Thermal Protective Switch

The invention relates to a thermal protective switch having a contact assembly made up of moving contact with a counter-contact, a bimetallic sensing element for moving the moving contact clear of the counter-contact, and a resistor.

When a fault develops, the first action of known thermal switches is to interrupt the load circuit, for example the circuit of a radiator. After the bimetallic sensing element has cooled down the contacts are moved together and the radiator load circuit is closed again. If the fault or other cause for the thermal protective switch turning off is not taken care of, the switch will open again. Because this may take place at an ever increasing rate, so that the switched-on intervals become shorter and shorter, the bimetallic strip and the contact faces are likely to be damaged by the heating effect of the electric current, the contact assembly no longer being able to take up and dissipate such heat in the desired way. Frequent switching at excessive temperatures may cause the contacts to weld together. Such undesired switching of a thermal protective switch so that it is repeatedly switched on and off is referred to as "hunting". If the contacts are welded together temperature control by thermal protective switch will no longer be possible, because the radiator will be permanently connected with the electricity supply and it will not be switched off by the said thermal protective switch. The load and any other devices in circuit will be irreparably damaged. There will furthermore be a danger of fire because of the failure of the controller.

A further form of known thermal protective switch has so-called thick film resistors in parallel with the contact assembly for keeping the switch from hunting. These thick film resistors are purely ohmic resistors which are produced by screen printing resistance ink on a thermally stable material and baking. The thermal isolation of such resistors and the putting of the metal parts in position is a very complex operation and is frequently not possible. Further shortcomings are that because the resistor is not fully thermally separated or isolated from the contact assembly the resistor has to be run at higher temperatures, which are likely to be responsible for thermal overloading of the resistors themselves and they may then be damaged by the heat generated in them, while further resin components as frequently used in protective switches will become brittle and develop cracks so that they may fracture. The first-noted form of danger is even likely in a design in which an ohmic resistor, which heats itself up, is connected by leads between the two sides of the contact assembly.

Generally speaking, and without paying particular attention to these shortcomings, the use of PTC resistor elements is quite well known. The German unexamined (Offenlegungsschrift) specification No. 2,927,475 is with respect to a controller housed in a glass bulb with a PTC resistor element responding to heat. The resistor element has holes through which electrode leads may be threaded and it is kept in place by such leads. There is no possible chance of being able to dismount the element for replacement. Because of the glass bulb, use for the purpose noted above is not possible. More specially, use in a forced air flow heater would not be possible, because the glass bulb would cut down the heat transfer excessively. The German unexamined specification No. 2,606,201 is with respect to an actuating member with a bimetallic element and a PTC element such that mechanical holding functions such as locking functions are to be possible. Here as well the heat sensitive resistance element may not be replaced. Moreover the flow of heat and the thermal coupling of the PTC element is badly designed in respect of the lever motion. The German unexamined specification No. 2,907,763 is with respect to timer switch for the delayed switching on and off of electrical devices and may be compared with the function of a two pole, two throw switch, it being fitted with a ceramic resistance element functioning as an electromagnet winding. In other respects this design has the same shortcomings as the said German specification No. 2,927,475 because it is encapsulated so that the device is generally not able to be used in the way noted hereinbefore. The East German specification No. 119,497 relates to a bimetallic switch for the thermal protection of electrical devices, in which a homogeneous resistance element is placed one sidedly against the moving part of the bimetal spring and by way of a U-like clip produces an electrical contact with the stationary part of the bimetallic switch and at the same time acts as a support for the resistance element. The resistance element is a thermally stable elastomer with a limited electrical conductivity, more specially silicone rubber. The form of pressing effect is poor in this case, because the u-like bridgepiece loses its gripping effect on being heated. Furthermore the moving part of the controller has a supporting function so that the accuracy of control is likely to suffer.

On the other hand the purpose of the present invention is to further develop a protective device of the sort noted at the start in such a way that while taking care of the shortcomings noted, it is not possible for the device to the switched on again before the fault has been put right, the insulation of the heat energy being produced by simple means in the best possible way in the least space, while at the same time the temperature of the surroundings is kept from rising when there is a fault.

In keeping with the present invention this purpose is effected inasfar as the resistor is a PTC resistor placed on the contact assembly. By using a PTC component as in the invention a dependable, safe and fast transfer of the heat produced by a flow of current to the contact assembly is made certain of. The temperature-resistance function of such a PTC component firstly makes certain that as compared with the opening of the contact assembly because of an excessive ambient temperature the conductivity of the PTC component is so small that there is more or less no further flow of current. When the surroundings cool down the current through the PTC element and the release of heat dependent thereon become stabilized at a value such that the contact assembly is reliably kept open until there is an external interruption of the current, for example through turning off the device or an electrical appliance plug being pulled out. It is only some time later, that is to say only after checking the load and if necessary repairing it before turning on again after the undesired switching off of the load by the protective switch, that the contact assembly of the protective switch will act.

Further useful effects and details of the invention will be seen from the claims and the following detailed account of working examples of the new thermal protective switch.

FIG. 1 is a view of a first working example of the new thermal protective switch from the side.

FIG. 2 is a section taken through the switch of FIG. 1 on the line II--II.

FIG. 3 is schematic of a new thermal protective switch.

FIG. 4 is a view of a further thermal protective switch in keeping with the present invention.

FIG. 5 is a section taken on the line V--V of FIG. 4.

FIG. 6 is a view of a further thermal protective switch in keeping with the invention.

