Thermostat

The present invention relates to a thermostat comprising a bimetal switch element reacting at selected temperatures which on one of its ends is fastened on a frame and on its other end is provided with an electric contact adapted to engage a fixed contact provided on the frame.

As thus described, such a thermostat is generally known.

Usually in thermostats a bimetal switch element is used as a temperature sensing member. This switch element is moveable in dependence on the ambient temperature either with continuous creep movement or with snap-action. The bimetal itself can conduct a current in the electric circuit. In dependence on the selfheating of the switch element by such current, the thermostat can also be current responsive. Said current sensibility can be increased by applying additional heating means on the thermostat. The switching time is highly influenced by the heat mass and heat transfer in the thermostat and, of course, also by the temperature at which the switch element will open or close the electric circuit in which the thermostat is incorporated.

Since it is easier to add mass to the thermostat than remove mass both for the fixed thermostat function and the current sensing function, a light mass is usually desirable. However, the risk of permanent deformations during manufacture, adjustment and use of the thermostat is increased thereby.

Frequently upon manufacture of said thermostat, differences in materials and dimesions will cause an inaccurate circuit response, said inaccuracies being difficult to correct. The larger the number of parts of the thermostat, the harder said correction will be.

Usually for this kind of thermostat a large number of model types is also required. Such models differ not only with respect to the desired switch operating temperature and with respect to the various sensibilities to temperature and current of the bimetal and the overall thermostat, but also with respect to the mechanical differences in the connecting terminals in the electric circuit.

This makes an accurate structure more difficult too.

The present invention comprises a thermostat as described above, giving an appropriate solution to the abovementioned problems, whereby a great accuracy, even at high ambient temperatures, is obtained and simple, versatile manufacture in large volume is enabled.

The invention is characterized in that the frame comprised two mutually parallel sheet-like metal contact carriers forming a base unit through which two parallel ceramic pins are extending for insulating 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 and are maintained permanently mutually parallel during such mounting, each of said contact carriers preferably having an aperture for lateral exposing of nearly the complete bimetal switch element.

Since the switch element is the only moving part and has to provide the right temperature sensing after adjustment, this switch element is positioned between both metal contact carriers whereby a good protective mechanical screening of the bimetal environment is guaranteed. Owing to the lack of plastics in the base unit, a high ambient temperature is admissible. Since the terminals connecting the thermostat in the electric circuit will be arranged so they have relatively little mechanical effect on the operation of the thermostat, they can be chosen freely as to their shape, e.g. as a quick connect terminal, or a welded, screwed, riveted or swaged connecting terminal, or an inserting pin for sheets with printed circuits, etc.

The thermostat can be made more responsive to current by the choice of the bimetal material and by manufacturing one or both contact carriers from a selected resistance material. The resistance sensibility of the contact carriers can be modified by the resistivity of the material itself, by the choice of the material thickness, and by the section and length of the path of current in the contact carriers. Owing thereto no additional heating means for the thermostat are required, there is a direct heat conduction from the contact carriers to the switch element, and the mass of the thermostat is slight which results in short times for switching on and off.

The thermostat can also be made further sensible to current by using an additional heating means. This means is positioned against the thermostat then and is spaced and supported by the ends of said ceramic pins.

In this way a good electric insulation between the heating means and thermostat is obtained and also a fixed distance between the heating means and switch element is guaranteed. By additionally applying a heat sink or well, e.g. a ceramic bar with current wire winding in a housing or a U-shaped gutter with a current-carrying wire winding, the switch behaviour can be further influenced. The heat excited by the heating means is partly stored in the heat well. After switching off of the thermostat this heat is liberated and in this way it keeps the contacts open for a long time. Since the heating means is supported along its full length this means can be subjected to a maximum current load without risk of sagging or bagging out. For the heating means very thin material may be used, whereby a very current-responsive thermostat is created. On account of the cooling effect of the heat well on the heating means the latter will be less likely to burn through quickly so that the thermostat will be more able to stand peak loads.

Furthermore the thermostat may be provided with a bimetal switch element such that it can only be reset to its normal position manually when the ambient temperature has returned to a determine value.

It is preferred that a push button is arranged for accomplishing resetting by pushing on the convex side of a dished portion of the bimetal switch element so that with proper dimensioning of the parts, operation of the pushbutton creates an always-safe, open contacts situation and only upon again releasing the reset button will the desired switching operation of the switch element be restored. In that way, the safety function of the thermostat cannot be circumvented by continued operation of the reset button.

The present thermostat is operable at high temperatures, has a very narrow temperature tolerance during switching on and off, and has a very wide timing range. Further the forces exerted on the ceramic pins are divided very equably, which reduces the risk of breakage to a minimum.

The thermostat is versatile in that a large number of configurations of the connecting terminals on the contact carriers and the application of adjusting buttons, heating means, cases and housings is possible, while the base unit of the thermostat remains unmodified thereby.

