US2280550A - Thermal switch - Google Patents

Description

W. C. SMITLEY THERMAL SWITCH April 21, 1942.

Filed Aug. 12, 1939 His Attovr-mey.

- starting time intervals for the Patented Apr. 21, 19.42

, um rao. STATES-.1 PATENT OFFICE THERMAL swrrcn Wayne 0. -Smitley, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation-of New York Application August 12, 1939, Serial No. 289,897 A z'ol ims. (01. 250-275) My invention relates in gcneral'tothermal switches, and in particular to switches employing a bimetallic element to make and break the "interengagement of the switch contacts. Still more particularly, my 'invention relates to thermal switches of the glow discharge type in. which the heat for actuating the bimetallic element is generated by a glow discharge in an ionizable -medium. Such thermal switches are particularly applicable for use in connection with the starting and operating of electric discharge devices such as, for instance, fluorescent lamps of the well-known type now in commercial use.

Certain types of electric discharge devices, such as the aforementioned fluorescent lamps, employ a pair of thermionic electrodes which require a certainamount of preheating before.

the arc will strike therebetween. For this purpose, it is customary to interconnect the two electrodes in series across the line by means of an auxiliary switch, one form of which is a thermal switch of the glow discharge type referred to above. After the thermionic electrodes of the discharge device have been preheated to dis- A feature of my invention is the mounting of a U-shaped bimetallic strip around a wire electrode so as to surround the same, the said bimetallic strip and electrode being mounted within a small hermetically sealed tubular envelope so as to extend transversely thereof. A further feature of my invention is the provision of a wire switch contact secured to the free end of the bimetallic strip and so shaped as to provide a cam surface to thereby insure the return of the said switch contact to its normal position following the sealing-in of the switch envelope.

Further objects and advantages of my invention will appear from the following description of a species thereof and from the accompanying drawing in which:

Fig. 1 is a top view of a thermal switch comprising my invention with the switch envelope shown in section; Fig. 2 is an elevation, partly in section, of the switch shown in Fig. 1; Fig. 3 is an elevation, partly in section, taken at right angles to Fig. 2; Fig. 4 is a fragmentary perspective view of the switch shown in Figs. 1 to 3; and Fig. 5 is a diagrammatic illustration of the circuit employed for starting an electric discharge device having thermionic electrodes with the thermal switch comprising my invention.

Referring to the drawing, the thermal switch according to the invention comprises asmall hermetically sealed bulb or envelope l0 preferably of glass and tubular in shape. Secured to Then, too, in some of the fixtures proposed for the mounting of fluorescent lamps, there is a limitation on the size of the auxiliary switch so that it will not interfere with the insertion of a thermal switch of the glow discharge type which is of exceptionally small size and of relatively long life.

Still another object of my'invention is to prooperating electric discharge devices having thermionic electrodes, said switch having relatively short and substantiallyequal startingand rearc discharge in the discharge device. x

7 vide a thermal auxiliary switch for starting and one end of the envelope III in any suitable manner, preferably by cement, is a base II of the bayonet type, and comprising a metallic shell l2 and a bottom center contact l3 secured together by suitable insulating material l4. Obviously, if desired, the base I I may be of the screw-threaded or any other suitable type. A pair of leading-in wires l5, l6 are electrically connected to the base shell l2 and bottom contact I3 respectively, and extend into the glass envelope l0 through a relatively short stem I! at the base end of the said envelope. To permit the use ofan envelope In of exceptionally short length, the said stem [1 is preferably of the type shown and described in United States Patent No. 2,128,173, issued August 23, 1938, to Arthur J. White and assigned to the assignee of the present application.

The portion of the leading-in wire l6 extending interiorly of the envelope I0 is preferably made of molybdenum wire approximately 20 other leading-in wire I5. The straight wire portion I8 extends to a point closely adiacent the wall of the envelope I and serves as one of the switch contacts, as well as an electrode during the time the switch functions as a glow discharge device.

A U-shaped bimetallic element or strip I9 is secured at one end to the other leading-in wire IS in such a manneras to extend transversely of the envelope III in substantially the same direction and the same transverse plane as the wire contact I8, so that the bimetallic element in effect partly surrounds the said wire contact. The U-shaped bimetallic strip I9 is disposed with the open end of the U facing in the same direction as the wire contact I8 extends, so that the ends of the bimetallic strip are more or less adjacent the free end of the said wire contact. The free leg portion of the bimetallic strip I8 is extended a short distance beyond the other or main leg portion 2|, as shown particularly in Figs. 1 and 2. The bimetallic strip I9 is preferably so arranged that the free leg portion 20 separates from the main leg portion 2I under the influence of heat.

