Thermal cut-out for gas-heated glass ceramic cooking surfaces

A thermal cut-out for gas-heated glass ceramic cooking surfaces has a temperature sensor extending from a switch housing which contains a snap switch. The sensor consists of a tube with a rod therein, the tube and at least one portion of the length of the rod having differing coefficients of thermal expansion. The temperature sensor is surrounded by an enveloping tube which is at a distance from the temperature sensor.

FIELD OF THE INVENTION 
The invention relates to a thermal cut-out for gas-heated glass ceramic 
cooking surfaces. 
BACKGROUND OF THE INVENTION 
Cooking surfaces of this type have been disclosed in German Auslegeschrift 
No. 24 40 701 and German Offenlegungsschrift No. 26 41 274. They have a 
gas-heated radiant burner comprising a burner plate which is provided with 
a radiant surface which is arranged at a small distance from the glass 
ceramic plate and heats it. The gas burner is operated intermittently, 
i.e. is turned on and off at a predetermined rhythm depending on the 
desired power supply. It is necessary to light the gas each time and, in 
addition, the gas flame has to be monitored so that no unconsumed gas is 
able to escape. The supply of gas is controlled by means of a magnetic 
valve which is controlled by means of a pulsating power control 
instrument. 
Glass ceramic surfaces are sensitive to over-heating, but must be brought 
to a temperature which is as near as possible to but just below the 
permitted maximum temperature in order to ensure that sufficient power is 
transmitted through them. 
In order to limit this temperature and to keep it as accurate as possible, 
experiments have been carried out using thermal cut-outs of the type 
disclosed in German Offenlegungsschrift No. 24 22 625. A thermal cut-out 
of this type, arranged between the radiant surface and the glass ceramic 
cooking surface, has not produced satisfactory results since the switching 
frequency is too high and unstable intermediate positions occurred with a 
relatively high adjustment and increase required in the contact distance 
and this prevents satisfactory switching and, in particular, good 
cooperation with the electronic control means for ignition and monitoring 
of the ignition. 
BRIEF SUMMARY OF THE INVENTION 
The object of the invention is therefore to provide a thermal cut-out for 
gas-heated glass ceramic cooking surfaces which allows both a sufficiently 
low switching frequency and control of the temperature which is as 
accurate as possible to the critical glass ceramic temperature, and which 
switches in a reliable and clearly defined manner. 
According to the invention there is provided a thermal cut-out for 
gas-heated glass ceramic cooking surfaces comprising a switch housing, a 
snap switch arranged in the said housing, a temperature sensor extending 
from the switch housing and cooperating with the snap switch, the 
temperature sensor comprising a tube with a rod arranged therein, the tube 
and at least one portion of the length of the rod having differing 
coefficients of thermal expansion, and an enveloping tube which surrounds 
the temperature sensor at a distance therefrom. 
The rod can consist partially of a material having the same thermal 
expansion as the tube in order to prevent the snap switch system from 
being over-stretched too much in the cold condition. 
The enveloping tube, which preferably consists of metal, and more 
preferably of stainless steel, can be kept at a distance from the 
temperature sensor, for example by beads or mouldings in the shape of 
studs, and can be pushed loosely over the temperature sensor. It need not 
be sealed at the ends in this arrangement. It protects the temperature 
sensor from the direct radiant heat, but does not insulate the sensor so 
much from the temperature to be detected as to make it impossible to 
control accurately the maximum temperature of the glass ceramic. In 
addition, the temperature sensor is protected from the immediate influence 
of the flames, which, under certain circumstances could lead to corrosion 
or a one-sided thermal loading of the sensor. Stainless steel with its low 
thermal conductivity is particularly suitable. Moreover, the enveloping 
tube distributes the heat uniformly over the entire sensor so that any 
inactive portions of the rod cannot have a detrimental effect under 
extreme loads. 
In any case, it has been found that with the thermal cut-out according to 
the invention it is possible to limit the temperature to a relatively 
constant value without an initial temperature peak, after a heating up 
phase, without the switching frequency being unacceptably high. 
The temperature sensor can advantageously be arranged between the glass 
ceramic cooking surface and a burner plate of the gas heater designed as a 
radiant surface, projecting substantially diametrally beyond it. The 
switch housing and the free end of the temperature sensor containing, on 
the one hand, the switch and, on the other hand, an adjusting means, can 
preferably project beyond the burner plate, and the enveloping tube can be 
arranged only in the region of the radiant surface. This allows assembly 
and maintenance of the thermal cut-out to be carried out in a particularly 
simple manner.

