US2224828A - Electric discharge lamp and starting device therefor - Google Patents

Description

Dec. 10, 1940. Y MCCARTHY 2,224,828

ELECTRIC DISCHARGE LAMP AND STARTING DEVICE THEREFOR Filed Sept. 1, 1939 HENRY J MCARTIH INVENTOR.

ATTORNEY.

Patented Dec. 10,1940

PATENT OFFICE ELECTRIC DISCHARGE LAMP AND START- ING DEVICE THEREFOR Henry J. McCarthy, Danvers, Mala, asslgnor to Hygrade Sylvania Corporation, Salem, Mala, a corporation of Massachusetts Application September 1, 1939, Serial No. 293,070

Claims.

This invention relates to electric gaseous discharge lamps, and in particular to such lamps having thermionic cathodes, and to apparatus for starting and operating such lamps.

5 An object ,of the invention is to provide an electric gaseous discharge lamp with thermionic cathodes and with apparatus for allowing the cathodes to preheat for a definite-interval before the discharge is started between them. A

further object is to provide a switch with the proper time delay to allow preheating of the cathodes under all possible conditions of starting and restarting.

A feature of the invention is a switch actuated by a bent bimetallic strip heated by a filament. A further feature is the delay secured by the initially opposing action of the two arms of the bent strip, and by the necessity for heat conduction from one arm to the other to complete the action of the switch. i

, Other objects, advantages and features of the invention will be apparent from the following description, taken with the accompanying drawing in which:

Figure 1 is a schematic drawing of a lamp and switch in circuit, according to the invention, with the contacts closed.

Figure 2 is a schematic view of the same switch with contacts still closed; and

Figure 3 is a schematic view of the switch with the contacts open; and t Figure 4 is a schematic side view of the switching device of which Figs. 1, 2 and 3 give a top view.

In Figure l, a bimetallic strip I, each of whose sides is of a diiierent metal, one of iron and one of copper, for example, is bent in the shape of a U, one leg of the U preferably being slightly shorter than the other and attached firmly to the lead-in wire 2. A wire 3 projects into the bend of the U. A contact 4 is carried by the free end of the U, and the tension in the U is adjusted so that this contact 4 is normally closed against contact 5, which is rigidly supported.

The wire I, is placed close to the support side I.

of the U, and is spaced from the free side. A

filament ll, preferably of tungsten is supported by the lead-in wires I and I, so that it is close to the lower leg I, of the U but shielded from the upper leg 8. t

An electric discharge lamp II, has the tubular glass envelope I! with the electrodes it, comprising a coiled-coil of tungsten wire coated with one or more of the alkaline earthoxides, sealed through each end of the tube by the lead-in wires I4, II. The discharge tube Ii, preferably contains an inert gas and mercury vapor, and may have a coating of fluorescent material on the interior surface of its envelope. A wire it may be connected to the lead-in wire I 4, and extend along but out of contact with the fiament ll, toward the other lead-in wire ii. The lead-in wires Ii at opposite ends of the lamp are connected to a suitable power supply line by the conductors i1 and i8, conductor ll having the reactance coil is in its circuit to ballast the lamp during operation and to provide an inductive voltage surge at starting. The same conductor has the tungsten filament ill of the switching device connected in series with it, al- 15 though this filament It may be connected to the other side of the line, if desired. It should not,

. however, be connected in such a way that the reactance coil I9 is between filament l0 and the lamp, as in that case-the voltage across the 20 reactance coil will exist between filament iii and the contact circuit, and cause arcing in the switching device, particularly at the instant of breaking the contacts. The lead-in wires H, are connected together through the normally closed 25 contacts 4 and 8 being closed a current will flow through the reactance coil iiland the cathodes ll, heating the latter to an electron-emitting temperature and allowing a small current to flow from the wire l6 to the end of the filament 0 ll nearest the'lead-in wlre'l5, as described for example, in my copending applications, Serial No. 269,423 filed April 22, 1939, and Serial No. 272,246 filed May 6, 1939. To permit such a discharge, the voltage across the filament must be 5 trode II of the discharge tube is heating up, the

filament I! of the switch device also is heating up, and is adjusted so that when the electrodes ll 45 reaches the proper e ectron-emitting temperature and is surround by the ions produced by the small discharge to the wire It, the strip holding the contact 4' has Just been heated sufllciently to separate the contacts 4 and 5, thus producing 50 a voltage surge by discharging the inductance across the tube ll, starting the lamp.