A switch contact 22, preferably consisting of a length of molybdenum wire, is secured to the end of the free leg portion 20 of the U-shaped bimetallic strip I9 so as to extend transversely across the legs of the- U and either overlie or underlie the straight wire contact I8. This wire contact 22, beyond the straight wire contact I8, is bent or offset towards the said straight wire contact as indicated at 23, the plane of the bend or offset extending substantially transversely of the straight wire contact I8, The extent of the offset 23 is preferably just sufficient to insure positive engagement with the straight wire contact I8 as the wire contact 22 is moved transversely across the contact I8 by the movement of the bimetallic element I9 when heated. The offset or bend 23 is preferably in the form of either an upright or inverted V (depending upon whether the contact 22 overlies or underlies the straight wire contact I8) so that the legs of the V act as a cam which will enable the offset to ride or spring over the contact I8. This is of particular importance during the manufacture of the switch since it permits the contact 22 to return to its normal position following the sealing-in operation, during which operation the V-shaped offset or bend 23 is drawn across and over onto the opposite side of the contact I8 as a result of the excessive heating of the bimetal I9. 'While I have indicated that the offset or bend 23 is preferably of V-shape, it is obvious that any other form of offset may be employed which will provide the desired result. v

The heating of the bimetallic element I9 is effected by a glow discharge within the device. For this purpose, the envelope I0 is filled with a suitable ionizable gas or mixture of gases at a suitable pressure, while the bimetallic element I8 and the switch contact I8 are coated with a suitable activating material. The gas filling preferably consists of neon containing approximately 0.5 per cent of argon at a pressure of about 40 mm. of mercury, while the activating material preferably consists of magnesium which is vaporized or flashed onto the wire contact I8 and bimetallic element I9 (which act as electrodes during the time the switch functions as a glow discharge device) as well as on the inner surface of the envelope itself, as indicated at 24. The magnesium deposits or coatings within the envelope perform two important functions; first,

the deposit on the bimetallic element II and the cleaning up any impurities within the envelope,

such as residual and occluded gases.

To provide the deposits of magnesium on the inner parts of the switch, a small cap or disc 25,

, tubular lamp envelope.

preferably of nickel, is secured to the extended and bent end of leading-in wire I5 so as to be disposed directly above the bimetallic element IS. A small pill 26 of magnesium is secured to the under surface of this cap in any suitable manner, such as by means of a wire net or strip welded, clamped or otherwise secured to the cap 25. After the envelope I0 has been evacuated during the sealing-in operation, the cap 25 is heated to a temperature sufficient to volatilize the magnesium pellet 26 thereon. The heating of the cap is preferably accomplished by placing the entire device within a high frequency inductive field. The volatilized magnesium deposits on the bimetallic element I8 and the electrode I8 to form a coating thereon, as well as on the inner surface of the envelope III. The heating of the cap 25 is discontinued after the inner surface of the envelope I0 is coated with magnesium. Following the vaporization of the magnesium, the envelope is filled with the desired gaseous atmosphere and then sealed off in the conventional manner.

It will be apparent that by mounting the bimetallic strip l9 and switch contact I8 with their lengths extending transversely of the envelope I0 instead of longitudinally of the same as heretofore, the overall length of the device is reduced to a minimum. The short length of the device is further assured by the use of the short stem I! made according to the disclosure in the White Patent No. 2,128,173 referred to above. Such a small switch is obviously desirable, and in some cases is necessary in order to enable its use in certain types of fixtures for electric discharge devices,

Fig. 5 illustrates diagrammatically the circuit preferably employed for starting and operating, by means of a glow discharge switch I0 comprising my invention, an electric discharge device such as the well-known fluorescent lamp 21 now in commercial use and having a pair of thermionic electrodes 28, 29, one at each end of the Referring to Fig. 5, the two leading-in wires I5, I6 of the glow type thermal switch I0 are connected by leads 30 and 3|, respectively, to one end of the opposite lamp electrodes 28, 29. A small condenser 32, preferably having a capacity of about 0.006 mfd., is connected across the leading-in wires I5, I6 of the switch I0 so as to shunt the contacts thereof. The end of electrode 29 opposite to that end con nected to the switch I0 is connected by lead 33 to one side of the line, while the corresponding end of the other lamp electrode 28 is connected to the other side of the line through lead 34, a choke coil or ballast 35, lead 36, a main switch 31 and lead 38. The thermal switch I0 thus serves to connect the lamp electrodes 28, 29 in series across the line.

The operation of the thermal auxiliary switch I0 is as follows: when the main switch 31 is closed, the potential across the electrodes I8, I8 within the thermal switch breaks down or ionizes the gas filling therein, thus causing a glow discharge to take place between the electrodes I 8, I9.

