US 3846729 A
A current limiter is provided comprising a casing housing a first and a second terminal, the first terminal being electrically connected with a thin blade-shaped bimetal. The bimetal is energized with a bias force acting lengthwise thereof so that it assumes a pair of stable positions in which its plane lies on opposite sides of a critical line. In one stable position, a contact attached to the bimetal is held in abutment against the second terminal. When the current flow through the bimetal across the first and second terminals exceeds a given value, Joule heat from the bimetal causes it to deform, inducing its snap action movement to the other stable position located on the opposite side of the critical line.
Description (OCR text may contain errors)
United States Patent [191 [111 3,846,729 Sorimachi Nov. 5, 1974 CURRENT LIMITER Primary Examiner-J. D. Miller  Inventor, Mitsuo Sorrmachr, lsezaki, Japan Assistant Examiher Fred E- Bell  Assigneez Tokyo Hoshiden Kabushiki Kaisha, Attorney, Agent, or Firm-Elliott l. Pollock lsezaki-shi, Gumma-ken, Japan  Filed: Mar. 27, 1973 57 ABSTRACT  Appl. No.: 345,397
A current limiter is provided comprising a casing housing a first and a second terminal, the first terminal being electrically connected with a thin blade-shaped bimetal. The bimetal is energized with a bias force acting lengthwise thereof so that it assumes a pair of stable positions in which its plane lies on opposite sides of a critical line. In one stable position, a contact attached to the bimetal is held in abutment against the second terminal. Wheri the current flow through the bimetal across the first and second terminals exceeds a given value, Joule heat from the bimetal causes it to deform, inducing its snap action movement to the other stable position located on the opposite side of the critical line.
9 Claims, 9 Drawing Figures BACKGROUND OF THE INVENTION The invention relates to a current limiter adapted to interrupt a circuit in response to an overcurrent.
Current limiters of the kind described heretofore used a cantilever beam comprising a bimetal secured at one end. The only available deformation of the bimetal was that caused by Joule heat alone and such deformation has been utilized to drive a separate movable mem her. This required the use of a relatively thick bimetal, which prevented the use of a thin bimetal having sufficient resistance to allow its operation at a small current level of say 1 to amperes.
Therefore, it is an object of the invention to provide a current limiter incorporating a bimetal which allows SUMMARY OF THE INVENTION In accordance with the invention, there is provided a current limiter comprising a casing housing a first and a second terminal as well as a blade-shaped bimetal. The bimetal is electrically connected with the first terminal at its one end and is energized with a bias force acting lengthwise thereof. The bias force causes the bimetal to assume either of a pair of stable positions in which its plane lies on opposite sides of a critical line. In one stable position, the bimetal is held in abutment, with a contact attached thereto, against the second terminal. When the current flow through the bimetal across the first and second terminals exceeds a given value, the bimetal is caused to deform from its position adjacent to thesecond terminal toward the other stable position, and as the deformation goes through a critical point, the bias force applied thereto is effective to cause it to move into the other stable position by a snap action, thereby interrupting the circuit connection between the first and second terminals.
. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front view of one embodiment of the current limiter according to the invention,
FIG. 2 is a top view of FIG. 1,
FIG. 3 is a left-hand elevational view of FIG. 1,
FIG. 4 is a similar view of FIG. I with the front cover removed,
FIG. 5 is a plan view of a bimetal,
FIG. 6 is a perspective view of an arcuate spring,
FIG. 7 is a longitudinal section of another embodiment of the current limiter according to the invention,
. FIG. 8 is a cross section taken along the line A--A shown in FIG. 7, and
FIG. 9 is a plan view of another bimetal.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Initially referring to FIGS. 1 to 4, there is shown a casing I moulded from synthetic resin and having a front cover la detachably mounted thereon as illustrated in FIG. 4. A thin blade-shaped bimetal 2 extends horizontally across the central area of the interior of the casing l, and is fixedly mounted in the right-hand sidewall 1b of the casing 1 to be electrically connected with a first terminal 3. In the example shown, this end of the bimetal 2 is lap welded with one end of a plateshaped terminal 3, with the weld zone being fitted in a slit 4 formed extending through the sidewall lb to permit the other end of the terminal to project externally.
