Thermal protective switch

Thermal protective switches (1) that are used for limiting the temperature of electrical appliances by automatic opening of the circuit should only reset themselves again after a large change in temperature. In this respect, when the load (9) is overheated, the contact (5') fixed to the bimetallic strip (5) is moved clear of the counter-contact (4) into its open position so that the current through the contact assembly is interrupted. If the fault is not put right there is continuous switching or hunting and there is a danger of the contacts of the thermal protective switch becoming welded to each other. For this reason, for stopping the contacts closing again, the design is such that in the open position the contact assembly, is shunted by the PTC resistor (6). The parts are kept in the open position until the load (9) is turned off by hand.

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.

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.