Resistive heating track with bridge fuse

Details Resistive heating track with bridge fuse Resistive heating track with bridge fuse

1999-01-25

2001-03-27

Walberg, Teresa (Department: 3742)

Electric heating

Heating devices

With power supply and voltage or current regulation or...

C219S504000, C219S552000, C219S553000

Reexamination Certificate

active

06207938

ABSTRACT:

The present invention relates to electric resistances and heaters and in particular to electric resistances and heaters of the type comprising a resistive track provided on an insulating substrate.
Such resistances are used, for example, in controls for electrical appliances, such as motor, fans, etc. while such heaters are used or have been proposed for use in a variety of applications, for example in domestic appliances such as water heating vessels, water heaters and irons. Typically a glass, ceramic, or glass ceramic insulating layer is provided on a metallic base such as a plate (which may for example form a part of the base of a liquid heating vessel) and the resistive track laid down on the insulating layer, usually by a printing technique. A further electrical insulating layer may be applied over the track to protect it and prevent corrosion and oxidation.
It is clearly important that the resistance or heater should not be allowed to seriously overheat in a fault condition since this may cause substantial damage not only to the device or appliance in which it is being used, but also, potentially, to users thereof. Accordingly, in liquid heating vessels, it is common to provide a resettable overheat protector which operates in the event that the heater of the vessel overheats, for example if it is switched on without liquid in it or if it boils dry. Typically, this comprises a bimetallic actuator arranged in thermal contact with the heater and which operates at a given temperature, above the normal operating temperature of the vessel to open a set of contacts in the supply to the heater. However in the event that this protector should fail to operate it is also known to provide a back-up protector, for example a thermal fuse which will operate in the event that the temperature of the heater rises above a predetermined value.
In WO-A-94/18807, for example a thermally deformable fuse member is spring loaded against a part of the heater. When the heater temperature rises above a given temperature, the thermally deformable fuse member softens and deforms under its spring force, so as to open a set of contacts in the electrical supply to the heater, thereby disabling it. However, it is preferable to provide a heater or resistance with built-in protection.
It has been proposed therefore to include a thermal fuse in the track itself. In one arrangement, used in resistances for controlling fans in car heating systems a solder bridge is formed over a gap in the heating track. The solder is chosen to melt at a predetermined temperature, thereby opening the gap in the track, to break the electrical supply. This type of fuse has, however, several disadvantages. Firstly it is difficult to manufacture and in particular to obtain the required current carrying capacity in the fuse. Secondly, it is relatively slow to operate, as it relies upon surface tension effects in the molten solder to separate the fuse. Thirdly, solders can only be used over a limited temperature range, thereby limiting their range of operation. Also, since these solders are eutectics, over time they may change their crystalline structure which may result in the operating temperature varying. Finally, they are easily damaged for example in transit, storage or assembly, since any flexing of the substrate can break the electrical contact to the fuse.
The Applicant has now devised a new form of resistance or heater which attempts to address the above problems. It has been recognised by the Applicant that the electrical insulating properties of glass, ceramic or glass ceramic materials (collectively hereafter termed “glasses”) may be used in the overheat protection of resistances or heaters. In particular, the electrical resistance of glasses changes as the glass temperature rises. Whilst a glass may be an insulator at room temperature or at normal operating temperatures, its electrical resistance may drop considerably, indeed by several orders of magnitude, at higher temperatures approaching its melting point. By choosing a glass material with the appropriate resistance characteristics, and applying it between selected portions of a resistive track rated to operate with a given supply current, the Applicants have found that a predetermined portion of the track can be made to overheat before the whole heater or resistor overheats. A portion of the normal track can thus be made to operate, effectively, as a thermal fuse.
In particular, at normal operating temperatures the glass will act substantially as an electrical insulator, leading to a very small leakage current between the track sections. However, when the temperature of the heater track rises above normal (as would happen in an abnormal over-heat condition), the glass temperature will rise, thereby leading to a reduction in its resistance. This in turn will lead to an increase in the leakage current between the track sections. This will lead to a greater current flowing through the track sections, which increases the heating effect and so on. This is a self-perpetuating process which will result in the glass being heated internally by virtue of the leakage current flowing therethrough, which very quickly leads to the leakage current between the track sections running away, leading to the current in the track sections bridged by the glass exceeding its design rating, so that that part of the track will vaporise thereby disabling the resistance or heater.
From a first aspect, therefore, the invention provides a resistance or heater of the type comprising an electrical resistive track provided on an insulating substrate, two predetermined sections of said track having a predetermined current carrying capacity being bridged by a glass material, the configuration of the track, and the glass material being chosen such that at a predetermined temperature, the leakage current between the track sections rises to the extent that it causes a current to flow through one or both of said sections which is substantially above its current carrying capability, whereby a section fails.
The invention thus provides a self-fusing resistance or heater which does not rely upon external safety devices and which obviates the need for the use of solders, as described above. By choosing an appropriate glass material and track configuration, a track designer may predetermine where, when, and at what temperature, the track will fail in a controlled manner.
From a second aspect, therefore the invention provides a method of manufacturing an electrical resistance or heater of the type comprising an electrical resistance track provided on an insulating substrate, comprising providing a bridge of glass material between two selected sections of the track capable of carrying a normal predetermined current, the position of said bridge being predetermined such that above a predetermined temperature the leakage current between said sections will rise to the extent that the current flowing through a section of the track rises above its normal predetermined current, causing it to fail.
The glass may be applied merely as a discrete bridge between the selected track sections. Preferably, however, for ease of manufacture, the glass is applied over the track sections as an overglaze. The overglaze may be local to the track sections to be bridged, but preferably it extends over a substantial portion, most preferably substantially the whole of the track so as in addition to protect the track eg. from corrosion and oxidation in normal operating conditions. This is particularly so when the overglaze is one which will become conductive at high temperatures, e.g. 850° C.-900°C., where the track would otherwise oxidise and fail.
The leakage current between the track sections through the glass material will depend both on the voltage gradient between the track sections and the temperature of the glass. The glass temperature at a given position is at least initially determined by the local temperature of the heater or resistance. This temperature in turn will depend on the local power density of the heater or resi

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