US2725447A - Electric blanket resistor - Google Patents

Description

NOV. 1955 B. T. VOGLER 2,725,447

ELECTRIC BLANKET RESISTOR Filed Aug. 20, 1955 IN V EN TOR.

BENJAMIN 7T VOG'L [R United States Patent ELECTRIC BLANKET RESISTOR Benjamin T. Vogler, Mount Airy, N. C., assignor to Essex Wire Corporation Application August 20, 1953, Serial No. 375,390

1 Claim. (Cl. 201--63) My present invention relates to wire used in electrically heated fabric such as blankets, sheets, and the like, and particularly to blanket wire composed of multiple conductors one of which is usually the heating element and the remaining conductor or, conductors, the control circuit.

The multiple conductor type blanket wire has been developed to eliminate the necessity of using thermostats in the body of electric blankets as circuit breakers. These thermostats are used in series with a single resistance wire, and when a hot spot develops in the blanket the thermostats are actuated to interrupt the flow of current in the blanket thus avoiding any harm to the user through burning of the blanket. Due to the likelihood of hot spots developing in any part of the blanket, as by the overlapping or folding of the blanket at any point, the use of thermostats for safetys sake is fairly profuse and represents objectionable bulk. The development of the multiple conductor type blanket wire eliminates the need for thermostats contained in the body of the blanket. In this type of blanket wire one of the conductors is used as the heater wire and the remaining conductor or, conductors, make up the control circuit. The control circuit and the resistance element are separated by a thermosensitive insulating material the electrical resistance of which decreases as temperature increases. When a hot spot develops in an electric blanket equipped with this type of wire, the temperature of the insulating material increases and it becomes conductive. Current then flows through the insulation to the control circuit and thus actuates a suitable control mechanism which interrupts the blanket heater current. It can be readily seen that because of the intimate association of the control circuit with the resistance element infinitely more protection is achieved with the multiple conductor wire than is possible using thermostats. Wherever a hot spot develops in the blanket the thermosensitive insulation becomes conductive and conducts current to the control circuit which by means of the control mechanism interrupts the flow of current to the blanket. U. S. Patent No. 2,581,212 to Spooner et a1. is illustrative of the application of this principle to electric blanket wire and describes the various insulating materials of practical interest.

Several of the objections to previously available multiple conductor blanket wires are centered about the stilfness of such wire and the poor flex life of the wire. The usual construction of such wire is as illustrated by the aforementioned Spooner patent. One conductor is helically wound on a messenger strand and a thermosensitive insulating material, usually nylon, is thereafter extruded onto the messenger and conductor. The second conductor is then helically wound on the extruded nylon coating and a final insulating coating is then applied. This construction represents a fairly large and stiff tubular form of poor flex characteristics. Flexibility in blanket wire is essential as is well illustrated by one widespread and common household practice. Blankets are frequently folded back near the head end of the bed so that the underlying sheet is turned back over the blanket for a short distance. This repetitious operation requires wire that is extremely flexible and pliable. If the in- 2,725,447 Patented Nov. 29, 1955 sulation has a short flex life and cracks, the permanent crosses that develop will render the electric blanket circuit useless.

In addition, it is difiicult to make a blanket equipped with stiff wire lay flat and smooth in a natural manner.

It is, therefore, a principal object of this invention to provide a multiple conductor blanket wire of improved flexibility. The improved flexibility operates in turn to increase the flex life of the wire and life of the blanket, and to improve the lay characteristics of the fabric and its tendency to lay flat and smooth.

With reference to the drawings:

Fig. 1 is an elevation partly in sections showing a typical construction of an embodiment of this invention.

Fig. 2 is a cross section of the multi-conductor wire 11 illustrated in Fig. 1.

Fig. 3 is a view in the direction indicated by the arrows 33 of Fig. 2.

For a general understanding of this invention, reference is made to Fig. 1. In this embodiment of the invention, the resistance Wire which serves as the heater wire and a second conductor associated therewith is helically wound on a supporting strand. The turns of wire per linear unit are based on the heat requirements. An outside layer of insulating material usually a vinyl plastic is applied to the helically wound wire. In addition, it is sometimes desirable to put a protective coating over the thermosensitive material as for example in a case where the material is subject to moisture pickup. The drawings depict a two conductor type blanket wire, however, the invention can be practiced using any number of conductors. The ultimate selection of the number of conductors will be based on the control mechanism used. The usual number of conductors required is two or three.

In Fig. 1, an inner supporting strand is indicated at 10. The insulated conductors are indicated generally at 11 and the covering insulation at 12. The wire 11 is sectioned to show the parallel relationship of conductors 15 each of which is located so that it is adjacent the inner messenger strand.

Figs. 2 and 3 are separate views of the nylon insulated conductors prior to the helical wrapping of wire conductors on the inner supporting strand 10.

It can be seen by references to Fig. 2 that the amount of thermosensitive material separating the two conductors can be varied by displacement of the conductors in a lateral direction. As the conductors are spread apart the over-all shape of the Wire will vary from substantially circular to elliptical. Thus, there is substantially no change in the dimension of the wire along the normal axis of the two conductors. Inasmuch as the wire is helically wound on the messenger so that each conductor is adjacent the messenger, this means that the over-all diameter of the wire need not be increased in order to increase the amount of effective thermosensitive material. It is possible, therefore, to obtain a helically wound multiple conductor wire with a minimum over-all diameter. Mechanically speaking, the column is slender and as a result flexible.

Having thus described my invention, I claim:

An electric wire for use with electrically heated fabrics comprising a plurality of conductors, thermosensitive insulation which maintains said conductors in spaced relationship and which is conductive at elevated temperatures, an inner supporting strand on which said conductors are helically wound in parallel spaced relationship so that the conductors are equidistant from the inner strand, and a cover insulation.

References Cited in the file of this patent FOREIGN PATENTS 627,403 Great Britain Aug. 8, 1949

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