The thermally responsive device of this invention is adapted to be applied as a thermostat or protector in hair dryers, car wind screen wiper motors, refrigerator compressor motors, panels with printed circuits, light starters, fan heaters, toy transformers, etc.

In this way of mounting and adjusting, it is also guaranteed that the mutual sheets and position of the contacts can be accurately maintained upon manufacture, so that the optimum shaping with respect to the switch behaviour (mechanical wear, formation of sparks and the like) can also be realised upon the intended wholesale manufacture.

Finally, additional modifications are possible in dependance on the embodiment and position of the connecting terminals and a selected timing can be obtained by positioning a direct heat capacity means, e.g. ceramic material, a coating or a fastened metal onto the contact carriers. Also a heating element, preferably with PTC-characteristic, can be positioned between the connecting terminals. This heating element, which is connected in electrically parallel relation to the switch element, is put into operation upon opening of the contacts and keeps the contacts open through the heat then generated. Only by switching off the main current is the thermostat brought to resetting. Of course combinations of the abovementioned measures are possible.

The invention will be further elucidated now from the drawing.

FIG. 1 shows a perspective view of a base unit according to the invention.

FIGS. 1A-1E show alternate embodiments of connecting terminals on the base unit of FIG. 1.

FIG. 2 shows a plan view of a base unit according to the invention with an embodiment of a heating means shown partially in longitudinal section fastened thereon.

FIG. 3 shows a plan view of a base unit according to the invention and a partial section view of another type of heating means for the base unit diagrammatically illustrating use of the heating means with the base unit.

FIG. 4 shows an end view of the heating means of FIG. 3.

FIG. 5 shows a partial plan view of an embodiment of one of the contact carriers in which a terminal end portion of the carrier is provided with perforations.

FIG. 6 shows a partial plan view of an embodiment of one of the contact carriers in which a terminal end portion of the carrier is provided with zigzag-like path of current between its ends.

FIG. 7 shows an embodiment of the invention having a push button structure for resetting the switch element to its normal position.

FIG. 8 is a partial section view along line 8--8 of FIG. 7.

The base unit 1 in FIG. 1 comprises two mutually parallel, sheet-like metal contact carriers 2 and 3 of steel or the like which, with a tight fit, are fastened on two parallel, dimensionally stable, ceramic pins 4 and 5 electrically insulating the contact carriers from each other. The pins extend through two deep-drawn holes in the contact carriers. The contact carriers are press-fitted on and are slidable to and fro on the ceramic pins to be disposed in selected location relative to one another and are maintained mutually parallel during such assembly and adjustment of the thermostat unit. That is, each carrier has two holes surrounded by integral flanges 4.1, 5.1 which are deep-drawn from the carrier material and the ceramic pins are press-fitted into those holes to frictionally engage the deep-drawn flanges to hold the carriers parallel to each other as they are moved into a selected spaced relation to each other on the pins. The contact carriers also comprise connecting terminals 6, 6 which in base unit 1 are provided on opposite ends of the base unit and are aligned with one another to extend in opposite longitudinal directions from the base unit (FIG. 1).

On the inner side of one of the contact carriers 3 a thermostat metal switch element 7 is fastened at 8 on the relevant contact carrier by e.g. welding. Preferably for example the carrier is embossed at 8.1 and an end of the thermostat metal element has a weld slug 8.2 welded on one side. That end of the element is welded to the carrier embossment 8.1. The switch element preferably comprises a bimetal having an indenture or dished portion 9 which moves to an inverted dished configuration with a snap-action when the bimetal is heated to a predetermined temperature. In some embodiments of the element 7, the indenture 9 returns to its original configuration with snap action when the element cools to a reset temperature and in some embodiments that reset temperature is selected to be very low so the element requires resetting by hand to its normal position when the ambient temperature has a determined value such as normal room temperature. The dished portion 9 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 10 is resiliently biased into a closed circuit position engaging the fixed contact 11 by the thermostat metal element 7 when the element indenture 9 has one configuration as shown in FIG. 1 and is in an open circuit position disengaged from the contact 11 when the indenture has its opposite configuration. (See FIG. 7). Each contact carrier preferably has an aperture 12 (FIG. 1) for the lateral exposing of nearly the complete bimetal switch element 7 to be more rapidly responsive to changes in ambient temperature. Furthermore each contact carrier may be manufactured from a resistance material, whereby the current sensibility of the thermostat is increased. The contact carriers may be manufactured from the same material or from two different materials. A contact may comprise three layers, e.g. from Ni-Cu-Ag (CdO). It is also possible that the switch element has not been provided with an indenture and moves with creep action.

Each contact carrier may also comprise an end portion having a number of perforations 19 (vide FIG. 5), or a serpentine zigzag-like path of current (21) (FIG. 6) of reduced cross section. These measures are applied for varying the resistance and the heat generating and holding capacity of a contact carrier in order to obtain therewith a modification of the switching times of the thermostat.

