US 3789266 A
A lighting system comprising a low pressure mercury vapor discharge lamp with means for supplying an intermittent current flow therein. The lamp efficiency is increased by operating it with a maximum pressure of 6 Torr, a duty cycle in the range between 0.35 and 0.65 and at a special switching frequency in the order of 30-60 KHz. A further feature includes means for periodically reversing the direction of current flow in the lamp.
Claims available in
Description (OCR text may contain errors)
nited States Patent [191 Polman et a].
ARRANGEMENT PROVIDED WITH A LOW-PRESSURE VAPOUR DISCHARGE LAMP Inventors: Jan Polman; Jan Evert Van Der Werf; Peter Cornelis Drop, all of Emmasingel, Eindhoven,
Netherlands Assignee: U.S. Philips Corporation, New York, NY. 7
Filed: May 2, 1972 Appl. No.: 249,526
Foreign Application Priority Data May 8, 1971 Netherlands 7106345 US. Cl 315/194, 3l5/DIG. 5, 3l5/DIG. 7 Int. Cl. H05b 37/02 Field of Search 313/185; 3l5/DIG. 5, DIG. 7,
[ Jan. 29, 11974 [5 6] References Cited FOREIGN PATENTS OR APPLICATIONS 793,582 4/1958 Great Britain 3l5/DlG. 7
Primary Examiner-Roy Lake Assistant Examiner-Lawrence J. Dahl Attorney, Agent, or Firm-Frank R. Trifari [5 7] ABSTRACT A lighting system comprising a low pressure mercury vapor discharge lamp with means for supplying an intermittent current flow therein. The lamp efficiency is increased by operating it with a maximum pressure of 6 Torr, a duty cycle in the range between 0.35 and 0.65 and at a special switching frequency in the order of 30-60 KHz. A further feature includes means for periodically reversing the direction of current flow in the lamp.
5 Claims, 1 Drawing Figure ze k ARRANGEMENT PROVIDED WITH A LOW-PRESSURE VAPOUR DISCHARGE LAMP This invention relates to an arrangement provided with a low-pressure mercury vapour discharge lamp having an oblong discharge vessel of circular crosssection and including at least one rare gas, in which two input terminals of the arrangement are connected by a series arrangement of at least the lamp and a switching element. The switching element is connected to a control device which controls said switching element in such manner that an intermittent current flows through the lamp. During a time interval ofW seconds a current flows through the lamp and is followed by a substantially currentless time interval having a duration of R seconds. The current flowing through the lamp is reversed in direction from time to time.
A known arrangement of the kind mentioned is described, for example, in U.S. Pat. No. 3,422,309.
One advantage of the known arrangement is that an accumulation of mercury near one end of the lamp is prevented by varying the direction of the lamp current from time to time. A drawback of this known arrangement is that the repetition frequency (I/W-l-R) of the current pulses is rather low so that the efficiency, ex-
pressed in lumens per Watt, also is relatively low.
An object of the invention is to provide an arrangement in which the said efficiency is relatively high.
According to the invention there is provided a lowpressure mercury vapour discharge lamp having an oblong discharge vessel of circular cross-section and including at least one rare gas, and in which two input terminals of the arrangement are connected by a series arrangement of at least the lamp and a switching element. The switching element is connected to a control device which controls this switching element in such a manner that an intermittent current flows through the lamp. The a time interval having a duration of W seconds followed by a'substantially currentless time interval having a duration of R seconds, the current through the lamp being reversed in direction from time to time. A feature of the invention that the maximum pressure of the rare gas (the rare gases) is 6 Torr and that the diameter of the discharge vessel is so large that TE TD is satisfied, the ratio WIW+R being between 0.35 and 0.65 and the duration of a period (W+R) of the lamp supply being between the values 211- TE and 2 1r 1- in which:. 1,; is the energy relaxation period (in seconds) of the electrons in the discharge vessel, and
r is the diffusion period (in seconds) of the electrons in the discharge vessel.
The energy relaxation period is understood to mean l/A B in which: A is the mean fractional energy loss per impact of an electron (in this respect fractional is understood to mean that the energy loss is expressed in the energy of the electron before the impact) and B is-;t.he average number of impacts per second of an electron with other particles, which are not electrons, in the discharge vessel.
In this respect diffusion period is understood to mean the period (expressed in seconds) which is needed on average by an electron to reach the wall of the discharge vessel.
An advantage of the arrangement according to the invention is that the efficiency (for example, expressed in lumens per Watt) is relatively high.
A proposal in a copending U.S. application, Ser. No. 86,471, states that a filler gas pressure of, for example, 5 or 10 Torr in combination with a ratio WIW+R 0.4 can, when supplying neon lamps, lead to both a satisfactory efficiency and an acceptable lamp lifetime.
However, the efficiency of the conversion of electrical energy into ultraviolet radiation, which is important in low-pressure mercury vapour discharge lamps, is dependent on the current density in the lamp in contrast to the situation where visible light in neon lamps is generated. In the low-pressure mercury vapour discharge lamps considered herein the mentioned dependence is such that a smaller current density leads to a larger efficiency. A reduction of W/W+R leads, however, to a larger efficiency too. However, a smaller ratio W/W+R involves a larger current density when the power of the lamp remains the same. In the case of the low-pressure mercury vapour discharge lamp considered all this leads to the fact that a high efficiency is only reached at ratios of W/W+R which are located about the value 0.5, namely in the region of from 0.35 to 0.65. Such a ratio for W/W+R, which is not so very small thus implies a current density which is not too high and this in turn implies that the filler gas pressure in the lamp need not be so large to counteract sputtering of the electrodes and hence maintain the lifetime of the lamp acceptable.