The free end of the bimetal 2 is biased toward the fixed end thereof by a spring 5 which may comprise a semi-elliptic spring formed of a leaf spring. The semielliptic spring 5 engages, at its one end, a detent groove 35 formed in the left-hand side wall 10 opposite to that in which the one end of the bimetal 2 is secured and extending in a direction perpendicular to the sheet of drawing, and engages the free end of the bimetal 2 at its other end. Such engagement between the spring 5 and the bimetal 2 may be achieved, for example, between a crosswise elongate slot 6 adjacent the free end of the bimetal 2 (FIG. 5) and an integral tab 7 (FIG. 6) extending centrally from that end of the spring 5 which engages the bimetal 2. A shoulder 8 defined between the tab 7 and the body of the spring bears against the surface of the bimetal 2.
The semi-elliptic spring 5 energizes the bimetal 2 with a bias force which tends to compress it lengthwise, so that it will be seen that the free end of the bimetal 2 remains unstable when it lies on a line, referred to herein as critical line, joining the point of abutment of the semi-elliptic spring 5 against the left-hand wall 1e and the point of anchorage of the bimetal 2, but that it assumes a stable position on either side of the critical line 9 as viewed normal to the plane of the bimetal. A second terminal 10 is arranged to bear against the free end of the bimetal 2 when the latter is in one of its stable positions. In the example shown, the second terminal I0 is located adjacent the bottom wall 1d of the easing 1, and a plate-shaped second terminal 10 is inserted through a slit 1] formed in the sidewall lb adjacent the boundary with the bottom plate 1d, the second terminal being inserted from the exterior so as to extend it externally on the same side as the first terminal 3. Within the casing 1, the second terminal 10 is bent at an angle extending toward the free end of the bimetal 2 and has its free end 10a bent reversely to run substantially parallel to the bottom plate 1d. The free end of the bimetal 2 has a contact 12 secured to its side nearer the bottom plate 1d, and by virture of the bias imparted thereto by the spring 5, this contact is urged against the free end 10a of the second terminal 10. The terminal I0 is secured in position by being held between a ramp 13 formed on the bottom plate 1d and a projection 14 formed on the right sidewall 1b.
As soon as the joint between the bimetal 2 and the spring 5 moves even slightly to the side of the critical line 9 which is nearer the top wall If of the casing l, the free end of the bimetal 2 is immediately urged toward the position adjacent the top wall If as shown in dotted line in FIG. 4, where it becomes stabilized, whereby the contact 12 is removed from the second terminal 10. To reset the bimetal from this position to the original position to reestablish the conduction between the first and second terminals 3, 10, a reset button 15 is provided. A through opening 16 is formed in the top wall 1 f oppositely to the free end of the bimetal 2, and a rod-shaped reset button 15 is placed therethrough. The opening 16 has an enlarged portion 16a of increased diameter intermediate its ends, and the button 15 has an integral flange 17 which is received in the enlarged portion 16a. A coil spring 18 is disposed around the button 15 and abuts against the lower face of the flange as well as the end of the enlarged portion 160 to urge the button 15 upwardly or in a direction away from the casing 1. When the button 15 is pushed inward against the resilience of the .spring 18, its inner end is urged against the bimetal 2 to cause it to be depressed below the critical line 9.
To provide the convenience of actuating awarning circuit such as a warning lamp or buzzer when the bimetal 2 moves away from the'second terminal 10, a
third terminal 19 is inserted into the casing 1 through a slit 20 formed in the sidewall adjacent to its boundary with the top wall 1 f, and has its free end positioned so as to be opposite to the free end of the bimetal 2 on the opposite side from the terminal 10. As the bimetal 2 deflects toward the top wall 1 f beyond the critical line 9, a contact 21 carried by the free end of the bimetal 2 is urged into abutment against the third terminal 19. A warning circuit may, for example, be connected across the terminals 3 and 19. As will be noted in FIG. 2, a pair'of brackets 22 and 23 are provided to extend laterally from the casing 1 in order to permit its attachment to.an equipment with which the current limiter is associated.
In operation, a circuit to be protected is connected across the terminals 3 and 10. While the current flow across these terminals is below a given value, the bimetal 2 remains on that side of the critical line 9 adjacent to the bottom wall 1d even though the bimetal is subject to Joule heat. However, when the current flow across the terminals 3 and 10. exceeds the given value, Joule heat produced in the bimetal 2 becomes sufficient to cause it deform in such a manner that it moves, by'snap action, beyond the critical line 9 toward the top wall If, whereby the contact 12 moves away'from the terminal 10 to interrupt the circuit across the terminals 3 and 10. At the same time, the circuit across the terminals 3 and 19 is completed. Subsequently, the bimetal 2 can be restored to its stable position abutting against the terminal 10 by pushing the reset button inward.
