Electric heating – Heating devices – Combined with diverse-type art device
Patent
1986-02-28
1987-08-04
Goldberg, E. A.
Electric heating
Heating devices
Combined with diverse-type art device
219529, 219549, 219505, H05B 354
Patent
active
046847855
DESCRIPTION:
BRIEF SUMMARY
This invention relates to electric blankets and is particularly concerned with heating elements used therein. The expression "electric blankets", as used herein, encompasses not only electrically-heated overblankets and electrically heated underblankets but also electrically heated pads, electrically heated clothing, and any other electrically heated article of a flexible sheet-like form.
Heating cables or elements used in electric blankets (or pads) conventionally comprise at least one resistive heating conductor that will generate heat when a current passes through it. (The expression "resistive heating conductor", as used herein, means a conductor whose resistance is substantially so high that, when current is passed through it, it will produce sufficient heat to warm the blanket. Such a conductor is typically formed from so-called "resistance wire", e.g. of nichrome, as distinct from low resistance wire (e.g. of copper).) It is generally, desirable that means be provided to regulate the degree of heating, in order to achieve a desired level of heating and/or to protect against excessive heating. An established and effective technique of accomplishing this is to associate with the resistive heating conductor a layer of a material (e.g. polyvinyl chloride) having a Negative Temperature Coefficient (NTC) of resistance or impedance. This material, referred to hereinafter as "the NTC material", may for example physically separate first and second conductors, one of which is a heataing conductor and the other of which is a low resistance (substantially non-resistive) sensor conductor. The impedance or resistance of the NTC material is monitored, for instance by sensing current flowing through it. At normal temperatures, the NTC material is a good insulator. At elevated temperatures, while the NTC material generally remains an insulator, a small but discernible current flows through it. This current can be monitored and used to regulate heating by regulating the current supplied to the resistive heating conductor(s). Cables of the above-outlined type are referred to hereinafter as "NTC cables".
Recently, interest has been expressed in a different kind of heating cable for electric blankets or pads, referred to hereinafter as a "PTC (Positive Temperature Coefficient) cable". Superficially, a PTC cable resembles an NTC cable in that it comprises two conductors separated by a material (in this case a PTC material) whose resistance varies with temperature. In fact, however, the difference between the two types of cable is much more radical. As explained above, in an NTC cable at least one of the conductors is a resistive heating conductor and generates sufficient heat to warm the blanket, the NTC material being an insulator that becomes less insulative as the temperature increases to enable the temperature to be monitored. In contrast, in a PTC cable, the conductors act only as electrodes to connect the PTC material to a power supply, and are therefore of a substantially non-resistive nature, and heat is generated in the PTC material (rather than in the electrodes) by current flowing through the PTC material, from the supply, via the electrodes. (Since the electrodes are, of course, not perfectly conductive, a small amount of heat will be generated in them. However, the amount of heat is very small relative to that generated in the PTC material and is insufficient in itself to provide any substantial degree of heating of the blanket). The PTC material typically comprises carbon black embedded in a polymeric matrix. Examples of PTC cables are disclosed in UK Patent Specifications Nos. GB-A-1 456 047 (Rayachem Corporation), GB-A-1 456 048 (Raychem Corporation) and GB-B-2 079 569 (Sunbeam Corporation).
In a PTC cable, the resistance of the PTC material increases, as the cable heats up from cold, thereby reducing the heating power until the temperature stabilises at a value which, for a particular cable and a particular supply voltage, will be constant. That is to say, PTC cables can be considered "self-regulating" o
REFERENCES:
patent: 2914645 (1959-11-01), Wallace
patent: 4251718 (1981-02-01), Cole
patent: 4309596 (1982-01-01), Crowley
patent: 4329726 (1982-05-01), Middleman
patent: 4575620 (1986-03-01), Ishii
Dreamland Electrical Appliances PLC
Goldberg E. A.
Walberg Teresa J.
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