description

BACKGROUND OF THE INVENTION

This invention relates to packages and mounting means for positive temperature coefficient resistors (PTCRs), commonly made of doped barium titanate, and more particularly to such packages intended for use in delayed-action contactless electrical switching applications.

For example it is known to employ a thermally independent PTCR in series with the start winding of a single phase motor whereby the motor is aided in starting by a current flowing in the start winding through the low "cold" resistance of the series connected PTCR. After a short time this current heats the PTCR to the anomaly temperature of the PTCR at which point the PTCR resistance abruptly increases several orders of magnitude causing the current to be diminished accordingly. This is designed to occur just prior to the motor having achieved normal running speed.

Typically the desired start period is 0.5 second and the desired PTCR recovery period (for restart) is three minutes. Conventionally packaged PTCRs having a high thermal resistance between the PTCR elements and the environment, are capable of achieving the desired short start time. However, the recovery period characterizing such packages is typically much longer, on the order of ten minutes. A premature attempt to restart the motor will result in improper starting. Known methods for obtaining thermal coupling between the PTCR and the PTCR package environment, and thus reducing the recovery period, include the use of an all metal housing to which the PTCR is connected, or from which the PTCR is electrically insulated by a thin insulating material, as described in copending application Ser. No. 366,931 by Manfred Kahn.

It is desirable, for the purposes of PTCR package standardization that the PTCR package user be able to easily adjust this thermal resistance for his particular application. Furthermore, when two or more PTCR elements are contained in the same package, it is often desirable that the user is able to adjust the thermal coupling of each element independently of the other. For such applications, known packages are not suitable.

It is an object of this invention to provide a PTCR package that reduces the thermal recovery time of a PTCR body enclosed therein.

It is a further object of this invention to provide a PTCR package for motor start applications having a normal start time and a significantly shorter recovery time compared with conventional PTCR packages.

It is a further object of this invention to provide an electrically insulative package for one or more PTCRs having an efficient thermal coupling means between the PTCRs and the package environment.

It is a further object of this invention to provide a package for one or more PTCRs having a broad capability for varying the thermal coupling of each such individual PTCR with the package environment.

SUMMARY OF THE INVENTION

A PTCR package, according to the present invention, has one or a plurality of stacked PTCR elements having at least two electrical film terminals on opposite faces of the stack. At least one metal heat conductor contacts one of the terminal faces. An insulating housing encloses the PTCR elements and the heat conductors. The heat conductors are each in broad thermal contact with a portion of the inner wall of the housing such that thermal coupling is realized between the package environment and each PTCR element that is contacted by a heat conductor. A heat sinking metal part may contact the outer wall of the housing, opposite to the inner heat conductors so as to further enhance the thermal coupling to the environment. The electrically insulating housing contains and provides physical support for one or more PTCR elements while simultaneously providing one or more heat conducting paths between selected regions in the PTCR body or elements assembly and one or a corresponding number of selected corresponding regions outside the package. The basic means by which heat is introduced into or removed from the PTCR body from the outside is through the electrically insulating housing and thence through metal heat conductors to the body. Furthermore, the user of the package of this invention may readily select the degree and location of thermal coupling by varying the position and size of outer metal clamp piece or pieces while preserving electrical isolation in accordance with his particular requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side sectional view of a first preferred embodiment of the present invention.

FIG. 2 shows a full end view of the package shown in section in FIG. 1.

FIG. 3 shows a side sectional view of a second preferred embodiment of this invention.

FIG. 4 shows a side sectional view of a third preferred embodiment of this invention.

FIG. 5 shows an isometric view of the third preferred embodiment shown in section in FIG. 4.

FIG. 6 shows a side sectional view of a fourth preferred embodiment of this invention.

FIG. 7 shows in partial view a fifth preferred embodiment of the present invention, having a fusible metal link.

FIG. 8 shows the fusible link as taken in the plane 8--8 indicated in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A sectional view of the package of the first preferred embodiment of this invention is shown in FIG. 1 having been taken in plane 1--1 of FIG. 2. The end view of this package is shown in FIG. 2 having parts designated by corresponding numbers relative to FIG. 1.