FIG. 7 is a view of a further possible form of the thermal protective switch of the invention.

FIG. 8 is a section taken on the line VIII--VIII of FIG. 7.

FIG. 9 is a view of a still further thermal protective switch of the invention.

FIG. 10 is a view of the design of FIG. 9 in lengthways section.

FIG. 11 is a section taken on the line XI--XI of FIG. 10.

The open thermal protective switch 1 of the invention to be seen in FIG. 1 without a housing has connection and anchoring lugs 2, that are fixed in an insulating part 3 of the thermal protective switch 1. There is furthermore a contact assembly that is composed of a bimetallic strip 5 joined electrically with a contact lug 2 by which it is kept in place. On the end of the strip 5 a button-like contact 5' is fixed functioning with a a counter-contact 4, that is fixed on the other contact lug 2'. The contact lug 2 spring strip 7 is fixed by having a bent part 8 thereof spot-welded on to the contact lug 2. The spring strip 7 runs from this point to a position under the contact lug 2', it being acted upon by a spring force acting towards the contact lug 2'. Between the contact lug 2' and the spring strip 7 a PTC resistor bead 6 is placed, that is only pressed by the spring force of the spring strip 7 against the contact lug 2 and so kept in place. It is important that no further anchoring means be used so that the PTC resistor may be readily and simply replaced.

The thermal protective switch 1 of the invention is connected in series with a load 9 such as a heater or the like, as will be seen from FIG. 3. It will be clear from the circuit schematic that the PTC resistor 6 is placed in parallel to the contact assembly so that it shunts it. If the current is now turned on, it will go from one connection lug, for example 2, by way of the bimetallic part 5 and the contact 5' fixed thereto to the counter-contact 4 and from same to the connection lug 2' and then to the load 9, which will then for example be heated. If a fault develops, for example because the load 9 is excessively overheated, the bimetallic strip 5 of the thermal protective switch 1, which as is normally the case is placed next to the load 9, will switch off the contact assembly 4 and 5'; that is to say the contact 5' is moved clear of the counter-contact 4. For this reason the current flow through the contact assembly will be interrupted. Because of the effect of the ambient heat given off by the load 9, the PTC resistor as well will have such a low conductivity that it will hardly conduct any current to the load 9 so that the same will now no longer be loaded and the temperature therein will go down. With a fall in the temperature the conductivity of the PTC resistor will however go up so that there will be a self-heating effect therein and so the condition will become stabilized with a small current flow and a level of heating corresponding thereto, the release of heat to the surroundings, more specially in the present working example, to the counter-contact 4 and by way of radiation therefrom to the bimetallic strip 5 being such that the contact assembly 4 and 5' will keep open. For this reason the load may not be overloaded so that any material release of electrical power as heat from the load 9 of the radiator will not be possible. It is only later after turning off the load 9 by hand that the flow of current through the PTC resistor will cease so that same will cool down and the bimetallic strip 5 shuts the contact assembly 4 and 5' and after a certain time, more specially after checking out the fault, the load may be used again. The current still flowing through the PTC resistor 6 when the contact assembly is opened may in the respect be suitably selected.

Whereas in the design of FIGS. 1 and 2 the PTC resistor 6 is simply clamped between the connection lug 2' and the spring 7, it may be more securely fixed in place when this is necessary, for example when the appliance in which the thermal protective switch 1 is placed is subject to vibrations. In keeping with the design of FIGS. 4 and 5 a piece of hose 10 with shrink properties is slipped over the connection lug 2', the PTC resistor 6 and the end of the spring 7 holding it and then shrunk onto the parts so that there is now a better heat insulating effect.

In the design of FIG. 6 the PTC resistor 6 is bonded by adhesive 11, as for example silicone adhesive, on the contact lug 2'. This as well makes for a more secure anchoring effect.

Another way of fixing is that of the construction of FIGS. 7 and 8. In this case a resin sleeve is drawn over the PTC component 6 to keep it in place and to press it against a contact lug 2'.

A still further preferred form of the thermal protective switch of the invention is to be seen in FIGS. 9 to 11. Inasfar as the parts are the same they are marked with the same part numbers as in the earlier working examples of the invention, to which attention is to be given with respect to parts which are the same and which are not covered here in detail. Unlike the earlier forms of the invention in the design of FIGS. 9 to 11 the spring strip 7 pressing the PTC element 6 against the contact lug 2' has a curved convex embossed part 12 under the PTC element, said part 12 alone touching the PTC element 6; with it the spring 7 presses the PTC element against the contact lug 2. The PTC element 6 may for this reason tip about the pivot-like embossed part 12 and so adjust itself that it is rested flat and evenly on the contact lug 2' on the level. This on the one hand gives a very good thermal insulating effect and on the other an optimum electrical contact so that the function of the thermal protective switch of the invention is fully enabled by this further development thereof, even although the PTC element may nevertheless be simply and readily replaced. In the design of FIGS. 9 to 11 the spring part 7 has furthermore has a bell-like resin part molded round it, the mass of the resin part 13 following the embossed part 12 freely in the bell-like hollow of the part 13. The PTC element is simply placed inside so that it is held at the side by the bell-like form of the resin part 12 against the contact lug 2'. The spring part 7 has its connection part 8 spot welded at 14 to the connection lug 2. The resin used for making the part 13 is Ryton.

The features of the invention given in the above specification, in the figures and in the claims may be used separately or in suitable combinations to put the invention into effect in the different examples thereof.