US2922916A - Electrode for fluorescent lamps - Google Patents

Description

A. J. SHARKEY ELECTRODE FOR FLUORESCENT LAMPS Jan. 26, 1960 Filed No v. 10. 1953 lnveption relates to sue 0nd len of wires.

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INVE N TOR.

HIS J'I'TORNEYJ' nited States Patent 2,922,916 ELECTRODE FOR FLUORESCENT LAMPS Arthur J. Sharkey, Haworth, NJ., assignor to Duro Test gorpioration, North Bergen, N.J., a corporation of New Application November 10, 1953, Serial No. 391,221 9 Claims. (Cl. 313-344) The present invention relates 'to electrodes for fluorescent lamps or other lamps of the low pressure positive column discharge type.

More particularly the invention relates to a coil type of electrode which is usable with fluorescent lamps designed for operation in instant start, rapid start, and preheat starting circuits.

Fluorescent lamp electrodes are coils of tungsten wire coated with an electron emissive coating in order to supply electrons to the arc. Since such lamps are generally operated on alternating current each electrode thereof serves as a cathode during one half cycle and as an anode during the succeeding half cycle. Such electrodes must meet ,a number of requirements. As a cathode the electrode must be heated sufliciently to furnish electrons while having a low cathode drop; the electrode area presented to the arc must be large enough to dissipate 2 apparent when the following description is considered in connection with the annexed drawings, in which,

F1gure 1 is an enlarged elevational view showing a partially completed electrode for a fluorescent lamp;

Figure 2 is a similar view showing a modification of the electrode;

Figure 3 is a view showing a still further modification of the electrode in its partially completed state; and

Figure 4 is a front elevation of a completed electrode, the particular electrode shown being a completionv of the form of Figure 2.

Electrodes in general use in preheat and instant start fluorescent lamps are either coiled coils or, in some instances, triple coils and have from eight to twelve turns in the finished electrode. In the 40 watt lamp used hereinafter as a basis of comparison, these electrodes have the heat generated by the electron bombardment when the electrode serves as an anode, and the surface area of the tungsten wire coated with an electron emissive coating, must be suflicient so that the lamp life will'be relatively great.

At the present time electrodes which meet the requirements above for one or two of the types of starting circuits mentioned do not meet the requirements for the remaining type or types and it is therefore necessary for a lamp manufacturer to produce one, two orthreetypes of fluorescent tube electrodes and for the manufacturer and distributor to stock these types of lamp in each size. The present invention makes it possible to produce fluorescent lamp electrodes which are usable in lamps to be used on any one of the three types of startingarrangements above-mentioned, and thus vastly simplifies the manufacturing procedure as well as decreasing the stock of electrodes necessary to be maintained.

It is an object of the invention to provide an electrode for fluorescent tubes which is usable in lamps operating in all three of the presentcommonly used circuits, that is, instant start, rapid start, and preheat.

It is another object of the invention to provide such an electrode which, as a cathode, operates at a sufliciently elevated temperature to furnish electrons while at the same time having a low cathode dropand which also functions properly as an anode since the areapresented to the arc is large enough to dissipate the heat generated It is a further object of the invention to provide -an electrode'for a fluorescent tube lamp which is readily manufactured and which renders a lamp of a particular wattage rating usablewithout distinction in any one of the three circuit arrangements above-mentioned.

' Other objects and features of the invention will be;

voltage ratings of from 10 currents.

However, in the recently developed rapid start system the ballasts are designed to provide a heating current in the range of 3 /2 volts. It is necessary to draw a current of approximately 350 milliamperes during electrode heating since the starting voltage across the lamp is varied with variation of the heating current. Because of these two requirements the resistance of the electrode is fixed; additionally it is essential that a wire size be used which will heat the cathode to emitting temperature at the 350 milliampere current supplied.

Rapid start lamps have been made using a triple coil arrangement such for example as that shown in US. Patent 2,306,925. Lamps employing this triple coil, in the 40 wattsize, have only four turns in the final Winding in order to meet the low voltage rating of 3 /2 'volts. Such 'a small coil greatly decreases the area available to dissipate the heat resulting from the electron bombardment during its functioning as an anode and also decreases the surface area of the material from which the electrode is made. In other words, small electrodes presently utilized in rapid start lamps, while suitable for use with the rapid start circuits, cannot economically be used in preheat and instant start circuits. When used in preheat circuits the lamp life willbe decreased and greater discoloration of the lamp will occur. When used in instant start circuits the same results follow to an even greater degree.