The thermal cut-out 11 illustrated in FIG. 1 has a housing 12 consisting of 
insulating material, for example steatite, in the form of a rectangular 
plate which is chamferred on one edge. The housing defines a recess 13, in 
which a snap switch 14 is arranged, the recess being made in its lateral 
face lying at the top in the drawing. The snap switch has a holder 15 
shaped from strips of sheet metal which at the same time forms one of the 
switch terminals and is inserted from the side lying at the top in FIG. 1 
into corresponding slots 16 in the housing 12 which are adjacent to the 
recess 13 and thus secure the holder. A fixed contact 17, whose holder 
forms the second terminal, and an abutment 18 which determines the contact 
distance are similarly provided. 
One side of a catch spring 19 is fixed on the holder 15 and the other side 
carries a moving contact 20. A spring contact tongue 21 which is stamped 
out of the catch spring rests on a support bearing 22 which is bent from 
the holder 15. A spring arm 23 is fixed to the catch spring 19, one end of 
the arm 23 holding a contact member which ensures that mechanical contact 
is made between a ceramic rod 24 and the actuation point of the catch 
spring 19. 
A groove-like recess 25 runs from the recess 13 to one of the longitudinal 
narrow sides of the housing 12. A temperature sensor tube 27 is fixed in 
the recess 25 by two sheet metal flanges 26 which are pushed over it and 
bonded to it, and the temperature sensor tube 27, together with the 
ceramic rod 24 running inside the temperature sensor tube forms the 
temperature sensor 28 of the thermal cut-out 11. The temperature sensor 
tube 27 is reduced in diameter at its end 29 and bears at that end an 
internal screw thread into which is screwed an adjusting screw 30 which 
rests on the rod 24 and by means of which the limiting temperature can be 
adjusted. In a preferred embodiment, the rod 24 consists of two portions 
37',38. The portion 37' is composed of a special steel which has the same 
coefficient of thermal expansion as the tube 27. In this way, one part of 
the sensor 28 is "inactive" so that the catch spring 19 is not bent so far 
upwards in the cold condition. The portion 38 of the rod 24, which is 
designed as a separate member, is made of ceramic material and has a 
different coefficient of thermal expansion from the tube 27. 
The temperature sensor 28 is surrounded by an enveloping tube 31 which is 
made of stainless steel and is a few millimeters larger in diameter than 
the temperature sensor. An air gap is thus formed between the enveloping 
tube 31 and the temperature sensor 28, which is kept constant since the 
enveloping tube has mouldings 32 at both ends in the form of beads or 
studs. FIG. 2 shows that three mouldings are provided on the circumference 
in each case. The air gap between the enveloping tube and the temperature 
sensor is also open on both sides and the enveloping tube can move freely 
on the temperature sensor in the longitudinal direction. If desired, the 
gap between the temperature sensor and the enveloping tube can be filled 
with insulating material. 
FIG. 3 shows a gas heating unit 33 having an upper burner plate 34 acting 
as a radiant surface. The plate 34 is made of perforated ceramic material 
through which the gas issues and burns substantially without flames. The 
burner plate is made to glow in the process and radiates its heat upwards 
where it penetrates through the glass ceramic plate 35 illustrated in FIG. 
4 and heats cooking containers placed on it. A waste gas pipe 36 is 
connected to the circular burner plate. Apparatuses for igniting and for 
monitoring the ignition are also provided in this position. 
The temperature sensor 28 is arranged between the upper radiant surface of 
the burner plate 34 and the glass ceramic plate 35, and projects 
horizontally and exactly diametrally over the gas heating unit in the 
embodiment illustrated. The sensor is mounted in a retaining ring 37 in 
such a way that the housing 12 whose recesses 13 and 25 are sealed by an 
insulating plate, is arranged outside the ring. The end 29 of the 
temperature sensor 28 also projects outwards through the retaining ring. 
The portion of the temperature sensor 28 located above the burner plate 34 
is surrounded by the enveloping tube 31 which thus has a smaller length 
than the temperature sensor. It protects the temperature sensor arranged 
about half way between the burner plate and the glass ceramic plate from 
the immediate effect of the heating gases and the radiation of the burner 
plate 34. 
Numerous modifications of the embodiments illustrated and described are 
possible. Thus, for example, a different type of regulator can be used 
provided it has an elongate rod-shaped or tubular temperature sensor. The 
temperature sensor can also be made of different materials although the 
combination of stainless steel and ceramic (steatite) is particularly 
suitable as sensor material. The fact that the ceramic rod is protected in 
the stainless steel tube also makes this embodiment favorable. It would 
also be possible, however, to arrange the material having the highest 
coefficient of expansion inwards, although this is not necessary owing to 
the advantages of the invention. The arrangement of the temperature sensor 
can differ from the diametral arrangement, although the diametral 
arrangement illustrated covers the most representative area. 
The metallic portion 37' of the rod preferably lies at the free end of the 
temperature sensor, i.e. at the adjusting screw. Owing to the two-part 
design of the rod it is possible to select the ratios of expansion 
independently of the length of the sensor needed for spatial reasons and 
with the materials which are most suitable at any time.