Thermostat

A thermostat, operating on the basis of current generated heat, rather than ambient temperature, operates as an equivalent of a snap-action thermostat, though without the necessity for provision of a snap-action dimple, by use of two bimetallic arms, the motion of one being restricted by the casing and a calibration dimple in or on the casing. The two bimetallic arms move, as in a creep action thermostat, upon current generation, with separation of the arms being accomplished when the restricted arm can no longer move to follow the unrestricted arm. Such thermostats are particularly valuable where a high initial inrush of current is experienced, as with an incandescent lamp fixture, when initial opening of the device, on the high current inrush, is not desired.

BACKGROUND OF THE INVENTION 
It has long been known in the thermostat art that creep action is to be 
avoided. The primary reason for the avoiding of creep action has been to 
prevent damage to the contacts caused by arcing as the contacts just begin 
to separate, and to prevent other undue wear on these contacts caused by 
arcing and chattering. 
A solution adopted by the prior art to avoid creep action has been the use 
of the snap-action thermostat. In particular, this is accomplished by the 
formation of a dimple in a bimetallic member, the dimple snapping from a 
convex to a concave, or concave to convex, shape when the preset 
temperature is reached. This snapping of the dimple causes a rapid 
movement of the bimetallic arm in which the dimple is formed, resulting in 
a sudden separation of the two contacts in the thermostat. Thus, creep is 
avoided. 
The use of snap-action thermostats, as described, however is not entirely 
free from problems. In particular, thermostats formed with snap-action 
members are about twice as expensive as those formed with creep action 
members. Further, when formed in automatic assembly equipment, as an 
extremely large number of thermostats are today, there is a substantial 
loss of thermostats employing the snap-action, dimpled bimetallic arms. 
Frequently, as many as 50% of the snap-action arms are lost in testing 
following formation; by comparison, there is generally about a 96% yield 
of creep action type thermostats. 
Attempts have been made by the industry to obtain the benefit of the 
snap-action type of system, desired for its operation and equipment wear, 
while utilizing, essentially, a creep action type of thermostat. In 
general, however, these have been hybrid systems, such as shown in U.S. 
Pat. Nos. 3,789,339, 3,851,288, and particularly 4,319,214, all assigned 
to the assignee of the present invention. While each of these thermostats 
provides an adequate solution for the problems it is to solve, it does not 
really provide for a thermostat giving the benefits of snap action, while 
avoiding the costs and assembly losses experienced with that type of 
thermostat. 
In addition, recently, an increased emphasis has been placed on thermal 
protectors for incandescent light fixtures. Because of the nature of the 
service, there is a high initial inrush of current, as the incandescent 
fixture is illuminated, followed, quite rapidly, by a substantial drop in 
current. If the thermostat employed is not able to accept this initial 
surge without breaking the circuit, it is difficult, if not impossible, to 
illuminate the incandescent fixture. Accordingly, the industry has also 
sought a thermostat for an incandescent fixture which will accommodate the 
high initial current surges without breaking the electric circuit, while 
still providing sufficient protection to interrupt the circuit should 
problems develop during regular operation of the incandescent fixture. 
BRIEF DESCRIPTION OF THE INVENTION 
In accordance with the present invention, two bimetallic arms are mounted 
within a thermostat casing, each of the arms having mounted thereto a 
contact, the two contacts mating to complete the circuit. One of the 
bimetallic arms is, essentially, simply cantilever mounted from the 
insulation in the open end of the casing, and bends about that insulation. 
In accordance with the present invention, the second bimetallic arm, which, 
preferably, has a higher electrical resistance than the first bimetallic 
arm, while also cantilever supported from the insulation at the open end 
of the thermostat, is restricted from movement because it is biased 
against the thermostat casing and, additionally, bears against a 
calibration dimple formed in or on the casing. This dimple acts as a 
fulcrum and prevents free movement of the second bimetallic element, 
except about the fulcrum. 