The present invention provides an electrode and a means for winding such electrode whereby the electrode has substantially the same cross-sectional area and surface area as standard fluorescent lamp electrodes, while having the voltage drop essential for use in rapid start c1rcu1ts.

The electrode of the present invention comprises a coil having substantially the same number of turns as is usual in an instant start circuit but with each turn consisting of two filamentary wires wound in parallel. By means of this arrangement smaller wire sizes may be utilized the combined length being such as to yield a coil with the usual number of finished turns while keeping the wires in close proximity for proper heating efliciency. The lengths of the two parallel Wound wires to 15 volts at proper heating may be equal or difierent and the size of the wires may be the same or different.

Onefcrm of the first winding step is shown in Figure 1. In that figure two wires 10 and 11 are shown. These .chosen that the resistance of the two wires 10 and 11 in parallel is identical with that which would be produced by a single wire when wound in the normal fashionof a coil. As an illustration of specific values which 3 secure the desired result for a 40 watt lamp the following is given.

A 9.85 milligram (per 200 mm.) tungsten wire is Wound on a 7 mil molybdenum mandrel the pitch being 105 turns per inch, this wire being that indicated at 10 in Figure l. Utilizing the wire 10 and the underlying 7 mil molybdenum mandrel as a mandrel, 9.85 milligram tungsten wire is wound thereon in a helix having 248 turns per inch, this second wire being indicated at 11. As is clear from Figure l, the wire 10 lies uniformly on the molybdenum mandrel, but wire 11 lies on the mandrel at some points and crosses the wire 10 at some points, consequently having a non-uniform diameter. The partially completed coil of Figure 1 consists there'- fore of a short length of the wire 10 and a longer length of the wire 11. After the winding has been completed to this point it is Wound on a .040" mandrel as indicated in Figure 4 yielding a finished coil of 8 turns. This finished coil is usable in lamps intended for preheat, instant start orrapid start circuits since its resistance. is such as to yield a proper voltage drop for the rapid start circuits, while at the same time its physical dimensions including its cross-sectional area and its surface.

area are such as to cause proper electron emission and to withstand the electronic bombardment and yield a sutficient lamp life.

It will be understood that molybdenum mandrels are not shown in either Figure 1 or 4 since these mandrels are removed or eliminated in any suitable manner after the completion of the winding of the electrode.

Referring now to Figure 2, there is shown therein a partially completed filament or electrode for a 40 watt lamp which is in accordance with this invention. In this instance, a'conductor 12 which is 9.85 milligram tungsten wire is wound on a 7 mil molybdenum mandrel the helical pitch of the coil being 125 turns per inch. Thereafter a second conductor 13 identical with the first is wound on the same mandrel with its turn interpositioned with those of the first coil, the conductor 13 being of the same size and being wound at the same pitch as conductor 12. There is thus formed a parallel wound coil having equal lengths of the same size wire. This coil is then wound as a unit upon a .040" mandrel'to give a finished coil of 8 turns as illustrated in Figure 4. 'At this point it should be noted that all of the values given are for a 40 watt lamp electrode and further that Figure '4 indicates the mode of winding the partially completed coil of Figure 1 into a completed coil but shows with-exactitude the finished product resulting from the winding of the partially completed coil of Figure 2.

In Figure 3 there is shown an arrangement wherein a triple coil is utilized. In this case the wire 14 is a tun'g- V sten wire of 0.85 milligram, and is wound at a helical pitch of 5 82 turns per inch on a mandrel consisting of --5 milligram tungsten wire v15 together with a 3 mil m'olybdenum wire laid side by side. Then as a separate operation 9.85 "milligram tungsten Wire is wound on a 7 mil molybdenum mandrel at a helical pitch of 105 turns per inch. Next, using the 7 mil molybdenum mandrel the first'wound coil, that is, the coil consisting of conductors 14 and 15, is wound 'at' 105 turns'per inch in the spaces between the turns of the coil consisting of conductor 16.

The coil thus formed consists electrically of the three conductors 14, 15 and 16 in parallel, there being a longer length of the 0.85 milligram conductor 14 and approximately equal lengths of the 5 milligram conductor 15 and the 9.85 milligram conductor 16. This coil is then wound on a 0.040 mandrel in the same manner as has" been described above in connection with Figures 1 and 2 in order to give a finished coil of 8 turns as generally indicated in Figure 4.

