Automatically Resettable Thermal Switch

Abstract

A thermal switch is provided which uses a bimetal element to provide the force to open and close a set of contacts. The bimetal element is initially restrained from opening the contacts by a latch mechanism which uses an eutectic material for determining the temperature at which the latch mechanism will release. Upon a sufficient drop in the ambient temperature, sufficient force is generated in the bimetal element to cause the bimetal to slip past a shaped surface on the latch mechanism thereby resetting of the switch.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to thermally actuated switches and more particularly to thermal actuated switches which are automatically reset. Thermal switches are useful or necessary as safety devices in any appliance or apparatus which generates a considerable amount of heat in operation. For example, thermal switches are useful in appliances such as clothes dryers in order to prevent excessive overheating. Thermal switches also find wide use in preventing overheating in lamp ballasts.

It is common practice in the lamp ballast are to encase (or pot) the components of a ballast apparatus in an encapsulating material having suitable sound dampening characteristics and good dielectric properties. For example, asphaltic compounds have been widely used as a constituent of such encapsulating materials since asphaltic compounds are relatively inexpensive and posses suitable sound-dampening and electrical characteristics. Various resinous materials, such as polyesters and epoxy resins, also may be used as constituents of encapsulating materials in ballast apparatus.

Failure of components, such as a ballast capacitor or a ballast transformer winding, is generally accompanied by the emission of high heat. This heat is transferred to the encapsulating material and can cause overheating of the material. Depending on the properties of the particular materials used, the encapsulating material may tend to smoke, burn, expand or liquefy upon overheating. These conditions may occur singularly or in combination. For example, when an encapsulating material containing an asphaltic compound overheats, the asphaltic constituents tend to liquefy and expand. Since asphaltic encapsulating materials are usually encased, the overheated material tends to come out of the case and may drip off the case. Encapsulating material containing resinous constituents also may burn, smoke and drip. These conditions are objectionable in a ballast apparatus since the apparatus is usually placed in an overhead lamp fixture or at other locations where the driping of the encapsulating material may damage property.

Therefore, it is desirable to have a thermal switch which will positively open the circuit supplying current to the ballast apparatus upon some predetermined degree of overheating. However, often it is not desirable to permanently remove a ballast from the line merely because it has overheated. For example, an air conditioning failure in a plant or factory may cause a temporary overheating of the lamp ballast without any failure of ballast components. In such a case, it would be desirable to have a thermal switch which would automatically reset or reclose after the ballast temperature has dropped to a temperature significantly lower than the ballast temperature at which the thermal switch contacts are opened. Likewise, it may be desirable to have a slight delay in the opening of the thermal switch contacts and to produce a snap action effect when a predetermined temperature sufficient to melt an alloy is reached.

2. Description of the Prior Art

Various types of thermal switches have been known in the prior art. An automatically resettable thermal switch using an elongated bimetallic element and a meltable alloy has been known in the prior art. For example, one prior art device uses a lever connected to a disc mounted in a casing containing a meltable alloy. A contact is mounted on the lever and motion is imparted to the lever by means of the bimetallic element when the melting point temperature of the alloy is reached. The lever is held in position by a spring and a ratchet mechanism connected to the casing in which the alloy is located. The casing for the alloy would be free to rotate in the absence of the ratchet mechanism.

Other prior art devices utilize temperature responsive arms which are held in position by a meltable alloy until a predetermined temperature is reached. At that time the alloy melts and the arms separate. Such devices normally are of the single operation type or must be reset manually, if they are resettable at all.

SUMMARY OF THE INVENTION

Briefly stated, in accordance with one embodiment of the present invention, a bimetallic element provides the force for operating a set of contacts and a latch means allows the bimetallic member to move in one direction upon the sensed temperature reaching the melting point temperature of an eutectic material. In an exemplification mechanism, a spring latching member is cemented to a rigid member by means of the meltable alloy. The end of the spring latching member is shaped, curved or slanted so as to allow the bimetal member to move past the latching member and reclose the contacts upon the occurrence of a sufficient drop in temperature.

The aforementioned and other features and objects of this invention and the manner of obtaining them will become more apparent, and the invention itself will be better understood by reference to the following description taken in conjunction with the accompanying drawing, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic elevation view of a thermal responsive switch mechanism incorporating one embodiment of the invention;

FIG. 2 is a diagram of the acurate movement of certain members shown in FIG. 1: and

