US3673380A

US3673380A - Glow switch relay for blanket overheat control switch

Info

Publication number: US3673380A
Application number: US192090A
Authority: US
Inventors: George C Crowley
Original assignee: Northern Electric Co Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1971-10-26
Filing date: 1971-10-26
Publication date: 1972-06-27
Anticipated expiration: 1989-06-27
Also published as: AU4531572A; CA932010A; GB1411124A; JPS4850240A

Abstract

An overload protection circuit for an electric heating blanket is provided. A sensing element in the blanket is coupled to a gas filled tube which has a pair of contacts that are opened when an overload condition is sensed, causing a bimetallic element to open a second pair of contacts connected in series with the heating element.

Description

United States Patent 1151 3,673,380

Crowley 1451 June 27, 1972 54] GLOW SWITCH RELAY FOR BLANKET [56] References Cited OVERHEAT CONTROL SWITCH UNITED STATES PATENTS [721 Invent W 3,356,825 12/1967 Mills et al. ..219/494 x Elem-c 1 3,361,941 1/1968 Fickweiler ..317/132 [73] Assume Northern Company Chlcago' 3,445,631 5/1969 Fickweiler ..317/132 x [22] Filed: Oct. 26, 1971 l pp No 192 090 Primary Examiner-James D. Trammell Attorney-George R. Clark 52 us. 01. ..219/212, 219/494, 317/4012, [571 ABSTRACT 317/132 An overload protection circuit for an electric heating blanket [51] I'll. Cl. ..H0lh 37/00 is provided A ensing element in the blanket is coupled to a [58] Field of Search ..3l7/ 132, 40 R; 307/1 17; gas filled tube which has a pair of contacts that are opened 219/494, 212 when an overload condition is sensed, causing a bimetallic element to open a second pair of contacts connected in series with the heating element.

4 China, 1 Drawing Figure 10 26a. "1 5; 26b I 32 5; 124-: F 1 I I I I- J l J? 32; I

ao if PATENTEDJUH27 1972 1120 221471: We g'owg' W 7/? GLOW SWITCH RELAY FOR BLANKET OVERHEAT CONTROL SWITCH BACKGROUND OF THE INVENTION Electric heating blankets are commonly constructed with a heat-sensitive sensing wire or element and a heating wire or element. One frequently encountered electric heating blanket construction consists of a flexible insulating core around which the heating wire and sensing wire are wound. The heating and sensing wires are separated by a layer of heat sensitive material. The resistance of the heat-sensitive material varies inversly with the temperature of the heating wire and this variation is sensed by the sensing wire to provide for overload protection. An insulating covering is preferably supplied over the sensing wire.

Control for a blanket of the type described is conventionally achieved by a means of a thermally sensitive switch which has a pair of contacts coupled in series with the heating wire. The sensing wire may be coupled to the control circuit of a second thermally sensitive switch which has a pair of overload .contacts in series with the heating wire. The second thermal switch provides overload protection for the blanket. The bimetallic thermal control switches which are commonly used to control the operation of the overload contacts are expensive and are capable of handling only a very small amount of power. Moreover, these switches tend to be overly sensitive and are difiicult to manufacture.

It is therefore an object of the present invention to provide a relatively inexpensive and uncomplicated thermal overload circuit.

It is another object of the present invention to provide an overload protection circuit for an electric blanket wherein a gaseous thermally sensitive tube controls the opening and closing of a pair of overload contacts connected in series with the heating wire of the electric blanket.

It is a further object of the present invention to provide an electric blanket control in which a gas-filled control tube is coupled in series with the sensing wire of the blanket so that the gas in the tube is ionized when the blanket is turned on causing a bimetal electrode, having a contact thereon, to heat up and to deflect towards a stationary electrode contact so that when an overload condition occurs the reduction of the resistance of the sensing wire insures that the firing electrodes of the tube are shunted by the sensing wire sufficiently to allow the bimetallic electrode to cool ofi; thereby breaking the electrical connection between the bimetallic electrode and the stationary cathode electrode and opening the heating wire circuit.

DESCRIPTION OF THE DRAWING The FIGURE is a schematic diagram of an embodiment of the present invention.

TECHNICAL DESCRIPTION OF THE INVENTION In order for an electric blanket to be suitable for personal use the blanket must be safe and easy to operate. An electric heating blanket may become overheated during use thereby creating a dangerous risk to the user of the blanket, therefore virtually all commercially available blankets provide for some form of overload protection. The various control circuits that areconventionally employed, however, are relatively expensive to manufacture and are relatively diflicult to manufacture. A reliable and inexpensive overload control circuit is provided by the present invention.

An electric blanket which incorporates the present invention is indicated by the reference number 10 of the FIGURE. The electric blanket 10 is constructed in a somewhat conventional manner in that it has a heating wire 12 and a sensing wire 14 incorporated therein. The length of the sensing wire is approximately equal to one-half the length of the heating wire 12, which is shown schematically in the Figure by the two

half length sections

12a and 12b. The sensing wire 14 is wound through only one half of the electric blanket 10, but unlike most sensing wires the sensing wire 14 is able to carry enough electrical current to function as a heating wire also. The sensing wire 14 may be physically located in a portion of the heating blanket with either of the half-length segments of the heating wire in proximity to it. The resistance of the heating section which is in proximity to the sensing wire 14 is slightly different from the resistance of, the other section of heating wire in order to provide for temperature equalization across the heating blanket l0.