A PTCR body is comprised of two coaxially stacked cylindrical PTCR elements 10 and 12, each having on their flat end faces metallized films 18 and 17, respectively, of aluminum or other conducting metal. Alternatively, the body may be comprised of only one PTCR element or in general any number of PTCR elements held in good electrical and thermal contact with one another. Thin metal cups 15 and 16 have flat bottoms, the outside surfaces of which are in contact with the two outer terminals of the PTCR body.

A plastic housing 20 has a cylindrical shape and contains the PTCR body made up of elements 10 and 12 and the cups 15 and 16, that are mounted coaxially therewith. The metallized films 18 and 17 on the outer faces of PTCR elements 10 and 12 serve as electrical terminals for the PTCR body, and are thermally and electrically contacted by the metal cups 16 and 15 respectively. The flanges of the cups make broad contact with a portion of the inner wall of the housing, having a slide fit therein. Conductive grease may be used to enhance the thermal contact between the cup flanges and the housing. Flat metal leads 21 and 22 are tightly pressed into slots at the ends of the housing and the outer ends of these leads are formed so as to be readily connected to an electrical circuit by means of quick-connect press-on type connector hardware.

The housing is initially made of two complementary thermoplastic pieces having leads 21 and 22 pressed therein. The assembly is made by placing the PTCR elements 10 and 12 together, placing the cups 15 and 16 at each end of the PTCR body, placing springs 13 and 14 within the cups, and compressing these parts within and between the two plastic housing parts. Finally the two housing parts are ultrasonically welded together, by means well known to the art, thus forming an integral plastic housing 20 about the PTCR assembly. The springs 13 and 14 serve to hold the body and the cups 15 and 16 in compression between the end portions of the housing while also serving as the electrical connections between each lead (e.g. 21) and adjacent cup (i.e. 15). The PTCR body is thus made electrically accessible at leads 21 and 22.

A heat-sinking metal part, made of two pieces 23 and 24 being wrapped conformally about the housing 20 and welded together in regions 29 (FIG. 2), provides a means for conducting heat away from the package to any convenient metal frame whose temperature remains essentially at the temperature of the environment, or simply to the surrounding air. Thus the flanges of the heat conducting metal cups 15 and 16 and the heat sinking metal part 23-24 are in broad thermal contact on opposite surfaces of a wall of the housing 20, and the PTCR body is thermally coupled to the environment.

Tubular extensions 27 and 28 of the housing 20 have coaxial positions relative to leads 21 and 22, over which rubber boots (not shown) may be fitted to insulate the quick-connect junctions that may be made between the leads 21 and 22 and external wiring.

In FIG. 3 is shown a second preferred embodiment in sectional view. It will be recognized that the end view will appear as shown in FIG. 2, having the same relationship to FIG. 3 as it does to FIG. 1 although corresponding numbers for the various parts are not used in this instance.

The PTCR body is comprised of three coaxially stacked cylindrical PTCR elements 30, 31 and 32. The flat end surfaces of the body are metallized films of aluminum, 33 and 34, serving as the body terminals.

Thin metal cups 35 and 36 have flat bottoms, the inside of which are in contact with the two outer terminals 33 and 34 of the PTCR body such that these cups partially contain the body. An insulating tape 37 covers the outer periphery of PTCR elements 30, 31 and 32 so as to prevent electrical contact in these areas to the containing metal cups 35 and 36. Third and fourth metal cups, 38 and 39, have essentially the same geometry as cups 35 and 36. Cup pairs 35 and 36, and 38 and 39 are each pressed together so that their outer bottom faces make good thermal and electrical contact.

In all other respects the structure of FIG. 3 is similar to that of the first preferred embodiment shown in FIG. 1. Leads 41 and 42 extend through holes in the housing 40 and have bent extensions lying adjacent to the end inside surface of the housing 40. Metal springs are in compression between the bent extensions of leads 41 and 42 and the inner faces of cups 39 and 38, respectively, serving to compress the stacked PTCR elements and cups between the inside end surfaces of the housing and serving to make electrical connection between the PTCR body terminals 33 and 34 and the package leads. Tubular extensions 47 and 48 of the housing 40 provide means for mounting insulating rubber boots (not shown). Furthermore, the PTCR package may be mounted by means of metal brackets fastened to extensions 47 and 48, permitting the use of heat sink pieces such as 23-24 shown in FIG. 2 being coupled to the environment only by exposure to the surrounding air.

