US 3033960 A
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May 8, 1962 T. GELLER THERMOSTATIC SWITCH 2 SheetsSheet 1 Filed May 11, 1959 MUM,
ATTORNEY y 8, 1962 T. GELLER 3,033,960
THERMOSTATIC SWITCH Filed May 11, 1959 2 Sheets-Sheet 2 INVENTOR. THOMAS L. 6 ELLER mWW ATTORNEY United States Patent 3,033,960 THERMOSTATIC SWITCH Thomas L. Geller, West Bend, Wis, assignor to The West Bend Company, a corporation of Wisconsin Filed May 11, 1959, Ser. No. 812,356 1 Claim. (61. 200-138) This invention relates to a thermostatic switch which is particularly adapted for use in an electric heating circuit for automatic coffee percolators and similar appliances.
The object of this invention is to provide a thermostatic switch of this type which is simple and relatively inexpensive to manufacture and, although of small size, is durable and can be easily and accurately calibrated.
Another object is to provide a switch having the above advantages which is highly sensitive to provide accurate control of the heating circuit in which it is used.
These objects are achieved by a switch comprising a tubular body closed at one end with an insulating ringshaped member with an axial opening mounted in the open end of the body. A pair of elongated bimetallic contact elements are mounted in the member and extend inside the tubular body. A wedge-shaped spacer is inserted into the axial opening in the member between the bimetallic elements to hold them securely in a pre-, determined position and thereby provide the desired thermal calibration. An adjusting screw can be provided to provide more accurate calibration if desired.
The thermostatic switch is not designed to respond directly to variations in liquid temperature as is generally the case. With the contacts closed, current will flow through the bimetallic elements causing a temperature rise in the elements due to the resistance of the elements themselves. The heat generated will deflect the elements and open the contacts at a predetermined ambient operating temperature. The bimetallic elements are small and therefore cool quickly permitting the contacts to close again providing a cycling action. The percentage on time of the switch will -vary with the rate of flow of heat from the bimetallic elements into the surrounding mass which, in turn (upon proper positioning of the switch), will vary with the temperature of the liquid in the percolator. Thus, the lower the temperature of the liquid the greater rate of heat flow from the elements and the greater the on time of the switch.
Other objects and advantages will be pointed out in, or be apparent from the specification and claim, as will obvious modifications of the single embodiment shown in the drawings, in which:
FIG. 1 is a plan view of a plug-in type unit for an electric percolator embodying the thermostatic switch of my invention;
FIG. 2 is a section taken along line 2-2 of FIG. 1;
FIG. 3 is a fragmentary sectional view taken through a percolator constructed to receive the thermostatic unit shown in FIGS. 1 and 2;
FIG. 4 is an enlarged plan view of the thermostatic switch of this invention;
FIG. 5 is a sectional view taken along line 55 of FIG. 4;
FIG. 6 is an end view of the switch shown in FIG. 4; and
FIG. 7 is a partially schematic and simplified wiring diagram of a thermostatically controlled heating circuit using the switch of my invention.
Referring to the drawings (FIGS. 4,5 and 6), the thermostatic switch includes a tubular body 19 preferably of metal having one end closed as shown. A pair of flat elongated bimetallic elements 12, 12 with contacts 14, 14 are mounted in body it) by a ring-shaped member 16 3,d33,%0 Fatented May 8, 1962 ice having a flanged end 17 and a serrated outer surface 19. The ring is inserted into the open end of the body with the annular shoulder formed by flange 17 abutting the end of body it} to position it in the body. Bi-metal elements 12, 12 extend through the axial opening in the ring and are held securely in assembled position against fiat walls 18, 18 on the inner surface thereof by a wedgeshaped spacer pin 20 wedged into the ring between the elements as shown. Ring 16 and pin 20' are preferably made from molded phenolic plastic, and elements 12, 12 and pin 20 are sealed in ring 16 by epoxy resin 24 as shown.
One or both flat inner walls 18, 18 are tapered slightly (only one wall tapered in FIG. 5) and pin 20 is similarly tapered to conform with the angle of the walls to thereby insure a tight fit between ring, elements and pin. In adition, it is important to note that by the use of a ring and pin assembly of predetermined contour, elements 12, 12 can be uniformly positioned to provide switches having a specific thermal calibration. That is, by keeping the slope of walls 18, 18 in ring 16 constant the pressure at contacts 14, 14 of the switch elements can be held within close limits.
To permit even more accurate calibration of the switch, a trim screw 22 (FIG. 2) threadably mounted in thermostat housing 26 and extending through an opening 23 in body it} is provided to pre-stress the bimetal elements after assembly of the switch.
As shown in FIGS. 1 and 2, the thermostatic switch is mounted in a plug-in type housing 26 and connected by suitable leads 28 to female terminals 39, an indicator light 31 and a line cord 32. This thermostatic switch assembly is adapted for plug-in connection with a percolator 34 (FIG. 3) having a sealed heating element 36 with male terminals 38 positioned below a sealed open end recess td in the base of the percolator. The electric heating circuit (FIG. 7) controlled by the thermostatic switch is completed for operation of the percolator by plugging the switch assembly into the appliance with terminals 38 mating with terminals 34 and body 10 of the switch extending into cavity 4i As shown in FIG. 7, with contacts 14 on the bimetal elements closed (as they normally are), curent will flow through the switch to heating element 36. The ohmic resistance of bimetal elements 12 is such that at a predetermined ambient operating temperature the current passing through them will produce sufficient heat to cause them to deflect and open contacts 14 and thereby deenergize heating element 36. Elements 12 being small (.025 inch thick X .156 inch wide x .750 inch effective length in preferred embodiment) will cool quickly causing the switch contacts to close and again energize heating element as and again initiate the self-heating action of the elements. Thus, the switch will cycle on and off at a given ambient operating temperature in response to the heat produced by the current fl w through the ole ments and the subsequent cooling of such elements when the contacts open. The amount of heat produced by the heating element during a given period of time will, of course, depend on the percentage of such period of time the switch is closed. The on time of the switch during which the heating element is energized will depend on the length of time it takes the elements to cool, i.e. the rate of heat flow from the elements into the surrounding mass. By positioning the switch in cavity 40 in close proximity with the liquid in the percolator, the rate at which elements 12 will cool can be made to vary with the temperature of the liquid. The lower the temperature of the surrounding mass the greater the heat flow from the elements and the greater the percentage of on time in the operation of the switch. It has been found that by properly balancing the length, width, thickness, thermal activity and electrical resistivity of the bimetal elements in conjunction with the physical location of the switch with respect to a given mass and wattage to be controlled, a highly sensitive and accurate control can be designed.
Although but one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claim.
A thermostatic switch comprising, a hollow body closed at one end, an insulating member mounted in the open end of said body and having an opening therethrough, a pair of elongated resilient contact elements mounted in said opening and extending inside said body, at least one of said elements being a bimetal element, said elements being straight over the length thereof and each having a contact mounted thereon, a spacer inserted into the opening in said insulating member between said elements to hold said elements securely in a predetermined position with respect to each other, said opening in said insulating member having a pair of oppositely facing flat walls against Which said contact elements are held by said spacer, at least one of said flat Walls sloping from one end of said member to the other, said spacer being tapered to conform to the angle of said fiat Walls of said member whereby said elements will assume a non-parallel pre-stressed relationship having said contacts thereon in contact with each other when the switch is subjected to normal ambient temperature conditions.
References ilited in the file of this patent UNITED STATES PATENTS