Thus, the only portion of the second bimetallic arm which moves, upon 
heating, is the free end of that arm on the opposite side of the fulcrum 
from the cantilever support. This second arm is preferably formed of a 
higher resistance material than the first bimetallic arm, so that upon 
high current loads, it tends to heat and move faster. Because of the 
restriction of its motion, it is able to continue to mate with the first 
bimetallic arm for only a portion of its travel. When the second arm is no 
longer able to follow the first arm, operation of the device is similar to 
a standard device, except that motion of the second bimetallic arm is 
prevented, so that there is, in effect, a snap action separation of the 
two contacts. 
As indicated, the thermostat device of the present invention is 
particularly useful for incandescent light fixtures. However, it may also 
be used in other areas where thermostats have been employed in the past, 
such as in motors, electrical appliances, and the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the present invention, and referring, particularly, to 
FIGS. 1 and 2, a thermostat 1 is illustrated having a casing 2 with an 
open end and a closed end, and an insulating support member 3 mounted in 
the open end of the casing. Mounted by the insulating support member 3 are 
a first bimetallic arm 4 and a second bimetallic arm 5. 
The materials of construction of the bimetallic arms 4 and 5 are the 
standards employed in the art. While the two bimetallic arms may have, 
essentially, the same electrical resistance, it is preferable that the 
second bimetallic arm 5 have an electrical resistance at least twice the 
resistance of the first arm 4. The second bimetallic arm 5, in addition to 
or instead of the increased electrical resistance, may be formed of 
materials to provide a higher deflection rate during the passage of 
electrical current than the deflection rate demonstrated by the bimetallic 
arm 4. Bimetallic arm 5 is provided with a movable contact 6, while 
bimetallic arm 4 is provided with a movable contact 7, the contacts 6 and 
7 being in electrical contact, as illustrated in FIG. 2, to complete an 
electrical circuit, now shown. 
For proper operation of the device, it is important that the bimetallic arm 
5 be biased toward the casing wall, such as toward the top 12 of casing 2. 
One means to accomplish this is to form the bimetallic arm 5 in the manner 
shown in FIG. 1. Here, employing prime numbers, the bimetallic arm 5' with 
the movable contact 6' is bent at the center, as shown at 7', to provide a 
left portion 8', and a right portion 9'. When the bimetallic arm 5' is 
inserted into the casing 2, the portion adjacent the insulating member 3 
being, essentially, parallel to the upper portion of the casing 12, the 
end 9' is forced upwardly, with substantial pressure, against the top 12. 
Depending upon the degree of the bend 7', the bend may actually not be 
apparent within the casing 2. 
In order to provide proper calibration of the thermostat in accordance with 
the present invention, as well as to provide a fulcrum, substantially 
separated from the insulating member 3, a calibration dimple 10 is formed 
in the upper portion 12 of casing 2 to bear against bimetal member 5. This 
placement of the calibration dimple 10 is one of the means of making 
certain that the bias of the bimetal member 5 is toward the casing wall 
and, if the bimetal 5 employed is in the form 5' illustrated in FIG. 1, 
may act to remove the bend 7' from the bimetal 5' as it is employed in the 
thermostatic device 1. Though the thermostat 1 is illustrated with a 
dimple 10 formed in the top 12 of the case, it should be apparent that, if 
desired, the calibration dimple can be preformed on the casing, as by use 
of a dot of solder, or other means, formed on the inside of the casing. 
Because the bimetal 5 is prevented from bending, except for the free end 
11, beyond the fulcrum 10, upon initial current passage through the 
device, movement of movable contact 6, according to arrow A, is 
approximately the same as the movement of movable contact 7, in accordance 
with arrow B. Because of this, contact is maintained between contacts 6 
and 7, without interruption of the electrical circuit and without arcing 
between the two movable contacts. This relatively equal movement of the 
movable contacts 6 and 7 is maintained for a portion of the travel of the 
movable contacts 6 and 7, as illustrated in FIG. 3. 