The heating wire 12 is connected in series with the thermal control element 16 of the thermal switch 18 across the input voltage line. The element 16 is a conventional bimetal thermal control element that is responsive to the current that flows through the heating wire 12 and it is adjustable by the user by means of a conventional adjustment screw 180. The switch 18 causes the connection between the

points

18a and 18b to be repeatedly made and broken in acyclic manner. The length of time that the connection is made during a given time period is determined by the setting of the adjustment screw.

In addition to the control switch 18 an overload switch 20 must be provided to insure that an overload condition does not damage the blanket and possibly result in a dangerous condition for the user of the blanket. When the blanket 10 is energized the overload switch 20 closes the heating wire circuit through the

points

20a and 20b since the On-Ofl switch 28 is in its On position and electrical connection is also made between the

points

28a and 28b. The bimetallic thennal control element 22 that controls the making and breaking of the contacts of the switch 20 is coupled to the control element 26. The control element 26 is a three element glow tube which has a cathode 260 that constitutes one firing electrode, a movable temperature-sensitive arm 26b which is preferably a bimetallic element and comprises the control electrode 280 and an anode electrode 26d. The glow tube 26 is filled with an ionizable gas such as neon. When the switch 28 is turned to its 0n position, the circuit between the

points

28a and 28b is completed and current flows through the

resistors

32 and 34 which function as a voltage divider.

The voltage that appears at the junction point of the

resistors

32 and 34 causes the gas in the tube 26 to be ionized due to the current flowing through its firing electrode path which consists of the firing electrodes 26a and 260. The bimetallic arm 26b is then deflected towards the cathode 26a and electrical connection is made between the

contacts

26a and 26c which causes sutficient current to flow through the bimetallic element 22 to hold the switch 20 in its closed position so that electrical connection is made between the

parts

20a and 20b. Current due to ionization then flows through the sensing wire 14 and the current limiting resistor 30 while current supplied directly by the line flows through the resistor 32, the element 22 and the contacts 260 and 26c.

The sensing element 14 is made of a material which has a decreased resistance when its temperature increases. When an overload condition occurs the resistance of the sensing element 14 is reduced and ionization current through the tube increases. Since a relatively constant impedance is presented to the power supply current through the element 22 and the contacts 26a and 260 therefore decreases. The bimetallic arm 26b I then cools off and is deflected toward the anode 26d so that the electrical connection between the

electrodes

26a and 26c is broken. Ionization current will now blow between the contacts 26a and 260 and through the bimetallic element 22 but this current is not sufficient to keep the switch 20 closed and the circuit between the

points

20a and 20b is therefore broken. The heating wire circuit will remain open until the sensing wire 14 has cooled and its resistance is increased to the point where the current through open contacts 260 and 26c is large enough to cause the bimetallic arm 26 to again deflect sufficiently to close the connection between the electrodes 26a and 260.

While a particular embodiment of the present invention has been described it is not intended that the invention be limited thereto since many modifications may be made and it is intended to cover any such modification that falls within the true scope and spirit of the appended claims.

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

1. An overload protection circuit for an electric blanket having a heating means and a sensing means which has a resistance that decreases with increasing temperature; the sensing means being located in said blanket in proximity to said heating means and electrically coupled in parallel with said heating means, comprising means for energizing said blanket, an electronic tube filled with ionizable gas which comprises a cathode, an anode, a cathode-anode path coupled in series with said sensing means and a temperature-sensitive movable arm having a control electrode thereon, said arm being positioned in a first position when said gas in said tube is deionized so that said cathode and said control electrode are spaced apart a predetermined distance, said predetermined distance being such that ionization of said gas will result when said blanket is energized and a potential is applied across the cathode-control electrode path of said tube if the resistance of said sensing means is above a predetermined minimum value, said arm moving from said first position to a second position following ionization of said gas to hold the cathode and the control electrode in contact with each other until said blanket is de-energized or until the resistance of said sensing means decreases below said minimum value, said circuit further comprising control means coupled to said tube and having a pair of overload contacts coupled in series with said heating means; said control means being constructed to control the opening and closing of said overload contacts in accordance with the position of said movable arm and means coupled to said tube to apply a potential across said cathode and said control electrode when said blanket is energized.

2. An overload protection circuit as set forth in claim 1 wherein said means for applying a potential across said cathode and said control electrode comprises a resistive voltage dividing network which is coupled across said heating means for supplying a voltage which is less than the potential of the input source across said cathode and said control electrode.

3. An overload protection circuit as set forth in claim 1 wherein said control means comprises a bimetallic control element that is coupled in series with said cathode-anode path and sensing means.

4. An overload protection circuit as set forth in claim 3 wherein said means for applying a potential across said cathode and said control electrode comprises a resistance voltage dividing network which has a first resistor and a second resistor that are coupled in series across said heating means for supplying the voltage developed by said second resistor across the series combination of said bimetallic control element, said cathode-anode path of said tube and said sensing means.