The continuance of this glow discharge accordingly heats up the bimetallic element or electrode I9, thereby causing the free leg portion 20 of the bimetallic element to gradually deflect or separate from themain leg portion 2|. The current flow through the switch l and lamp electrodes 28, 29 during the continuance of the glow discharge is exceedingly small, being of the order of 0.050 ampere or thereabouts. This amount of current is insuilicient to cause any appreciable preheating oi the lamp electrodes 28, 29. The glow .discharge, and resulting movement of the bimetallic element I9, continues until the oilset or bend 23 of switch contact 22, engages the straight wire contact or electrode l8, the latter being in the path of movement of .the oilset or bend23. The engagement of the switch contacts l8 and 22 immediately shorts the arc path between the switch electrodes, so that the glow discharge is terminated and further heating of the bimetallic element l9 discontinued. With the switch contacts l8 and 22 in engagement, the current flow through the switch I9 and lamp electrodes 28, 29 then amounts to approximately 0.4 to 0.6 ampere,

which is sufilciently high to effect a preheating of the lamp electrodes. Because of the slight lag in the movement of the bimetal behind the temperature rise, and further because of the tendency oi. the switch contacts to weld together'(which latter condition is particularly true when the instant of contact between-the switch contacts occurs-at or close to that point. of the cycle of current flow at which the current is a maximum), the switch contacts l8 and 22 remain in engagement with each other for a more or less extended period of approximately /2 to 1 /2 seconds or thereabouts. This time interval is sufllciently' long to enable .the current flowing through the lamp electrodes 28,29 to adequately preheat the same to arc discharge supporting temperature, i. e., to the temperature at which they become sufliciently electron emissive to support an arc discharge therebetween. Upon the opening or separation oi the switch contacts I8, 22, the arc discharge immediately strikes between the electrodes 28; 29 of the lamp 21, thereby shunting the thermal switch Ill. The are discharge occurs in the lamp 21 in preference to a restarting of the glow discharge in the thermal switch ill for the reason that the breakdown potential of the gas within the lamp 21 is somewhat less than the breakdown potential oi the gas within the switch ll, being approximately 50 to 60 volts for the lamp as compared to 75 to 90 volts for the switch.

Since the thermal switch "I is shunted by the arc discharge within the lamp 21, the switch is virtually out of the circuit so that little, if any,

- current flows through the switch. Consequently, the bimetallic element l9 returns to its normal position, and the switch It is then ready to restart the lamp even while the arc discharge in the restarting oi ,the arc discharge within the lamp. In this connection, it has been found that tb'estartandrestarttimeoitheglowswitchll ocordingto'myinvention. i.e.,thetotalelapsed time between closure of the main switch 36 and actual starting of the arc discharge within the lamp 21, is approximately 2 to 3 seconds or very close thereto.

While I have shown and described the preferred form of glow switch according to my invention, it is obvious that other modifications are possible within the scope of the invention. Thus. instead of employing a pill 26 of magnesium secured to the cap .25, the magnesium may be in the form of a strip extending over the inner surface of the bimetallic element l9 and secured thereto by suitable means, such as by a metal clamp preferably of nickel. This modified form of the invention has the additional advantage oi a somewhat longer life, for inasmuch as the entire quantity of magnesium is not flashed of! during the flashing operation, the remaining magnesium, being located within the glow discharge, is subsequently sputtered off during operation of the switch so as to replenish from time to time the deposits on the parts within the envelope I0.

A more or less continuous supply oi -activating material is thus supplied to the switch electrodes l8 and 19 throughout a long period oioperation of the device. As a result, the operating characteristics oi the device remain reasonably constant throughout the life of the device.

What I claim as new and desire to secure by Letters Patent oi the United States is:

1. A thermal switch comprising a hermetically sealed envelope containing an ionizable gaseous filling, a pair 01' lead-in conductors extending into said envelope, a substantially U-shaped bimetallic electrode element secured at one end to one 01 said lead-in conductors, a contact member on the free end of said bimetallic element, the other of said lead-in conductors being provided with an electrodeand contact portion located substantially between and extending substantially parallel to the legs 01 said bimetallic element to sustain a glow discharge between said bimetallic element and said electrode and contact portion,

said electrode and contact portion being normally spaced from said contact member but intersecting the path of movement of a portion of said contact member.

l 2. A thermal switch comprising a hermetically sealed envelope containing an ionizable gaseous filling, a pair or lead-in conductors extending into said envelope, a substantially U-shaped bimetallic electrode. element secured at one end to one of said lead-in conductors andbeing designed to open up when heated, the other of said lead-in conductors being provided with an electrode and contact portion located substantially between and extending substantially parallel to the legs of said bimetallic element to matain a glow discharge between said bimetallic element and said electrode and contact portion, and a contact member secured to the free end oi said bimetallic element and extending toward the supported end thereoi beyond said electrode and contact portion where it is provided with a cam-like oflset portion designed, upon abnormal heating or said bimetallic element, to engage and cross over said electrode and contact portion and to return to its normal position upon cooling of said bimetallic element. 1

WAYNE C. BMITLEY.

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