Thusit will be appreciated that in the current limiter of the invention, a deformation of the bimetal 2 is not used to drive another movable member, but the bimetal 2 is energized with a bias force by spring 5 to cause it to assume either of two stable positions into which it switches upon movement across the critical line 9 and themovement of the bimetal itself is directly used to interrupt its contact with the second terminal 10, so that the bias force to the bimetal 2 may below and a thin bimetal, for example, of a thickness on the order of 0.15 mm, may be used to increase its resistance, whereby the current limiter can be made to operate at a small current on the order of l to 5 amperes. Thebias force afforded by the spring 5 urges the contact 12 on the bimetal 2 into abutment with the terminal 10, thereby normally maintaining a good electrical contact therebetween. It will also be noted that the current limiter is constructed with a reduced number of components and in a simple configuration, and the assembly is greatly facilitated in that the terminals 3, l0 and 19 can be fitted into therespective slits 4, ll and from the front side when the front cover la is removed and the semi-elliptic spring 5 is simply hooked between the bimetal 2 and the casing 1 without the need for set screws or the like, thus allowing the current limiter to be provided inexpensively and as a compact structure of reduced overall size.
While a compressive bias force is applied to the bimetal 2 in the above embodiment, a lengthening bias force may be applied to the bimetal 2 as well, as illustrated in FIGS. 7 to 9. In this embodiment, the casing 1 comprises a base 23 of an insulating material which forms the bottom wall, and a metallic cover 24 which covers various components mounted on the base 23. The first and second stationary terminals 3 and 10 are mounted on the opposite ends of the base 23 and extend therethrough. The blade-shaped bimetal 2 extends across these terminals within the casing 1 and has a slot 6 at its one end into which is inserted the inner'end of the first stationary terminal 3. The bimetal 2 has an opening 25 formed intermediate its ends, through which extends a column 26 formed integrally with the base 23 and serving as a fixture. In its surface facing the second terminal 10, the column 26 is formed with a detent groove 35 which extends substantially parallel to the plane of the bimetal 2. The semi-elliptic spring 5 engages the detent groove 35 at its one end and engages an edge 27 of the opening 25 in the second stationary terminal 10 at its other end, thereby applying a tension to the bimetal 2 which tends to lengthen it toward the second terminal 10.
It will be understood that the stable positions of the bimetal 2 will be on the opposite sides of the point of engagement between the semi-elliptic spring 5 and the column 26, that is, the detent groove 35, and more specifically when the plane of the bimetal is located on the side nearer the base 23 and on the side nearer the top plate 24a of the cover 24, respectively, with respect to this groove 35. The inner end of the second stationary terminal 10 is bent as shown to provide the contact 10a, and the free end of the bimetal 2 is resiliently urged against the contact 10a with its contact 12 when the bimetal is located so that its plane lies on the side nearer the base 23 with respect to the detent groove 35. The free end of the column 26 is formed so as to extend toward the second stationary terminal 10, thereby providing a stop 26a. As the bimetal 2 deflects beyond the detent groove 35 toward the top plate 24a, it ceases its movement by abutting against the stop 26a and the contact 12 moves away from the second terminal 10. To permit pivotal motion of the bimetal 2 between the abovementioned two positions, a detent recess 28 is formed in the surface of the first terminal 3 removed from the second terminal 10 and is engaged by the bimetal 2, thus preventing its dislocation.
The reset button 15 extends through the top plate 24a from the exterior at a position adjacent the column 26 on the side nearer the first terminal 3 and has its inner end formed in the configuration of an inverted T with the head of the T extending crosswise of the base 23. In an area opposing the reset button 15, the bimetal 2 has a reduced width, and a pair of helical springs 18a and 18b are disposed between the inner end of the reset button 15 and the base 23 on opposite sides of the bimetal portion having a reduced width. The opposite ends of the springs 18a and 1811 are received in recesses 29a, 29b formed in the base 23 and recesses 30a, 30b formed in the inner ends of the button 15, respectively. Thus the reset button 15 is normally biased outwardly through the top plate 24a.
