Electric heating – Heating devices – With current connection and/or disconnection means
Reexamination Certificate
2001-08-23
2002-10-29
Paschall, Mark (Department: 3742)
Electric heating
Heating devices
With current connection and/or disconnection means
C219S512000, C219S481000, C337S333000, C337S377000
Reexamination Certificate
active
06472646
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to thermally sensitive controls for electric heaters, and more particularly for thick film electric heaters used in liquid heating vessels.
2. Background Information
So called thick film electric heaters comprise an electrically resistive heating track deposited, for example printed, onto an electrically insulating substrate provided typically as a ceramic or an insulated metallic support plate. In liquid heating vessels the metallic support plate may be formed by the base of the vessel itself, or by a plate mounted in an opening in the base of the vessel. Examples of such heaters are shown, for example in WO96/18331 and WO96/17497.
Typically the heating track has a tortuous path terminating in a pair of terminals through which power is supplied to the track. The power supply to the track is normally controlled by a thermally sensitive control which acts to open a set of contacts in the control to disconnect the power supply to the track in the event, for example, that the heater overheats. Power leads may be soldered on to the terminals and connected to the control, or as shown in GB-A-2283156 the thermally sensitive control may be provided with sprung contacts which make electrical connection with the terminals when the control is mounted to the heater.
A recent proposal by the Applicant in GB 2322274 is to use the contact provided on the heater as one of a pair of switch contacts, the other contact being provided in a control or control unit mounted against the heater. In the event that the heater overheats, a bimetallic actuator arranged in good thermal contact with the heater operates to lift the control contact from the heater contact to interrupt the supply of energy to the heater, thereby preventing serious overheating of the heater and possible damage to the liquid heating vessel.
In the earlier proposals, the control mounted contact and the bimetallic actuator are separately mounted in the control. This, however, is inefficient in terms of assembly tolerance and costs. The present invention seeks to overcome these drawbacks.
DISCLOSURE OF THE INVENTION
From a first aspect, therefore, the invention provides a thermally sensitive control for a thick film electric heater comprising a bimetallic actuator arranged for making good thermal contact with the heater, and a movable electrical contact for engagement with a contact provided on the heater, said movable contact being operatively coupled to the bimetallic actuator such that in use upon operation of the actuator, it moves out of engagement with the heater contact to interrupt the electrical supply to the heater, said bimetallic actuator and said movable contact being mounted on common leaf spring means which act in use resiliently to bias the actuator into thermal contact with the heater and the movable contact into electrical contact therewith.
Thus in accordance with this aspect of the invention, the need for separate mountings for the movable contact and actuator is avoided, and a common leaf spring is used both to mount the actuator and contacts and resiliently to bias them respectively into thermal and electrical contact with the heater. The leaf spring means thus performs a dual function, avoiding duplication of parts. Furthermore, the contact and actuator may be preassembled before being mounted in a control unit, for example, thereby reducing assembly times and costs. It also allows accurate positioning of the respective components relative to each other. Furthermore, the leaf spring will have a relatively low thermal capacity, thereby reducing the amount of heat drawn out of the actuator so improving its thermal response.
The invention also extends to such a preassembly, so from a second aspect, the invention provides an assembly for use in a thermally sensitive control for a thick film electric heater, the assembly comprising a bimetallic actuator, and a movable electrical contact, said bimetallic actuator and said contact being mounted on common leaf spring means which in use act to resiliently to bias the actuator into thermal contact with the heater and the movable contact into electrical contact therewith.
The leaf spring means may in a simple embodiment comprise a single limbed leaf spring, with the actuator and contact mounted to different mounting locations on the leaf spring. In the preferred embodiment, however, the leaf spring means may comprise a leaf spring having at least two connected limbs mounting respectively the actuator and contact. This is preferred so that the biasing force of the respective components can be tailored to suit the required conditions by choosing the appropriate spring force in each limb.
Preferably the actuator is a bimetallic actuator, preferably a snap acting actuator, and most preferably one of the type described in GB1542252.
In the case of such an actuator, the actuator may typically be biased against the heater with a spring force of 25 g, although lower forces may be used. A biasing force of 25 g will ensure good thermal contact between the actuator and the heater and allow for thermal distortion of the heater, but is not so great as to change the operating characteristics of the actuator. The actuator is preferably mounted by its tongue on the leaf spring member, so as to engage the heater around its periphery, thereby giving a large area of thermal contact while avoiding the need for heat sink compound, which is expensive.
The contact force should be sufficient to ensure a good electrical contact between the movable contact and the heater contact, but not so large that it cannot be overcome by the actuator. For a pure silver contact, a contact force of log is adequate for electrical contact purposes, and easily overcome by the actuator operating force, which typically is in the region of 250 g. For silver alloy contacts, a higher spring force would be necessary for a satisfactory electrical contact.
In the preferred embodiment, the leaf spring member is generally U-shaped, with the contact and actuator mounted on the respective side limbs of the U. The limb mounting the contact member is preferably arranged so as to overlap a movable portion of the actuator such that when the actuator moves, it moves the contact mounting leg to move the contact. The contact carrying limb may have a folded up tab for engaging the actuator, thereby minimising the movement of the actuator required to lift the contact, and obviating the need for a separate push rod or the like to transmit the movement of the actuator to the movable contact, thereby reducing material and assembly costs.
The leaf spring member, and more particularly the base limb of the U shaped spring member may be formed with means to mount it in a control housing. For example, it may be provided with a resilient bow for fitting in a complementary slot in a control housing.
The leaf spring member is preferably also provided with a terminal, for example a spade terminal to allow its easy connection to an external power supply, for example a connector provided on the end of a wire of a power cable.
The leaf spring member can, as has been said above, be mounted in a control housing itself mounted to the heater or the liquid heating vessel. In other embodiments, however, it may be mounted directly, eg by bonding, to the heater itself, or by rivetting to a separate terminal piece.
The invention also extends to a combination of a control or assembly of the invention in combination with a thick film heater.
As mentioned above, having a separately mounted spring contact and bimetallic actuator does have the disadvantage of being inefficient from the point of view of assembly tolerances and costs, these disadvantages may be mitigated, to some extent, by having the separate components commonly mounted. For example, the respective components may be commonly mounted over one or more mounting posts so that they may be secured by common retaining means e.g. screws, nuts, rivets and so on, or, for example push fitted into a common mounting location.
Thus
McCormick Paulding & Huber LLP
Paschall Mark
Strix Limited
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