It is often preferable, as is well known, to make thermal and electrical coupling to a PTCR body at the same opposite faces of the body. It is seen, that the package of the present invention additionally provides a solution to the attendant problem of providing safe and reliable electrical insulation while permitting the user a wide range of thermal coupling means between individual PTCR elements and the package environment.

A self heating PTCR body is normally maintained in the hot steady state "off" condition by the small electrical current that is permitted to flow through the high body resistance. When the electrical circuit is broken, the rate of cooling of the body to a temperature below the PTCR anomaly temperature determines the characteristic recovery time of the package. Recovery is greatly accelerated in the package of this invention wherein, with reference to the preferred embodiments, the thermal energy in the PTCR body is drawn away by the cups and thence through the wall of the housing to the environment.

A PTCR package according to the first preferred embodiment was built for series connection with a motor start winding to control the motor start characteristic and the delay for proper restart was C previously explained herein. The housing 20 was a polysulfone molded plastic having a cylindrical wall thickness of 0.060 inch. The outer diameter of the housing was 1.47 inches. The two PTCR elements 10 and 12 were identical and had an anomaly temperature of 125.degree.C. In Table I is given test data and performance results for various configurations of the package as follows: Configuration A is without cups 15 and 16 or heat sinking piece 23-24. Configuration B is with cups 15 and 16 but without heat sinking piece 23-24. Configuration C is with cups and heat sinking piece 23-24. The measured thermal resistance, T.R., between the PTCR and the environment is presented in Table 1. The time t is the time necessary for the PTCR body to cool from a steady state temperature of 150.degree. to 90.degree. in a 25.degree.C ambient. These configurations and associated performances correspond to the requirements in numerous practical applications.

TABLE I ______________________________________ Package Condition T.R. t ______________________________________ .degree.C/watt seconds A 32.5 780 B 16.2 490 C 11.0 240 ______________________________________

In FIG. 4 is shown a third preferred embodiment in sectional view. FIG. 5 shows an isometric view of the package of the third preferred embodiment with corresponding members assigned to each of the structural features in the two figures.

The PTCR body is comprised of a first cylindrical PTCR element 50 separated from a second cylindrical PTCR element 52 by an electrically insulating layer 51. The first PTCR element 50 has a significantly lower anomaly temperature than that of the second PTCR element 52. The desired degree of thermal coupling between PTCRs 50 and 52 may be achieved by choosing an appropriate thickness for layer 51. Layer 51 may be a plastic sheet, for example of polysulfone or polyimide material, or may be a layer of ceramic such as alumina.

The PTCR body is enclosed and held compressed within a housing 20 by metal cups 15 and 16 and with springs 13 and 14, and leads 21 and 22, as was the case for the first preferred embodiment as shown in FIG. 2. Corresponding numbers are used therefor.

Metal plates 53 and 54 are attached to the inner metallized terminal faces 55 and 56 of PTCR elements 50 and 52, respectively. Extensions 57 and 58 of metal terminals 53 and 54, respectively, extend through the housing 20. These extensions 53 and 54 serve as electrical leads to which quick-connect press-on type connector hardware may be connected. Both leads 57 and 58 are shown lying in the plane of the section in FIG. 4, for convenience in showing both in one drawing. It is preferred that they protrude in different radial directions from the housing so as to simplify external connection and avoid accidental shorting therebetween.

