US2716173A - Electric switches - Google Patents

Description

Aug. 23, 1955 w. M. RUSSELL ELECTRIC SWITCHES 5 Sheets-Sheet l Filed April 28, 1952 EN wv o om/imw uw mfm um Bygw Aug- 23, 1955 W. M. RUSSELL 2,716,173

ELECTRIC SWITCHES Filed April 28. 1952 5 Sheets-Sheet 2 Wnlham MoRms Russe y Attorney Aug. 23, 1955 W. M. RUSSELL 2,716,173

ELECTRIC SWITCHES Filed April 28, 1952 3 sheets-sheet 5 /nvent'or \^/\\\\m'\ Monms RUSSEH a MM Yg/a. y Attorney United States Patent O ELECTRIC SWITCHES William Morris Russell, Bromley, England, assignor, by mesue assignments, to General Mills, Inc., Minneapolis, Minn., a corporation of Delaware Application April 28, 1952, Serial No. 284,698

10 Claims. (Cl. 20G-138) This invention relates to thermally-operated electric switches for the thermostatic control of electrically-heated devices such as smoothing irons.

A known form of switch for this purpose comprises a contact carried by a leaf spring, whose position is controlled by a bimetal strip or other temperature sensitive member, and a second contact which is also carried by a leaf spring in such a manner as to urge it against the rst contact. The bimetal strip is arranged adjacent the heated sole plate of the iron, and as its temperature rises, it allows the rst contact to move in a direction away from the second contact, which however follows its movement and remains in engagement with it. The movement of the leaf spring carrying the second contact is limited by a stop, so that when the temperature reaches a certain value the first Contact moves away from the second, thus breaking the heating circuit. As the bimetal strip cools, the contacts meet again and the procedure is repeated. ln such a slow-break switch the gap between the contacts is small, and the switch consequently causes considerable radio-interference, owing to the high frequency of operation and the relatively long interference pulse produced at each operation.

The slow-break switch described can be modied to constitute a snap-action switch by the use of an overcentre or magnetic action, whereby there is provided a larger gap and a consequent lower frequency of operation, quite large temperature differentials being acceptable. Although the duration of the interference pulse is shorter, its amplitude is greater than in the slow break switch. This necessitates a large capacity condenser to suppress the radio-interference, whilst the larger gap requires a larger capacity to prevent arcing when operating on direct current.

The present invention provides an improved switch having a slow break but a large gap between the contacts, so that the interference pulses produced have both a small amplitude and a low frequency of repetition.

in a thermally-operated electric switch in accordance with the invention the separation and engagement of a pair of contacts is controlled by two temperature responsive devices, one device being arranged to effect initial separation of the contacts as its temperature increases, and the other device, the heating of which is eected by the current controlled by the contacts, being arranged to increase the gap between the contacts when this device cools due to said initial separation of the contacts.

it is preferred that said one device should be a bimetal strip, and that said other device should be a so-called hot wire. The hot wire is an elongated member having an electrical resistance and a coeicient of thermal eXpansion such that when the current controlled by the contacts passes through it, its length increases to an appreciable extent. The term wire is not intended to limit the form of the device which may, for instance, be a strip or tape.

ln one form of the invention, the thermally-operated electric switch comprises a pair of co-operating contacts, each biased for movement in the same direction, a bimetal iff 2,716,173 Patented Aug. 23, 1955 ice strip adapted to be indirectly heated by the current controlled by the contacts, which strip as its temperature rises permits movement of said contacts, a stop member arranged to arrest the movement of a rst one of said contacts at a predetermined temperature whereby initial separation of the contacts is effected, and a wire directly heated by said current and attached to the second contact, whereby thc contraction of the wire on cooling after said initial separation, causes further movement of said secon contact away from the rst contact.

Another form of switch in accordance with the invention comprises a pair of co-operating contacts, a bimetal strip, and a wire directly heated by the current controlled by the contacts and attached at one end to the bimetal strip and at the other to one of said contacts, said one Contact being mounted on the free end of a leaf spring anchored at its other end and tending to maintain the contacts in engagement. In this form of switch, the bimetal strip is heated directly and/or indirectly by said current whereby it effects movement of said one contact away from the other contact at a predetermined temperature, after which the contraction of the wire on cooling, effects further movement of said one contact to increase the gap between the contacts.

