Electric heating – Heating devices – With power supply and voltage or current regulation or...
Reexamination Certificate
2000-08-03
2001-10-30
Paschall, Mark (Department: 3742)
Electric heating
Heating devices
With power supply and voltage or current regulation or...
C219S212000, C219S497000, C219S481000, C338S0220SD
Reexamination Certificate
active
06310332
ABSTRACT:
This invention relates to what are termed herein as “blankets” and in the majority of cases this will be the apt description, but in fact the items may be more generally described as sheet materials as they will not necessarily in all cases be used as “blankets”. The sheet materials are however in all cases provided with electric heating elements and as the majority of sheet materials which have heating elements can accurately be described as blankets, only this expression is used hereinafter, in the interests of simplicity of description, but it is to be remembered that the invention can be applied to other sheet heating devices, such as pads, and seat warmers.
In any event the invention is more specifically concerned with heating elements for such blankets.
Conventionally, an electric blanket comprises a heating element in the form of a long tubular assembly comprising an inner core around which is wound a first or inner resistance heating conductor, a plastic (e.g. polythene) meltdown tube which overlies the inner conductor, a second or outer, resistance heating conductor which is wound on the plastic meltdown tube, and an outer cover tube. The meltdown tube therefore forms a meltdown layer between the conductors.
At one end of the assembly the conductors are connected or are for connection to an alternating current power supply, whilst at the other end, the conductors are connected through a one way rectifier, e. g. a diode, so that only half cycles of one type pass through the conductors. It is usual for only the positive half cycles of the power supply to pass through the conductors.
The plastic tube is called a meltdown tube as it forms the means of preventing overheating of the blanket, insofar as if the blanket element overheats, the plastic tube will melt, which has either the effect of causing a short circuit between the conductors if the melting takes place at the first end of the element, or of allowing the negative half cycle current to flow through the conductors, and in either case, the condition can be detected, and the power shut off.
Although there are shortcomings with the conventional blanket, virtually all electric blankets are constructed as described above. One of the shortcomings is that when a meltdown condition is detected, the heating element is irreparable and the blanket is useless. Clearly, this represents a considerable wastage.
Attempts have been made to provide that if an overheat condition takes place, there will be control of the power supply to prevent the blanket from being rendered useless, and in one such attempt, a third electric conductor (for example of the so called “tinsel” type) is used. This third conductor is placed in the inner core, and is separated from the adjacent inner conductor by a layer of specially doped PVC to provide a resistance between the inner conductors and the third conductor. This doped PVC has a negative temperature characteristic (NTC) as between temperature and resistance, or in other words as the temperature of the material increases, so the resistance of the material decreases. The NTC is furthermore of a high value in that the resistance between the inner conductor and the third conductor at room temperature might be in the order of mega ohms and at a temperature of say 70° C. would be in the order of several hundred ohms. In the electric blanket fitted with this third conductor, use is made of the NTC, by electronic means, to detect the change in resistance which takes place when overheating takes place, so that the power to the blanket can be modified before meltdown takes place, and therefore the blanket is not rendered useless when overheating takes place. This arrangement can also be used for temperature control purposes to achieve user select conform levels.
The three conductor system also has disadvantages, including that the addition of the third conductor and the NTC material tends to make the element, and hence the blanket, thicker and less flexible, and of course the blanket tends to be more expensive. As with the two conductor type of blanket, the three conductor type has to function in meltdown mode if the third conductor control system fails.
Even when known NTC layers are used in two conductor heating elements, it is difficult to achieve uniform characteristics along the whole run of the heating element so that calibration control is often required, which is expensive and time consuming.
At the heating devices are usually sold in different sizes, each device has a different length of heating element, so that the NTC values being fed back to the control unit are different for different devices and therefore calibration of each device is again required.
Another approach is the PTC (positive temperature co-efficient) method. This is an American system which uses a carbon impregnated polymer fed by two parallel busbars to form a self regulating element. In theory is a good system, but is has the disadvantages that it is expensive, is difficult to manufacture, is bulky which increases the bulk of the heating appliance, and it is prone to breakdown at European voltages in the order of 240 volts (as opposed to 110 in the USA)
It is also known to include bi-metallic strips to detect high temperature levels, but these add cost and bulk to the device and are invariably difficult to fit.
The present invention is concerned with the provision of an electric blanket which is of the two conductor type (only one of which need by a heating means), as opposed to the three conductor type, but wherein the detection of an overheating condition does not result in the destruction of the heating element and hence the blanket, whereby the blanket can be re-used.
In accordance with the invention there is provided an elongated heating element for an electric blanket comprising a first conductor means to provide heat for the blanket and extending lengthwise of the element, a second conductor means extending lengthwise of the element, and a meltdown layer between the first and second conductor means which is selected, designed constructed or otherwise formed so as to display a NTC, and including electronic control means set to detect a change in the resistance of the meltdown layer to provide a means of changing the power supply to the conductor means providing heat to the blanket to prevent destruction of the meltdown layer.
In a first preferred form, the second conductor means is also a heat providing conductor means, and both conductor means may comprise heating wires.
In an alternative arrangement, the second conductor means may be a detection or sensing conductor which serves to provide a current path in the event that the temperature of the blanket deviates from a pre-set value. Specifically, the sensing conductor may have a positive resistance characteristic (PTC), so that when it heats up its resistance increases and this is used by the electronic control to control the power to the heating conductor means.
The sensing conductor may also provide a current path, which is also through the NTC layer, in the event of that layer showing a condition of too high a temperature, requiring the power to the heating conductor means to be switched off.
Preferably, the meltdown layer has a NTC and a meltdown characteristic low enough (typically 120-130° C.) so as to enable the blanket to pass the relevant safety tests required by current regulations, such as IEC regulations. In this connection, current doped PVC's do not have a low enough melting point, but modified PVC's which are softer are suitable. In one example, a soft PVC, which is doped with 20% by weight of Stantonin Antimony, is suitable.
The meltdown layer may be arranged to have a small NTC and the electronics may be arranged to detect extremely small resistance changes in the meltdown layer, before meltdown actually occurs.
Normally, in the three conductor blankets as described, as the electronics are simple, a large NTC is required to enable the control described to operate satisfactorily, and for plastics materials which could be used as meltdown ma
Paschall Mark
Winterwarm Limited
Woodard Emhardt Naughton Moriarty & McNett
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