A metal heat sinking piece 60 contacts the portion of the outer wall of the housing that is essentially opposte the flanges of heat conducting cup 15. Metal part 60 has extended portions not in contact with the housing 20 that are suitable for extending the surface area of contact between the ambient air and part 60. Alternatively the holes 61 and 62 permit screw attachment of the part 60 to, for example, a larger heat conducting structure that may be held at room or ambient temperature, thereby achieving stronger thermal coupling of PTCR 52 to the environment. On the other hand, PTCR 50 is in relatively weak thermal contact with the ambient air through cup 16 and the housing 20. An even weaker contact is readily achieved by removing cup 16. A stronger contact may be achieved by adding a second heat sinking part (not shown) to the outer housing opposite cup 16.

This third preferred embodiment is useful in a motor start control system having a definitive delay to restart function as is fully described in co-pending application by W. Moorhead and F. Kahn filed concurrently herewith.

In FIG. 6 is shown a fourth preferred embodiment of the present invention. The PTCR body is comprised of disc shaped PTCR elements 70, 71, 72 and 73 and a disc shaped metal plate 74 sandwiched in a coaxial stack. Outer metallized films 76 and 77 on outer ends of PTCR elements 70 and 73 serve as two terminals of the body. On the inwardmost end faces of PTCR elements 71 and 72 are metallized film terminals 78 and 79, both of which are contacted by metal plate 74. Plate 74 has an extension 75 that protrudes radially through the package housing 20 and serves as a third electrical lead for the package. A tubular extension 80 of the housing 20 provides a means for fitting an insulating rubber boot thereabout.

In all other respects the structure of FIG. 6 is similar to that of the first preferred embodiment shown in FIG. 1, and the following structural parts have corresponding numbers. Metal cups 15 and 16 contact the outer terminals of the PTCR body and provide thermal connection between the body and portions of the inner wall of housing 20. Springs 13 and 14 make electrical contact between cups 15 and 16, respectively, and leads 21 and 22, respectively. The springs additionally serve to hold the body and cups in compression between the end portions of housing 20. Thermal connection is thus made between the PTCR body and the air surrounding the housing.

It is essential that each face of a PTCR element that abuts another PTCR element has adhering to said face a thin metal film for efficient and broadly distributed electrical and thermal contact therebetween. In FIG. 1 are designated such abutting film pairs 91 and 92. Film pairs 93-94 and 95-96 serve this function in the package of FIG. 3. Similar film pairs 97-98 and 99-100 are shown in FIG. 6.

The package of the fourth preferred embodiment is particularly useful in situations where it is desired to obtain a low cold resistance PTCR package. It often occurs, for example, in the application of a PTCR for a motor start-control, that all specifications can be met with a given PTCR element formulation and geometry except for cold resistance. In the package of FIG. 6, the user may interconnect leads 21 and 22 so as to place PTCR elements 70 and 71 in parallel with elements 72 and 73, and thereby obtain in effect a single PTCR having only two leads (21-22 and 75) and having half the cold resistance of either pair of elements taken alone.

In a fifth preferred embodiment of the present invention, the electrical connection between each of the two end leads of the package and the corresponding body terminal is made by series connection through a fusible metal link. In FIG. 7 is illustrated in cross-section a fusible metal link 102 lying adjacent to an end inner wall portion of housing 20. A large diameter spring 104 is compressed and in contact between the outer extremities of the link 102, and the cup 15 that in turn is in electrical contact with terminal 17 of PTCR body 12. In FIG. 8 is shown the link 102 and spring 104 as taken in the plane 8--8 of FIG. 7. Another link (not shown) is similarly connected in series between the opposite lead and terminal. The assembly is accomplished in a similar manner as was described for the package of FIG. 1 except that the metal links are each placed coaxially against the inside end of one of the complementary housing pieces, and the lead 21 pressed through the combination holding the three pieces together. The lead 21 may advantageously have reverse barbs (not shown) provided therein so that it is irreversibly and firmly locked in the housing.

The links may be made of any of the well known fusible metals or low melting alloys such as the tin-lead solders. If, for example, a crack develops in the PTCR body 12, while electrical energy is applied, its temperature may become high enough to melt the plastic housing and cause shorts to a grounded metal heat sink. This may pose a hazard to the equipment such as a motor that may be connected in series with the PTCR package. The fusible link, in this case, will receive heat by radiation and convection from the over hot PTCR body and upon melting break the circuit before damage to associated equipment occurs. Thus in this embodiment a fail-safe feature is provided by the fusible links.