when current flows through filament I! on connecting the lamp circuit to the line, the lower part l of the U, nearestthe filament, heats first 55 can move no Iurther.

and moves downward bringing the upper part down with it, actually making the contact be-.

tween 6 and 5 more firm. When, as in Fig. 2, the upper part 8 of the U reaches the stop 3 it The upper part 8 of the U farthest from the filament Ill has then become heated itself, however, and moves upward to break the contact between 4 and 5, as shown in Fig. 3. A definite time delay is thus secured before the contacts open, since the initial expansion of the bimetallic strip tends to keep the contact 4 and 5 closed, and since time is required for the'heat to be conducted to the upper leg, the U-shaped strip should, of course-be arranged so that the ends of the U tend to move apart on expansion, which generally means that the metal of highest coeflicient of expansion is on the inside of the U; The contacts maybe of tungsten, but in many cases contact may be made directly from the bimetallic strip i, to con tact 5, which may be a nickel wire.

Figure d'shows a side view of the switching device oi Figs. 1, 2 and 3 with the switching unit placed in a glass bulb 20, having the stem at,

' as of smaller diameter than shown schematically in Figs. 1, 2 and 3.

In order to reduce the amount of power required by the filament to heat leg {of the bimetallic strip I, the leg 8 may be coated with a black heat-absorbing surface, such as a layer of carbon, for example, on the side nearest the filament 10. The other side of leg 8, and both sides of the leg 9, may have a shiny, reflecting surface if desired to further reduce heat loss since these parts do not absorb heat from the filament Hi.

It will be clearly seen, from the above description, that my switch gives a very definite preheating period before the switch contacts I and 5 open. This is due to the fact that the initial heating of the bimetallic strips I, by the filament I0, causes'the strip to move in the downward 50 direction of the drawing, which pulls'leg 9 tighter against the contact 5, holding the contact even more firmly. The heat absorbed by leg 8 is slowly conducted to leg' 9, but before that leg can move to open the contacts, the tension due to the legsbeing pulled downward by the bottom leg, must first be removed by the heating. The leg will then move, opening the contact.

To avoid the deterioration --of the filament ii in the discharge lamp,.the contacts I and 5 must be kept closed for the time required for the filsment II to heat sufllciently to allow the discharge. tostart instantly. The delay necessary may be only half a second but it must neverthe less be present if the lamp is to have any appre- *ciable life.

The positive delay action oi my switch is encotive even when the lamp of Fig. 1 is mrned on and then the circuit again clgsed before the bimetallic switch has had time to completely cool off. That is because the initial heating will, as before, tend at first to make the contact firmer, since the first leg 8 will be heated first. This prevents flickering of the lamp, which would occur in a switch which merely cooled off until the contact just closed and then immediately opened as the lamp current flowed through it, and opened so soon that the filaments of the lamp were not hot enough to start the discharge. Such a switch would produce a flash or filcker, through the lamp each time the contacts opened, due to the inductive discharge, and would ruin the cathode 13 without starting the lamp. My device avoids fiickering. I

By making the free leg 9 somewhat, but not much, shorter than supported leg 8, fairly good compensation of the device against ambient temperature changes may be achieved.

What I claim is:

1. A time delay electric switch, for discharge lamps, comprising a substantially fixed contact,

a movable contact attached to' a U-shaped bimetallic strip near one end of said strip, the other end being fixed to a support, a stop to limit the motion of the arm of the U nearest the support, and a heating coil placed outside of the U and shielded from the free leg of the U by the fixed leg.

2. The combination of. claim 1 in which the stop is placed inside the bend of the U.

. 3. The combination of cla'im 1, in which the stop is placed inside the bend of the U, and in which the fixed contact, the fixed end of the U, the stop inside the U, and the heating coil outside the U, are attached to support wlres extending from the stem of a sealed glass bulb entuating member, a -U-shaped bimetallic strip of temperature, a stop inside the bend of the U and'arranged to allow a small predetermined motion of the U-leg nearest the filament before stopping that motion and allowing the free leg to curl back to break-the contact circuit.

5. A time delay electric switch for discharge lamps, comprising a substantially fixed contact, a movable contact attached to a U-shaped blmetallic strip near one end of said strip, the

other end being fixed to a support, a stop to limit the motion of the arm of the U nearest the support, and a heating coll concentrated to heat the partof said U-shaped strip nearest the support.

HENRY J. MCCARTHY.

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