In that arrangement, the contact carriers 2,3 are slidable relative to each other on the ceramic pins 4,5. The pins remain parallel to each other during such sliding movement and the carriers are also maintained parallel to each other as they are moved on the pins. The thermostat element 7 is adjusted in position relative to the fixed contact 11 by that means and, with an element 7 of selected characteristics, the thermostat 1 is thereby calibrated so that the element 7 moves between open and closed circuit positions at precisely predetermined temperatures. The ceramic pins 4,5 are press-fitted into the noted carrier holes and there is substantial frictional engagement between the ceramic surfaces of the pins and the flanged portions 4.1, 5.1 of the carriers so that the carriers are thereby secured in desired spaced relation to each other.

The connecting terminals may have various shapes and positions. For instance the connecting terminals 6a in unit 1a of FIG. 1A may be provided on the outer sides of the contact carriers and extend in opposite directions.

In the FIG. 1B the connecting terminals 6b are provided on the ends of the base unit 1b and extend in the same direction perpendicularly to the longitudinal direction of the base unit.

In FIG. 1C the connecting terminals 6c extend longitudinally to the base unit 1c from one end of the base unit, said terminals being parallel and extending in the same direction.

In FIG, 1D the connecting terminals 6d extend perpendicularly to the longitudinal direction of the base unit 1d. They are provided on one end of the base unit, are aligned to one another and are directed oppositely.

In FIG. 1E the base unit 1a of FIG. 1A is used in an assembly in which an electrically insulating fastening means 22 is added.

In FIGS. 2 and 3 the base unit is fitted out with a heating means which is attached thereon in any conventional way. This heating means can be carried out as a winding 14 about a ceramic heat sink bar 15 both provided in an insulating housing 18. The bar 15 is engaged or supported by corresponsing ends of the ceramic pins 4, 5 for spacing the heating means from the element 7 while assuring desired electrical spacing of the heating means from the carriers 2,3. The ends of the winding 14 are secured to a carrier terminal 6 and to a heater terminal 14.1 by soldering or the like if desired. The housing 18 is secured to the base unit 1 by fitting of the terminals 6 into slots 18.1 in the housing or in any other conventional manner. In the FIGS. 3 and 4 a modified heating means 13 is applied. It only consists of a current wire winding 16 provided in a ceramic gutter 17 as is diagram-matically illustrated in FIG. 4. The gutter is attached to the base unit 1 in any conventional manner (not shown).

FIGS. 7 and 8 show an embodiment of a push button structure for manually resetting the switch element to its normal position. In that embodiment, the thermostat metal element 7 is selected to have an operating temperature at which the indenture 9 moves from its original dished configuration to an inverted dished configuration with snap action so the element moves the contact 10 sharply to an open contacts control position as shown in FIG. 7 and to have a reset temperature below room temperature or the like so its indenture 9 does not normally return to its original dished configuration after the element 7 has once moved to open the the device circuit. The push button 19 is then preferably arranged so that when it is pressed it will engage the indenture 9 on the side of the indenture which is then convex. The push button is mounted in a housing 20 fastened on the carrier 2. Preferably the housing is secured to the carrier by being pressed into the aperture 12 in the carrier. Manual movement of the push button against the convex side of the indenture 9 then serves to snap the indenture back to its original dished configuration for permitting subsequent snap acting movement of the element 7 back to closed circuit position when the push button is thereafter released. Preferably the push button is dimensioned and the element 7 is arranged as shown in FIG. 7 so that movement of the element by pressing of the push button holds the contact 10 safely in its open contacts position while the push button is retained manually depressed. In that way the safety function of the thermostat 1 in opening a circuit is not circumvented by holding the push button down. The element 7 is permitted to return to closed circuit position only when a pushbutton is manually released as noted above. Of course, if the element has not cooled below its operating temperature when the pushbutton is released, the element promptly moves to maintain its opened contact position. Preferably the pushbutton 19 is proportioned and located as indicated in FIG. 7 so that a portion 19.1 of the pushbutton is adapted to engage a stop such as a portion of the weld slug 8.2. In that way, when the pushbutton is manually depressed for resetting the thermostat. The engagement of that pushbutton portion limits the pushbutton movement and protects the element 7 from excessive pushbutton force which might damage the thermal response characteristics of the element.

If desired, a ceramic heater element of a material of positive temperature coefficient (PTC) of resistivity is electrically connected between the carrier terminals 6 in electrically parallel relation to the element 7 as indicated 21 in FIG. 7. In that arrangement, the heater is energized on separation of the contacts 10 and 11 and is proportioned for heating the element 7 to prevent resetting of the thermostat 1 until the thermostat is disconnected from the noted electrical circuit. The PTC characteristic of that heater renders the heater self-regulating to stabilize at a safe temperature.

It should be understood that this invention includes modifications of the above-disclosed embodiments of the invention falling within the scope of the appended claims.