Thermostat with ceramic mounting pins of resistive material

A thermostat comprises a bimetal switch element reacting upon changes in temperatures which on one of its ends is fastened to a frame and on its other end is provided with an electric contact adapted to engage a fixed contact provided on the frame. The frame comprises two mutually parallel sheet-like metallic contact carriers forming a base unit, through which two parallel pins are extending for connection of both contact carriers and on which pins the contact carrierrs, with a tight fit, are slidable to and fro relative to one another while being held in permanent mutually parallel relation during the sliding. The parallel pins in the thermostat are formed of a ceramic material of positive temperature coefficient of resistivity which serve to electrically space the two metallic contact carriers while also serving as a self-regulating external heating means for the thermostat.

BACKGROUND OF THE INVENTION 
This invention relates to a thermostat adapted to open a circuit in 
response to temperature change and relates more particularly to a 
thermostat having heating means for keeping the circuit open until the 
thermostat is manually reset. 
A known thermostat comprises a bimetal switch element reacting upon 
temperatures, which on one of its ends is fastened on a frame and on the 
other end is provided with an electric contact adapted to engage a fixed 
contact provided on the frame, said frame comprising two mutually parallel 
sheet-like metallic contact carriers forming a base unit, through which 
two parallel pins are extending for connection of both contact carriers 
and on which pins the contact carriers, with a tight fit, are slidable to 
and fro relative to one another to be held permanent mutually parallel 
during mounting, which thermostat may be provided with an external heating 
means and/or an internal heating means, and a selected heat capacity, as 
well as with various embodiments of connecting terminals for the contact 
carriers. 
Such a thermostat has been described in U.S. Pat. No. 4,528,540. 
A number of embodiments of the known thermostat are shown in the noted 
patent for influencing the current sensibility and/or the times of 
switching the thermostat on and off. The patent also shows various 
connection and mounting possibilities. 
For instance various embodiments of connecting terminals for the contact 
carriers are shown: an external heating element that is fed by the current 
to be switched is shown, possibly being arranged in or on means of a 
selected heat capacity; and the contact carriers, one or both of which 
being made in a resistance material, are shown being possibly combined 
with a special configuration of said carriers and/or an additional means 
of selected heat capacity. 
Furthermore the possibility is indicated in the patent of adding to these 
embodiments a heat source preferably with PTC characteristics, connected 
in parallel, for keeping the thermostat open after being opened until the 
main current is interrupted, so that a so-called manual reset function is 
obtained. 
A drawback of this structure is that an additional component (requiring 
additional space) and contact making means has to be added, whilst special 
care should be bestowed on stability, for instance in the event of 
vibrations. It is a further drawback that in the automatic reset function, 
that is without an additional heat source connected in parallel, the 
switch-back times highly depend upon the ambient temperature, and the 
range of switch-back times is restricted i.e. by the difference between 
the switch-back temperature and the ambient temperature. 
BRIEF SUMMARY OF THE INVENTION 
While maintaining the advantages with respect to the known thermostat 
mentioned in the above indicated patent, the present invention tends to 
abolish the above indicated drawbacks and for that purpose it is 
characterized in that at least one of the pins is made of PTC material 
(material displaying a positive temperature coefficient of resistivity 
which is preferably adapted to display a sharp increase in resistivity at 
a Curie temperature which is cracteristic of the material), from which 
pins, when energized, electrically generated heat can be transferred by 
convection and radiation to the temperature sensitive switching element of 
the thermostat. 
Because of the application of said feature the possibilities of use are 
enlarged by a broader range of switch-back times and reset functions, a 
simpler mounting without additional components is possible, and minimal 
room is necessary, said thermostat being reliable in a 
fabrication-technical respect and the time at which it switches back on 
again can be accurately influenced. 
One, or both, pins can be made of a PTC material with a predetermined Curie 
temperature with which a simple, solid structure is obtained. Heat 
generated in this or these PTC pins is transferred by radiation, 
conduction and convection to the switching member of the thermostat. 
Because of the close construction of the device the Curie temperature of 
the PTC pin, when energized, determines mainly the temperature of the 
switching member and provides self-regulation of the heating provided by 
the pin. At lower ambient temperatures it creates an artificial direct 
ambient temperature. When the Curie temperature of the PTC pin is chosen 
in such a way that this artifical direct ambient temperature lies between 
the switch-off and switch-on temperature of the thermostat, the latter 
cannot switch back on again when the ambient temperature falls. Only 
externally swtiching-off the current can switch the thermostat on again, a 
so-called manual reset function. When the Curie temperature is chosen in 
such a way that this artificial direct ambient temperature in the 
application at decreased ambient temperatures in the situation where the 
thermal balance is below and close to the switch-on temperature, a very 
long time for switching-on again is provided. This switching-on takes 
place automatically and the time of switching-on again depends less upon 
the ambient temperature. The closer the Curie temperature is to the 
switch-on temperature, the longer the cooling time of the thermostat will 
be. 