A specific embodiment of each of the two forms of the invention described above in general terms will now be described in detail and with reference to the accompanying drawings. These two particular switches are for the thermostatic control of electric smoothing irons; it will be appreciated that these embodiments are examples only and are described in order that the invention may be more fully and clearly understood.

In the drawings:

Figure l is a longitudinal sectional view of the sole plate of an electric smoothing iron fitted with a thermallyoperated electric switch, the switch being one embodiment of the invention, with the casing of the iron shown in chain-dotted lines;

Figure 2 is a plan view of the sole plate and switch of Figure 1, the casing of the iron being removed;

Figure 3 is a fragmentary view taken on the line III-III of Figure 2 and showing the arrangement of the hot wire device;

Figure 4 is a sectional view taken on the line IV-IV of Figure l;

Figure 5 is a circuit diagram for a smoothing iron tted with the switch shown in Figures l to 3;

Figures 6, 7 and 8 are underneath, side and plan views respectively of a thermally-operated switch which is another embodiment of the invention; and

Figure 9 is a diagram showing the circuit of an electric smoothing iron fitted with the switch shown in Figures 6 to 8.

In Figure l, there is shown an electric smoothing iron 10, which includes a sole plate 11. A heating element 12 comprising resistance wire between two mica sheets, rests on the sole plate. A mounting plate 14 for the thermally-operated switch is separated from the heating element by an asbestos sheet 13. The parts 12, 13 and 14 are secured to the sole plate 11 by means of screws itted with nuts. There are two such screw and nut arrangements 15 at the rear'of the iron (see also Figure 2) and one 16 towards the nose of the iron.

In a depression in the sole plate 11, there is a bimetal lstrip 17 with one end anchored at 18 to the sole plate.

The strip 17 is so arranged that when its temperature increases its free end moves upwardly from the position shown in Figure l. The free end of the bimetal strip is mechanically linked to one of the switch contacts so that, as explained below, its upward movement permits the contacts to open when the strip reaches a predetermined temperature.

The dome-shaped switch contacts 19, are riveted to the free ends of metal leaf springs 21, 22 respectively, the other ends of the springs being fastened to an insulating plate 23. This plate is secured to the mounting plate 14 by means of screwed rods 24 (Figure 2) welded to plate 14 and provided with nuts 25. The insulating plate 23 is spaced from plate 14 by sleeves 26 (Figure 1) on the rods 24.

Two Screws 27 are employed to secure the leaf springs to the insulating plate. Each screw 27 carries a metal washer 28 and a mica washer 29, it then passes through holes in a terminal strip 30 and the leaf spring 22. Mica washers fit into the holes in the strip 30 and spring 22, so as to electrically insulate these members from the screw. The screw then passes through the plate 23 and spring 21 from which it is similarly insulated, then through a mica strip 31 which insulates spring 21 from a further leaf spring 32. Finally the screw is received in a screw-threaded hole in a terminal bracket 33. It should be noted that terminal strip 30 is in electrical contact with spring 22, and terminal bracket 33 with spring 32, these springs being insulated from each other and from spring 21.

The insulating plate 23 shown in the drawing as a single member, may in fact comprise two laminated mica plates strengthened by a metal plate sandwiched between them. The holes in the metal plate for screws 27 will have to be sufficiently large to prevent electrical contact between the screws and plate.

A cross-piece 34 (seen more clearly in Figures 2 and 3) is secured to the end of the leaf spring 32, part of the cross-piece being bent up to leave a small projection at either end. A nickel-chrome tape 35 extends between these projections and is secured at its middle to a projecting tag 21a on the end of the leaf spring 21. This tape 35 constitutes the hot wire device and it will be clear that electrical connection is made from terminal bracket 33, through leaf spring 32, cross-piece 34, the two halves of tape 35 in parallel, and spring 21 to contact 19.

The leaf spring 21 is bent in the manner shown in Figure 1 so that at one point it approaches the spring 32 but is insulated therefrom by mica washers 36. The spring 21 is so bent that it is normally held under stress by the tape 35 and the contact 19 therefore tends to move downwardly away from spring 32.