FIG. 3 is a somewhat schematic elevation view of another switch mechanism incorporating the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now more particularly to FIG. 1 there is shown a terminal responsive switch mechanism including a base 10, which may be made of any suitable, basically rigid, insulating material. The base 10 has an extension or insulating support 11 projecting therefrom. An electrically conductive spring member 12, having a contact 13 mounted thereon, is firmly mounted through insulating support 11 in any suitable manner well known in the art. A terminal or lead may be connected to end 14 of spring member 12. An electrically conductive support member 15 is mounted through insulating support 11 and is secured therein. By way of example, support 15 may be secured in a suitable opening in support 11 by a body 16 of a thermosetting resin or other suitable cement. Use of a thermosetting resin provides certain advantages as will be discussed in more detail hereinafter. A electrically conductive bimetallic member 17 is soldered or otherwise mechanically and electrically connected to support member 15. The term "bimetallic" is used on its generic sense and includes any number of members which flex substantially upon change in temperature. "Bimetal" or "bimetallic," as used herein is intended to refer to all such devices, even through they may not be two dissimilar metals joined together. Bimetallic member 17 is provided with a contact 18 mounted thereon near its free or moveable end 24. A terminal or lead may also be connected to support member 15 thereby providing the other connection for an electric circuit, including spring member end 14 and support member 15 which is closed when contacts 13 and 18 are in engagement.

A rigid support member 19 and a spring latch member, in the form of a resilient arm or member 20, are also mounted through the insulating support 11 and held firmly therein by a thermosetting resin or other suitable cement 21, for example. A mass of eutectic material 22, which may be a thin layer, is located between rigid support member 19 and spring latch member 20. Spring latch member 20 has a shaped or curved surface or portion 23 which allows the free end 24 of bimetal member 17 to slide past the free end 25 of spring latch member 20 during the resetting operation. To this end free end 25 is return bent so that shaped portion 23 is smoothly curved away from the bimetal 24. Also end 24 of the bimetal is curved upwardly, as seen in FIG. 1, so that end 24 and shaped portion 23 are complimentary to aid the return movement of bimetal 17.

A thermosetting resin may be used advantageously in the exemplification thermal responsive switch in mounting support member 15, rigid member 19 and spring latch member 20 in insulating support 11 by eliminating the need for close tolerances in the manufacturing process. To this end the thermosetting resin may be placed around supporting member 15, rigid member 19 and spring latch member 20 in insulating support 11 and the whole assembly placed in an oven with the base 10 down. The temperature of the oven is set to be at or above the cure temperature of the thermosetting resin while, at the same time, below the critical or melting temperature of eutectic material 22. With this condition the thermosetting resin will cure with contacts 13, 18 in engagement and with free end 25 of spring latch 20 engaging free end 24 of bimetal 17. In this manner, the thermosetting resin may be used to compensate for any variations in dimension of the base or other members.

The material 22 may be one of any number of suitable available alloys which as a melting and freezing point temperature range (phase change temperature) equal to the temperature at which it is desired that contacts 13 and 18 open. In this regard "eutectic material" refers to material which is capable of repeatedly changing phase between solid and liquid as its temperature rises or falls through its phase change temperature. The most well known of such materials are the metal alloy solders. However, it will be understood that other materials, such as thermoplastic resins, having appropriate phase change temperatures could be used as the eutectic material. In the lamp ballast field, for instance, it is desirable to have contacts 13 and 18 separate at a temperature of between 113.degree. and 116.degree.C. An alloy which has a melting temperature range of 113.degree.-116.degree.C is one composed of 51.4 percent indium, 47.3 percent tin and 1.3 percent silver. This narrow melting temperature range is of special significance in fluorescent lamp ballasts protection since the increase in temperature of the case upon failure of the coil may present a fire hazard when the ballast is in close contact with wood or similar construction materials. Since it is desirable to fix the release or opening temperature of the switch as near as possible to the operating temperature of the coil, the liquid state of the alloy should occur at a temperature only slightly above the coil operating temperature. Therefore, the temperature range between the liquid state and the solid state of the alloy must be narrow to prevent the loss of mechanical strength of the alloy at the operating temperature of the ballast.

Referring now to FIG. 2, there is shown a diagram representing the arcuate paths of certain of the members of FIG. 1. Lines representing the parts in FIG. 2 are given the same numbers as the corresponding parts in FIG. 1. The arcuate path 30 of bimetal member 17 is seen to intersect arcuate path 31 of spring latch member 20 at point 32 when there is only a slight upward deflection. Spring support member 12 does not move very much. However, upon reclosing of contacts 13 and 18, that is when sufficient force is generated in bimetal members 17 by a drop in temperature in order to enable the end 24 of bimetal member 17 to slide past shaped surface 23 of spring latch member 20, spring member 12 may deflect to some degree due to the momentum imparted to contact 13 from contact 18. This deflection of spring member 12 is shown as arc 33. As may be seen from FIGS. 1 and 2, there is no problem with bimetal member 17 possibly slipping past spring support member 12.