This approximately equal movement of movable contacts 6 and 7 upon some 
current passage is particularly important when the thermostat of the 
present invention is employed as a circuit protector on an incandescent 
fixture. When an incandescent fixture is first illuminated, there is a 
high rush of current approximately 15 times the rated current for a few 
milliseconds. With a standard current activated thermostat, this would 
cause, particularly in the case of a snap action device, snapping of a 
bimetal arm, and, in any event, separation of the contact. In such a 
situation, since the current would immediately drop, the contact would 
reconnect, and there would be a rapid connection and disconnection of the 
contacts resulting in damage to the contacts through arcing and mechanical 
pressure, as well as a blinking of the incandescent fixture which is being 
illuminated. Because of the biasing of the second bimetallic element 
toward the wall of the casing, with only a portion of it free to move, 
this initial surge of current does not cause a separation of the contacts 
with the structure of the present invention. 
When the second bimetallic arm 5 is formed with an electrical resistance at 
least twice that of the first bimetallic arm 4, or the second bimetallic 
arm has a rate of deflection different from that of the first bimetallic 
arm, with or without a higher electrical resistance, the contacts 6 and 7 
are kept together during initial surge current with an even great effect 
than is caused by the structure. Because of the relative resistances, the 
bimetal 5 heats instantaneously much more rapidly than the bimetal 4, 
causing a more rapid movement of the free end 11 of that bimetal, so as to 
maintain it in contact with movable contact 7 of bimetal 4. This relative 
movement between the free end 11 of bimetal 5 and the movement of bimetal 
4 is also maintained on increasing ambient temperatures. 
If continued current flow through the thermostat 1 is excessive, indicative 
of some problem in the device being monitored, then the current flow 
continues to heat the bimetal arms 4 and 5 and to cause continued bending. 
Because bimetal 5 is free to bend only in section 11, even when it has a 
faster rate of heating, it is prevented from completely following bimetal 
4, so that at the calibration point, movable contact 7 separates from 
movable contact 6, as illustrated in FIG. 4. Because of the relative 
degrees of movement between these two movable contacts, the effect is 
similar to one achieved with a snap action thermostat. In particular, with 
the structure of the present invention, positive contact closing and 
opening is achieved, along with a differential between the opening and 
closing temperatures of the contacts 6 and 7. 
The action of the bimetallic arms in closing, in accordance with the 
structure of the present invention, is also important. As previously 
indicated, the relative position of the contacts 6 and 7 is shown in FIG. 
4. Obviously, when the contact is broken, as illustrated in FIG. 4, the 
thermostat begins to cool. Because of the construction of the thermostatic 
device, particularly when bimetal 5 has a higher resistance and/or a 
different rate of deflection than bimetal arm 4, bimetal arm 5 begins to 
return to its original position at a rate slower than bimetallic arm 4. 
These relative rates of return to the original position continue until 
bimetallic arm 4 "catches up" with bimetallic arm 5. This "catch up" is 
due to the different active lengths of the two bimetallic arms 4 and 5, 
and may be enhanced by the different resistances and rates of bending. 
Upon "catch up," the contacts 6 and 7 are again in a mating relationship 
and the circuit is re-established. 
The materials employed for casing 2 and insulating member 3 are standard in 
the art, and may be easily selected by those skilled in the art. The 
present invention is particularly directed to the biasing of the bimetal 5 
toward the casing wall, preferably when that bimetal has a higher 
electrical resistance than the bimetal 4, in combination with the fulcrum 
10, preventing full, free movement of bimetal 5 on current flow. As 
previously indicated, the cost of the instant device is substantially less 
than that of a snap action device and a much higher yield is obtained on 
assembly. 
While specific embodiments of the invention have been shown and described, 
this invlention should not be considered as limited to these embodiments, 
but only as limited by the appended claims.