A mechanical linkage from bimetal strip 17 to leaf spring 32 is provided by a rod 37 flattened and bent at its lower end so as to hook on to the free end of the bimetal strip through a hole therein. The rod, the upper part of which is screw-threaded, passes through apertures in parts 12, 13 and 14 and through holes in springs 22, 21 and 32, without touching any of these members. A anged sleeve 38 of insulating material fits in the holes in the upper springs, and a nut 39 is screwed on to the rod. The springs 22 and 32 bend upwardly when free so that they are held under stress by the mechanical linkage with the bimetal strip. Thus, when the bimetal strip bends upwardly due to increase in temperature, the springs 22 and 32 can both move up and the contacts 19 and 2t) also move up but remain engaged.

Opening of the contacts is brought about by preventing further upward movement of spring 22 and hence of contact 20 when a predetermined temperature is reached. To this end there is provided an insulating roller 40 freely rotatable on a pin 41 secured to the bent down end of spring 22. At the predetermined temperature the roller 40 meets a stop provided by a face cam 42. c

The cam is fixed on a shaft 43 mounted in a bearing secured to a bridge, the construction of which will be clear from the drawings (see more particularly Figure 4). By turning shaft 43 so that the roller 40 meets a different part of the cam 42, the position of the bimetal 4 strip and hence the predetermined temperature at which the contacts open will be altered.

Two further terminal brackets 44 and 45 (Figure 2) are secured by eyelets to the insulating plate 23. A source of electricity is connected to the terminal brackets 33 and 44 through a plug and socket connector or in some other suitable manner.

In the circuit diagram of Figure 5, the terminal strip 30, terminal brackets 33, 44 and 45, heating element 12, nickel-chrome tape (hot wire) 35, and contacts 19, 20 are illustrated diagrammatically. The bimetal strip is not shown since there are no electrical connections to it. Certain other parts appear in the circuit diagram which are not shown in Figures l to 3; these are a condenser 46 for suppressing radio-interference, a condenser 47 for suppressing arcing across contacts 19, 20 (provided only when the iron is operated from a D. C. source), and a shunt 48 for limiting the current through a pilot lamp 49.

The manner in which the switch operates to give thermostatic control of the iron is as follows. The cam 42 having been set by means of knob 50 fast on shaft 43 to the desired temperature for the material to be ironed, and the iron switched on, current ows from terminal 33, through hot wire 35, contacts 19, 20, heating element 12 and shunt 4S and lamp 49 to terminal 44. The length of the hot wire will increase and it becomes slack. The bimetal strip will bend upwardly so that contacts 19 and 20 move upwardly together until engagement of roller 41 with cam 42 prevents further movement of contact 20. Contact 19 however will continue to move slowly up after the slack in the hot wire has been taken up, so that the circuit through hot wire 35 and, of course, the heating element 12 is broken. The hot wire will cool quickly and therefore will pull the end of spring 21 up so that contact 19 continues to move away from contact 20 to produce a comparatively large gap before there is any appreciable cooling of bimetal strip 17. The latter will cool slowly and as it does so will pull contact 19 down until it meets contact 20 again so completing the circuit. The process will then be repeated.

To ensure that the required thermostatic control is obtained the position of nut 39 on rod 37 can be adjusted as necessary during manufacture.

Another embodiment of the invention is shown in Figures 6 to 8: this embodiment also is a switch for incorporation in an electric smoothing iron for thermostatic control.

ln Figure 7, there is shown at part of the sole plate of an iron with a sheet 61 of electrically insulating material on its upper surface. On top of this sheet there is arranged an insulating block 62 on which the parts of the switch are mounted. The bottom surface of the block 62 is provided with a shallow crossshaped groove into which one end of a bimetal strip 63 fits as will be seen in Figure 6. The block 62 is clamped rmly in position by a bridge member (not shown) arranged across the reduced portion 62a of the block and fastened by screws to the sole plate. This clamping arrangement also serves to hold securely between the block 62 and insulating sheet 61, one end of the bimetal strip 63. The other and free end of the strip 63 extends beyond the block 62. The strip is bent so that normally it is in the position of Figure 7, a sloping channel being provided in block 62 to accommodate the bent portion of the strip. The strip is arranged with the two metals so disposed that with increasing temperature, the free end of the strip moves downwardly.