Referring now to FIG. 3, there is shown another thermal responsive switch incorporating the present invention. There is shown a base 40 made of any suitable fairly rigid insulating material. The base 40 has a pair of spaced apart insulating supports 41 and 42 extending therefrom. A spring latch member 43 and a rigid member 44 are mounted in insulating support 41 by thermosetting resin or other suitable cement 52 in a manner as described above. An electrically conductive support member 45, having an elongated, electrically conductive bimetallic member 46, is mounted in secured thereto, insulating support member 42 using a thermosetting resin or other suitable cement 47 as described above. A contact 48 is mounted near the free end of bimetal 46. Contact 48 cooperates with contact 49 mounted on base 40. Contact 49 may be connected to any suitable lead or terminal as is well known in the art. A terminal or lead may also be connected to support member 45 in order to provide the other terminal for the switch. A body or mass 50 of eutectic material is provided between rigid member 44 and spring latch member 43. Upon a rise in ambient temperature sufficient to melt the eutectic material 50, spring latch member 43 is allowed to deflect upwards under the force of bimetal 46, opening contacts 48 and 49. A shaped, curved or slanted surface 51 is return bent on the free end of spring latch member 43 in order to allow the curved free end 59 of bimetal 46 to slip past the free end 53 of spring latch member 43 in the reclosing operation.

In operation, the switches shown in FIG. 1 and 3 operate in a very similar manner and therefore only the operation of the switch in FIG. 1 will be described in detail. The switch, as shown in FIG. 1, is in a closed circuit condition. As ambient temperature rises, the free end 24 of bimetal 17 will tend to deflect in an upwards direction but will be restrained from doing so by spring latch member 20. As the ambient temperature continues to rise, eventually the melting point temperature of the eutectic material 22 will be reached. When the eutectic material 22 melts, a major portion of the length of spring latch member 20 is freed from rigid member 19, thereby allowing free end 25 of spring latch member 20 to deflect in an upward direction under the force provided by bimetal 17. Eventually, the free end 24 of bimetal 17 will slip past the free end 25 of spring latch member 20 as shown in the diagram of FIG. 2 at point 32. As soon as the free end 24 of bimetal 17 slips past the free end 25 of spring latch member 20, spring latch member 20 springs back to its normal position of having a substantial portion of its length in contact with rigid member 19. Upon a subsequent decrease in ambient temperature to a temperature below the melting point temperature of eutectic material 22, the material 22 solidifies causing spring latch member 20 again to be structurally united with rigid member 19. As the ambient temperature continues to fall, a force is built up in bimetal 17 which tends to force the free end 24 of bimetal 17 in a downward direction. The free end 24 of bimetal 17 will at this time be in contact with the shaped, curved or slanted surface 23 of spring latch member 20. As the force continues to build up in bimetal member 17, eventually enough force will be created to cause bimetal member 17 to deflect thereby allowing the free end 24 of bimetal 17 to slip past the free end 25 of spring latch member 20 reclosing the contacts 13 and 18.

While each of the illustrative thermally responsive switches shown and described is a single pole, single throw switch, other types of switching actions are readily available. For instance, addition of another stationary contact above the movable contact, as viewed in the drawing would provide a single pole, double throw action. Also, simple rearrangement of the contacts will provide a thermally responsive switch mechanism which closes a circuit upon the occurence of a predetermined high temperature.

While, in accordance with the patent statutes, the presently preferred embodiments of this invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications maybe made therein without departing from the invention and it is intended in the appended claims to cover all such changes and modifications as come within the true spirit and scope of the invention.

Claims

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An automatically resettable thermal switch, comprising:

a base;

a resilient member having one end mounted to said base and a free end;

a rigid member mounted to said base and extending in juxtaposition with said resilient member;

an eutectic material, having a predetermined phase change temperature range, located between said resilient member and said rigid member for normally securing said resilient member to said rigid member; and

a bimetal member having one end mounted on said base and a free end, a first contact mounted on said bimetal member and cooperating with a second contact mounted in relation to said base;

said free end of said bimetal member being constrained from moving said first contact relative to said second contact by said free end of said resilient member until the ambient temperature reaches the predetermined phase change temperature range of said eutectic material.

2. An automatically resettable thermal switch as set forth in claim 1 wherein said free end of said resilient member is provided with a shaped surface which allows said free end of said bimetal member to slide past said free end of said resilient member in response to a drop in ambient temperature below a predetermined temperature.

3. An automatically resettable thermal switch as recited in claim 1 wherein said alloy comprises 51.4 percent indium, 47.3 percent tin and 1.3 percent silver.

4. An automatically resettable thermal switch, comprising:

a base;

an elongated, rigid member mounted to said base;

an elongated, resilient member mounted to said base;

said resilient member overlying said rigid member and having a free end portion projecting beyond said rigid member;

an eutectic material, having a predetermined phase change temperature range, positioned between said rigid member and said resilient member for normally securing said resilient member to said rigid member;

an elongated, bimetal member mounted to said base;

a first contact mounted to said bimetal member and cooperating with a second contact mounted in relation to said base;

said bimetal member including a free end portion positioned in interfering relationship with said free end portion of said resilient member for constraining movement of said bimetal member until the temperature of said eutectic material reaches the phase change temperature range of said eutectic material;

said resilient member then being free to flex for allowing said bimetal member to move past said resilient member in one direction.

5. An automatically resettable thermal switch as set forth in claim 4 wherein said free ends of said resilient and bimetal numbers are formed with complimentary surfaces to facilitate movement of said bimetal member past said resilient member in the other direction.