Claims

What is claimed is:

1. A PTCR package comprising

a. a PTCR body comprising at least one PTCR element, said body being cylindrical and having two flat end faces, said body having first and second metal film terminals being on said two end faces, respectively;

b. at least one metal heat conductor cup having a flat bottom, said flat bottom of said one cup being in broad contiguous thermal contact with said first terminal;

c. two electrical leads, each being electrically connected to one of said two terminals, respectively;

d. a housing of an electrically insulating material of cylindrical geometry completely enclosing said PTCR body and said cups, the flange of said at least one cup being in broad contiguous thermal contact with a curved portion of the inner wall of said housing, each said lead extending through another portion of said wall of said housing.

2. The PTCR package of claim 1 wherein said housing is made of a thermoplastic material.

3. The PTCR package of claim 2 wherein said thermoplastic is polysulfone.

4. The PTCR package of claim 1 wherein said flat bottom of a second of said cups is in broad contiguous thermal contact with said second terminal, said flange of said second cup being in broad contiguous thermal contact with another curved portion of the inner wall of said housing.

5. The PTCR package of claim 4 further comprising at least one heat sinking metal part being in broad contiguous thermal contact with a portion of the outer wall of said housing in a region opposite to at least one of said curved inner wall portions.

6. The PTCR package of claim 4 wherein said cylindrical body and said cups are mounted coaxially within said cylindrical housing and said two leads each extend through one of the two end portions of said housing, said package additionally comprising two metal springs mounted coaxially within said housing, each contacting the inner portion of one of said leads, and said springs holding said body and said cups in compression between said end portions of said housing, said compressed springs and cups serving as said electrical connections between said leads and said end terminals of said body.

7. The PTCR package of claim 6 wherein said thermal contacts with said first and second terminals are made to the bottom inside surfaces of said two heat conductor cups so as to partially contain said body within said cups.

8. The PTCR package of claim 6 wherein said thermal contacts with said first and second terminals are made to the bottom outside surfaces of said two heat conductor cups.

9. The PTCR package of claim 7 wherein the outside bottom surfaces of a third and a fourth of said heat conductor cups are in broad contiguous thermal contact with the outside bottom surfaces of said first and second cups, respectively, and said flanges of said third and fourth cups are in broad contiguous thermal contact with yet other curved portion of said inner wall of said housing.

10. The PTCR package of claim 1 wherein one of said PTCR elements has an anomaly temperature that is significantly higher than that of another of said PTCR elements.

11. The PTCR package of claim 10 having a heat sinking metal part in broad contiguous thermal contact with a curved outer surface portion of said housing, said part being positioned essentially opposite only to said flange of said at least one cup, said broad contiguous thermal contact between said at least one cup and said body being effected only to said one PTCR element having said higher anomally temperature.

12. The PTCR package of claim 1 wherein said PTCR body additionally comprises a layer of electrically insulating material positioned between two of said PTCR elements for reducing the thermal coupling therebetween and providing electrical insulation therebetween.

13. The PTCR package of claim 12 wherein said layer of insulating material is made of alumina.

14. The PTCR package of claim 12 wherein a third and a fourth of said terminals are on adjacent faces of said two PTCR elements having said layer therebetween, and wherein said package additionally comprises a third and a fourth electrical lead connected to said third and fourth terminal, respectively, and said leads extending through said wall of said housing.

15. The PTCR package of claim 1 additionally comprising two fusible metal links, each said connection between one of said leads and said corresponding body terminal being accomplsihed by series electrical connection through one of said fusible metal links.

16. The PTCR package of claim 1 wherein said body is comprised of at least two abutting PTCR elements, said elements having a metal film adhered to each abutting face.

17. The PTCR package of claim 6 wherein said body comprises two of said PTCR elements, said two elements being adjacent and having a third and fourth of said metal film terminals on their adjacent faces, respectively, said package additionally comprising a third electrical lead being connected to said third and fourth terminals and extending through said wall of said housing in a radial direction.