The hot wire device is a nickel-chrome tape 64 with its ends secured to the free end of the bimetal strip 63 and the free end of a leaf spring 65. This end of the leaf spring carries a contact 66 and is welded at its other end to a rigid L-shaped terminal 67. One arm of this terminal is secured by eyelets 68 to the block 62 and is cranked so that the part carrying the leaf spring is spaced above the block.

A second `contact 69 is mounted on a rigid bar 70 so as to co-operate with the contact 66. A screw 71 passes through a hole in one end of the bar 70; the screw is threaded through the block 62 and a second L-shaped terminal 72 secured by eyelet 73 to the block. A coil spring 74 and washer 75 are disposed on the screw between the terminal 72 and bar 70 which is held on the screw by a nut 76. The nut is provided with a screwdriver slot at the top and with teeth at the bottom which engage a portion of the bar 70 of inverted V-shape. This arrangement provides a convenient way of automatically locking the nut in a desired position on the screw. Turning of the nut enables the position of bar 70 and hence of contact 69 to be adjusted during manufacture. Electrical connection between the bar 70 and terminal 72 is ensured by the flexible metal strip 77.

The other end of bar 70 is mounted for sliding movement of a post 78, the hole in the bar being elongated (see Figure 8). The bar is cranked adjacent this end so as to provide a vertical portion which carries a pin 79 on which there is a freely rotatable roller 80 of insulating material. This roller engages a face cam 81 fixed on a shaft 82; this shaft is mounted on a bridge member 83 having feet which are secured to the block 62 by eyeletting. The roller 80 is held against the cam 81 by means of a coil spring 84 arranged on the post 78 between block 62 and bar 70.

A third terminal 85 is secured to the block 62 by means of an eyelet 86 and is electrically-connected to the fixed end of bimetal strip by a thin metal strip 87.

The electrical connections of an iron fitted with this form of the switch will now be described with particular reference to Figure 9. The following members of the switch which appear in Figures 6 to 8 form part of the electrical circuit and are shown diagrammatically in Fig-l ure 9: bimetal strip 63, hot wire 64, contacts 66, 69, and terminals 67, 72 and 85. Certain parts of the complete iron which are not shown in Figures 6 to 8, appear in the circuit diagram. These parts include the heating element 88 which is connected between terminal 72 and a further terminal 89, a pilot lamp 90 with its shunt 91 connected between terminals 67 and 85, and a condenser 92 for suppressing radio-interference connected between terminals 8S and 89. When the iron is operating on direct current a further condenser 93 connected between terminals 67 and 72 is necessary to suppressing arcing at the contacts 66, 69. The shunt 91 is dispensed with in irons for a supply voltage such that the voltage drop across the hot wire and bimetal strip alone is sufficient for lamp 90. The source of electricity for the iron is connected by any suitable means to the terminals 8S and 89.

The switch is set for a particular temperature by turning the shaft 82 so that via cam 83 and roller 80 the position of contact 69 is adjusted. The leaf spring 65 is bent so that it is normally under stress to hold contact 66 in engagement with contact 69. As appears from Figure 7, the hot wire or tape 64 is normally slack, and the extent of this slackness will depend on the setting of cam 81. The bimetal strip 63 is heated by the current flowing through it and also by the adjacent heated sole plate, and as its temperature increases it will take up the slack in the hot wire. At the same time the length of the hot wire will be increasing. When the upper value of the required temperature differential has been achieved, contact 66 will be pulled away from contact 69 so as to break the circuit through the heating element and the hot wire. The hot wire cools quickly, thereby causing the gap between the contacts to increase. The cooling of the bimetal strip is slower but as it cools it will permit contact 66 to move upwardly under the action of leaf spring 65. In due course and at the lower value of the temperature differential, the contacts will close again so as to complete the circuit and the procedure Will be repeated.

The switch has the characteristic, desirable in an electric iron, that the differential between the temperatures at which the contacts are opened and closed is greater for high operating temperatures than for low operating temperatures. This is due to the geometry of the system formed by the bimetal strip 63, hot wire 64 and leaf spring 65 and to the relatively increased effectiveness at low temperatures of the self-heatingof the system.