In any of the three examples given above, the e fiective area of the electrode subjected to bombardment when the particular electrode is functioning as an anode is approximately 14.6 square millimeters. Since the cfi'ective area subject to bombardment is the area extending in a plane perpendicular to the electron flow, it may be calculated by assuming that the area is equivalent to the aggregate cross-sectional area of the conductors in a plane parallel to the conductor axis and passing through that axis. Also, in any of the three examples given above the approximate surface area of the electrode which is coated with electron emissive material and which therefore emits when the electrode is serving as a cathode is approximately 95 square millimeters. A 40 watt lamp operates at a rated current of 425 milliamperes. Thus it will be seen that the lamp of the instant invention has a ratio of current expressed in amperes to effective bombardment area expressed in square millimeters of approximately 1:34. In a similar manner the ratio of rated current in amperes to surface area eifective for electron emission is approximately 1:225.

The rated current for lamps of other Wattages are diiferent and calculation shows that the range of ratios of current to bombardment area is from to V and the rangeof ratios of currentto emission area is from It may therefore be stated that my novel electrode comprises at least two conductors electrically in parallel, the conductors being of such lengths and diameters that the ratio of lamp current expressed in amperes to effective bombardment area expressed in square millimeters is from 1:34 to 1:40 and the ratio of current in amperes 'th'ese coils could be wound simultaneously with equal facility "and could also be wound in opposite directions. The result in each case would be the same, namely, two windings of greater resistance wound in a manner to produce a finished winding of lower net resistance forming an electrode which is interchangeable in lamps of a specific wattage rating whether those lamps be used in instant start, preheat or rapid start circuits.

While I have described preferred embodiments of my invention it will be understood that many other modifications may be made without deviating from the principles thereof; therefore I wish to be limited not by the foregoing description, butsolely by the claims granted to me.

What is claimed is:

1. An electrode for a fluorescent lamp comprising a helically coiled pair of air core helical coils of spaced turns of wire lying on the same helical axis and electrically connected in parallel, said Wires being of a size and length such that the ratio. of rated current expressed in amperes to area effective for the reception of electron bombardment expressed in square millimeters is in the range of from 1:34 to 1:40, and the ratio of rated current expressed in amperes to the surface area eflective for emission expressed in square millimeters is in the range of from 1:225 to 1:350.

2. A device as'clair'ned'in claim 1, characterized in that the said two wires are w'oundat different pitches whereby the turns of one wire overlie the turns of the other atspaced poi'nts therealong, said Wires being of the same diameter.

- laterally ofiset'with respect to adjacentturns.

4. A device as claimed in claim 1, characterized in that said two helically 'coiled wires are-of the same diameter'and are wound'atthe' same pitch, turns of one wire being interspersedwith'turns of the other.

5. A device 'as' claimed in claim 1,characterized in that the said twowires are 'wound at ditferent pitches where by the turns of one wire overlie the turns of the other at spaced points therealong, said wires being of different diameters.

6. An electrode for a fluorescent lamp comprising a first helically coiled conductor, a second composite helically coiled conductor, said second conductor comprising a main wire and a secondary wire helically coiled about said main wire, said composite conductor being coaxially helically coiled With said first conductor, said first and second conductors as a unit being again helically Wound to form the electrode, said conductors being connected electrically in parallel, the ratio of rated lamp current expressed in amperes to aggregate surface area of the electrode eifective to receive bombardment expressed in square millimeters being in the range of from 1:34 to 1:40 and the ratio of rated lamp current expressed in amperes to surface area of the electrode to emit electrons expressed in square millimeters being in the range of from 1:225 to 1:350.

7. An electrode as claimed in claim 6, characterized in that said first conductor and said composite conductor are of substantially equal developed length and are wound coaxially at the same pitch with the turns of said com- 6 posite conductor interspersed between the turns of said first conductor.

8. An electrode as claimed in claim 6, characterized in that said first conductor and said secondary Wire are of difierent developed length and are Wound at diiferent pitches and the turns of said composite conductor overlying portions of said first conductor at spaced points therealong.

9. The electrode of claim 1 characterized in that some turns of the helically coiled Wire are of greater diameter than other turns thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,106,689 Braselton Jan. 25, 1938 2,143,915 Jobst Jan. 17, 1939 2,258,836 Willner Oct. 14, 1941 2,273,762 Reerink Feb. 17, 1942 2,297,454 Berger Sept. 29, 1942 2,306,925 Aicher Dec. 29, 1942 2,479,193 Zabel Aug. 16, 1949

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