In operation, an electrical circuit to be protected is connected across the first and second terminals 3, with the bimetal 2 being in its normal position abutting against the second terminal 10. When the current flow across the terminals 3 and 10 exceeds a rating of the associated circuit element, Joule heat produced by the current flow through the bimetal 2 causes a sufficient deformation of the bimetal 2 to cause it to assume the position shown in dotted lines in FlG. 7, whereby the contact 12 moves away from the second terminal 10 to interrupt the current flow across the terminals 3 and 10. It will be appreciated that the semi-elliptic spring 5 functions to cause the bimetal 2 to pivotally move rapidly, by snap action, into the other position in which its plane is located nearer the top plate 24a with respect to the detent groove 35. To restore the bimetal from this interruption position, the reset button 15 may be depressed against the resilience of the springs 18a and 18b, whereby the bimetal 2 can be urged toward the base 23 and returned to its original position. The effects described in connection with the first embodiment can similarly be achieved with the second embodiment. By comparison, the bias force from the spring 5 functions to flex the plane of bimetal 2 in the first embodiment to increase the sensitivity, while in the second embodiment, securing one end of the bimetal 2 will cause a reduced sensitivity. This is avoided by the first embodiment while permitting to secure one end of the bimetal for ease of assembly.
A current limiter can be constructed in accordance 'with the first and second embodiments which satisfies the performance requirements for an overcurrent protector as provided in U.L. Standard 492 of the United States. Specifically, this demands a circuit interruption within two minutes'below 175 percent of a rating for a contact pressure of 20 grams, 2.0 ampere rating and at an ambient temperature of 25C 1*: 3C. A current limiter constructed in accordance with the invention provided a circuit interruption within 10 to 20 seconds, and the required operation was maintained up to an ambient temperature of 70C.
It will be noted that the spring used to energize the bimetal 2 with the required bias force is not limited to a semi-elliptic spring, but other springs may be used as well, and that the bimetal 2 may be secured at its end adjacent to the first terminal 3 in the second embodiment.
Having described the invention, what is claimed is:
1. A current limiter comprising a first and a second terminal mounted in a casing, a thin blade-shaped bimetal electrically connected with the first terminal within the casing and incapable of providing a contact pressure by itself, bias means for energizing the bimetal with a bias force acting lengthwise thereof to cause it to assume either of two stable positions in which the plane of the bimetal is respectively located on opposite sides of a critical line, said bias means serving to provide a contact pressure, a contact mounted on the surface of the bimetal and adapted to be urged against the second terminal in one of the stable positions and to move away from the second terminal in the other stable position, said bimetal when located in said other stable position being incapable of returning to said one stable position by itself, and reset means for selectively returning the bimetal from said other stable position to said one stable position.
2. A current limiter according to claim 1 in which one end of the blade-shaped bimetal is secured to the first terminal, said bias means supplying a bias force which acts from the other end of the blade-shaped bimetal toward the secured end thereof.
3. A current limiter according to claim 2 in which the bias means comprises a semi-elliptic spring of plate-like configuration engaging the casing at its one end and engaging the free end of the bimetal at its other end, said critical line being defined by a line extending from the point of engagement of the spring with the casing and to the secured end of the bimetal.
4. A current limiter according to claim 1 in which the bimetal is hooked onto the first terminal, the bias means being operative to exert a bias force upon the bimetal which maintains the hooked engagement.
5. A current limiter according to claim 4 in which one end of the bimetal is formed with a slot through which the first terminal extends, the bimetal being formed with an opening through which extends a column integral with the casing, said bias means comprising a semi-elliptic spring extending resiliently between the column and an edge of the opening located nearer the contact on the bimetal.
6. A current limiter according to claim 1 wherein said reset means comprises a manually operable reset button extending through the casing wall and adapted to be advanced to and retracted from the bimetal, and second bias means for normally urging the reset button away from the bimetal, whereby the reset button can be depressed against the resilience of the second bias means to urge the bimetal toward a position of contact with the second terminal.
7. A current limiter according to claim 1, further including a third terminal mounted in the casing for abutment with the contact on the bimetal as it moves away from the second terminal.
8. A current limiter according to claim 1, further including a stop which is engaged by the bimetal moving away from the second terminal.
9. The current limiter of claim 1 wherein the bimetal has a'thickness of substantially 0.15 mm.