The presence of electrical insulation between the sole plate 60 and the bimetal strip 63 provided by sheet 61 necessarily involves some degree of thermal insulation between these parts. However, the self-heating effect of the system due to the bimetal strip being included in the circuit is suiiicient to overcome the temperature lag which would otherwise be caused by the thermal insulation, so that hunting and overshoot of the switch are avoided.

In both embodiments described above, since there iS only a small amount of energy liberated at the contacts, the capacity ofthe condenser which must be connected across the contacts to suppress the arc when direct current is used is no greater than in the case of the known slow break switches, usually about 0.01 mfd., while the capacity of the condenser which must be connected across the heater circuit to reduce the radio frequency pulse to below an acceptable limit is about half that required in the case of the known quick break switches.

l claim:

l. A thermally-operated electric switch comprising a pair of co-operating contacts, and a bimetal strip 0peratively connected to one of said contacts by means of a wire which is directly heated by the current controlled by the contacts, the bimetal strip being also heated by said current, whereby it effects movement of said one contact away from the other contact at a predetermined temperature, after which the contraction of the wire on cooling, effects further movement of said one contact to increase the gap between the contacts.

2. A thermally-operated electric switch comprising a pair of cooperating contacts controlling an electric current, first biasing means for moving a rst one of said contacts, second biasing means for moving the second contact in the same direction as said first contact, a temperature responsive device the heating of which is effected by the current controlled by said contacts, means connecting said temperature responsive device to said first and second biasing means and so arranged that with increasing temperature said temperature responsive device permits Asaid first and second biasing means to move said contacts, a stop member arranged to arrest at a predetermined temperature movement of said first contact only, whereby initial separation of said contacts is eected, anchoring means arranged so as to normally move with said second biasing means, and a heat sensitive wire directly heated by said current and attached to said anchoring means and to said second Contact, the contraction of said wire on cooling due to the cessation of said current after said initial separation, causing further movement of said second Contact away from said first contact.

3. A switch in accordance with claim 2 wherein said contacts are secured to the free ends of leaf springs anchored and insulated at their other ends, and said wire extends from the free end of a third similarly mounted leaf spring to said second Contact, the bimetal strip being anchored at one end and operatively connected to the third leaf spring at the other end.

4. A switch in accordance with claim 3 wherein the free end of said third leaf spring is in the form of a cross-piece, the wire being anchored at its ends to the ends of said cross-piece so as to form a shallow loop, the middle of which is attached to said second contact.

5. A switch in accordance with claim 4 wherein the free end of the leaf spring carrying said first contact is provided with an insulating roller adapted to engage a cam which is adjustable to vary said predetermined temperature.

6. A switch in accordance with claim l wherein said wire is attached at one end to said birnetal strip and at the other to said one contact, which contact is mounted on the free end of a leaf spring anchored at its other end, and tending to maintain said contacts in engagement.

7. A switch in accordance with claim 6, wherein said wire is arranged at such an angle to said leaf spring that a small contraction of said wire produces a relatively large movement of said one contact.

8. A switch in accordance with clairn 7 wherein the other of said contacts is secured to a rigid strip intermediate its ends, one end of said rigid strip being adjustably fixed and the other end being provided with an insulating roller held by means of a spring in engagement with a cam, said cam being adjustable to vary said predetermined temperature.

9. A thermally-operated electric switch comprising a pair of cooperating contacts controlling an electric current, a temperature responsive device heated by said current, and a heat sensitive wire which is directly heated by said current and has one end attached to said temperature responsive device and the other end to one of said contacts, said temperature responsive device in response to increasing temperature effecting movement of said one contact away from the other contact and thereby interrupting said current at a predetermined temperature, the subsequent contraction of said wire due to its cooling consequent on the interruption of said current causing further movement of said one contact to increase the gap between said contacts.

l0. A thermally-operated electric switch comprising a pair of cooperating contacts controlling an electric current, a tirst temperature responsive device comprising a birnetal strip arranged to effect initial separation of said contacts in response to increasing temperature, and a second temperature sensitive device which, prior to said initial separation, is expanded through heating by the current controlled by said contacts, but which cools and consequently contracts after such separation due to the cessation of said current, and thereby causes the gap between said contacts to increase as it cools.

References Cited in the le of this patent UNITED STATES PATENTS 847,981 Brown Mar. 19, 1907

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