If one of said pins is made of PTC material, security is given in case, 
upon short circuiting of the pin, the latter would burn. The other pin, 
made of an insulating material, maintains the function of the thermostat 
and the mechanical structure of the whole.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The base unit 1 comprises two mutually parallel, sheet-like preferably 
metal contact carriers 2 and 3 which, with a tight fit, are fastened on 
two parallel pins 4 and 5, one pin or both pins thereof being made of a 
preferably ceramic PTC material. 
The pins extend through two deep-drawn holes in the contact carriers. The 
contact carriers are slidable on the pins to and fro relative to one 
another and are held mutually parallel by the pins during the composition 
and adjustment of the thermostat. Each contact carrier comprises 
connecting terminals 6, 6 which are provided on opposite ends of the base 
unit and are aligned relative to one another and extend in longitudinal 
direction of the base unit. 
On the inner side of one of the contact carriers 2, 3 a switch element 7 is 
fastened at 8 on the relevant contact carrier by e.g. welding. The switch 
element consists of a bimetal and preferably comprises a setting or formed 
portion 9 which enables independent switching with a snap action at a 
determined switch-on temperature and switches back at a lower temperature. 
If the direct ambient temperture remains above the switch-back temperature 
no switching-back takes place, unless resetting is performed mechanically 
or the direct ambient temperature is reduced. The switch-over temperatures 
are determined by the configuration of the setting and the manner of 
mounting of the bimetal in known manner. 
The setting or formed portion 9 of the bimetal is circular in the shown 
embodiment, however, it may have another shape too. 
The switch element comprises on its movable end a contact 10 adapted to 
engage a fixed contact 11 fastened on the inner side of the other contact 
carrier. The contact carriers may have any configuration. For instance 
they can have an aperture 12 as illustrated in FIG. 1 for laterally 
exposing almost the entire switch element 7. Furthermore each contact 
carrier can be made of a resistance material by which the current 
sensibility of the thermostat increased. The contact carriers can be made 
of the same material or of two different materials. A contact can consist 
of three layers, for instance of Ni, Cu, Ag (Cdo). It is also possible 
that the switch element has not been provided with a setting. The 
connection terminals can also have various configurations and positions, 
and a heat well can be used as well as an external or an internal heating 
means. 
In that arrangement, the terminals 6, 6 are adapted to be connected in an 
electrical circuit so that when the contacts 10, 11 are engaged the 
circuit is closed but so that when the bimetal 7 moves in response to a 
selected temperature change for separating the contacts 10, 11, the 
circuit is opened. As will be understood, the temperature at which the 
circuit opens is determined primarily by the temperature response 
characteristics or actuating temperature of the bimetal but the carriers 2 
and 3 are selectively movable relative to each other on the pins 4, 5 as 
described in the noted patent for calibrating or adjusting the actuating 
or circuit-opening temperature of the thermostat. As will also be 
understood, the bimetal is also adapted to reengage the contacts 10, 11 
for reclosing the circuit when the temperature of the bimetal falls to a 
reset temperature relatively lower than the noted actuating temperature. 
In accordance with this invention, the PTC pins 4, 5 are formed of a 
ceramic material or the like of positive temperature coefficient of 
resistivity which has a selected first, room temperature resistivity which 
s relatively low but which is preferably sufficient for the pins to 
display an electrical resistance greater than that displayed by the 
bimetal 7 and its associated contacts. However the pin material is adapted 
to display a sharp increase in resistivity at a temperature which is 
preferably selected to be between the actuating the reset temperatures of 
the bimetal such that when the pins are self-heated by directing 
electrical current thrrough the pins, the pins serve as a self-regulating 
electrical resistance heater and stabilize at a temperature which is 
sufficient to maintain the bimetal 7 at a temperature between its 
actuating and reset temperatures during variations in the general ambient 
temperature. 
In that arrangement, when the terminals 6, 6 are connected in a circuit, 
where the thermostat is disposed to sense normal temperature in a 
temperature zone to be monitored with the thermostat, the contacts 10, 11 
are in their normally engaged position for closing the circuit and no 
significant current flows between the carriers 2, 3 via the pins 4, 5. 
However, if the bimetal 7 is heated to its actuating or circuit-opening 
temperature in response to a predetermined increase in the temperature 
being monitored with the thermostat so that the bimetal moves to open the 
circuit, a significant current occurs briefly in the pins for rapidly 
heating the pin material to its high resistivity temperature to 
effectively complete opening of the circuit between the carriers 2, 3. 
However a small current continues to flow in the high resistivity pins to 
generate heat at the stabilizing temperature of the pins sufficient for 
maintaining the bimetal 7 above its reset temperature even though the 
temperature being monitored by the thermostat should return to its 
original or normal level. The thermostat is thus adapted to remain open 
until it is manually reset, by disconnecting it from the circuit for 
example, to permit the pins 3, 4 to cool to their original room 
temperature resistivity. 
It should be understood that although particular embodiments of the 
invention are described by way of illustrating the invention, this 
invention includes all modifications and equivalents falling within the 
scope of the appended claims.