Ratios of WIW+R of 0.5 have, however, been known per se, see for example the U.S. Pat. No. 3,422,309 (FIG. 5) mentioned in the preamble. A combination of this value of W/W+R with the very short period duration W+R (between 211' TE and 21: 'r,,) is found to lead, according to the invention, to an interesting increase in the efficiency of low-pressure mercury vapour discharge lamps.
The invention is' based inter alia' on the recognition of the fact that for the chosen time duration 'W+R the electron gas is cooled due to impacts, during the currentless time interval while the electron concentration has only slightly decreased during this interval. In fact, the extent of the decrease in concentration is determined by the diffusion period To which is longer than the time interval W+Rl2rr while the extent of the decreaseof the average electron energy is determined by the energy relaxation period 'r which is shorter than the said time interval W+R/21r.
The electron gas is again heated by means of the current pulse following a currentless time interval and it is found that a proportionally large number of fast electrons is obtained, that is to say, a larger number than in the corresponding situation with direct current operation of the lamp of the same mean current. Tee efficiency of a discharge is favourably influenced by the presence of a relatively larger number of fast electrons.
The fact that a satisfactory light efficiency of the lowpressure mercury vapour discharge lamp can be obtained during those relatively short periods (W+R) which fall within the interval 211 1-,; to 211- 1-,, can also be ascribed to a cooperation of the effect described in the previous paragraph with a second effect to be described below. This second effect means that the concentration of the radiating mercury atoms remains substantially constant and is adjusted to a value associated with the highest occurring electron temperature during In an electric arrangement according to the invention the lamp is preferably incorporated in an electric bridge and each of the bridge branches is provided with a transistor, while furthermore a further transistor is included in series with the bridge.
An advantage of this preferred embodiment is that one group of transistors (the bridge transistors) provides the change of direction of the lamp current while the further transistor provides the switching function to produce the intermittent current pulses. This embodiment simplifies the requirements regarding synchronisation of the control of the transistors.
In a further preferred embodiment of an arrangement according to the invention the pressure of the rare gas (or gases) is approximately 2 to 3 Torr and the lamp diameter is approximately 35 to 40 mm. In that case a high efficiency can be obtained with a lamp of a conventional type.
The invention will now be described in detail with reference to the accompanying drawing, the sole figure of which shows an electric circuit and a simplified view of a low-pressure mercury vapour discharge lamp to be fed thereby. The assembly constitutes a view of an arrangement according to the invention.
Reference numerals l, 2 denote input connection terminals for connection to a direct current source. Reference numeral 3 denotes a low-pressure mercury vapour discharge lamp having a length of about I22 cms and a circular cross-section having a diameter of 3.8 cms. The mercury pressure was approximately 6.10 Torr.
In addition to mercury the discharge vessel of the lamp also contained a filler gas consisting of 75 percent argon and 25 percent neon; total gas pressure 2.5 Torr. The normal lamp power at 50 Hz supply was 40 Watts.
The two ends of the lamp 3 are provided with internal electrodes 5 and 6.
The lamp 3 is incorporated in a diagonal of a transistor bridge including transistors 7 to 10 inclusive. A transistor 11 is incorporated in series with the bridge. The references a, b, i denote connections of inter alia the bases of the transistors. These-connections lead to connections having the same reference numerals of a conventional control box 12 which is energized through terminals 13, 14.
In the operating condition the transistor combinations (7, l0) and (8, 9) are alternately rendered conducting through the connections a to g inclusive so that the current flowing through the lamp 3 reverses its direction each time the transistors switch states. Transistor 11 provides for the pauses between the current pulses.
In the embodiment described the internal resistance of the direct current source was sufficiently large to limit the lamp current. If desired such a limitation may be alternatively obtained, for example, by incorporating a capacitor of low value in series with the lamp 3 in the diagonal branch of the bridge.
In a given system the frequency (l/W+R) was approximately 50 kHz and the ratio W/W+R 0.5.
For a comparison of this system with other supplies at which the lamp power was 40 Watts every time, reference is made to the following table. In this table the optimum lumens/Watt value of the lamp 3 with a supply from a 50 Hz supply line is fixed at I00 percent. Furthermore the table states the lumens/Watt values in the case the lamp is supplied from an 8 k Hz source, a
20 kHz source, a direct voltage source; and the last line provides data for the arrangement according to the invention.
The instantaneous voltages of the supply source of50 Hz, 8 kl-Iz and 20 kHz varied substantially sinusoidally with time. For sinusoidal alternating voltages of a frequency higher than 20 kHz there was no further increase of efficiency.
Lumcns/watt in Type of supply of the situation at 50 Hz supply 50 Hz I00 8 kHz approximately I08 Not according to the invention.
20 kHz approximately I08 direct voltage source 106 50 kHz According to the invention.
WIW+R 0.5 I I I In the given case according to the invention: 1 was approximately one p. second and 'r,, was approximately I000 y. seconds. The period duration W+R 20 a seconds thus was between 21:- 1,; which is approximately 6 y. seconds and 21r 7,, which is approximately 6280 .1. seconds. The table shows that the efficiency in the case of the invention is largest.
What is claimed is:
l. A lighting arrangement comprising a low-pressure mercury vapour discharge lamp having an oblong discharge vessel of circular cross-section and including at least one rare gas, two input terminals adapted for connection to a source of power for the lamp, a controlled switching element, means connecting the lamp and the switching element in series circuit across said input terminals, means connecting said switching element to a control device with alternately controls said switching element in a manner such that an intermittent current flows through the lamp during a time interval of W seconds followed by a substantially currentless time interval of R seconds, means for reversing the direction of the current flowing through the lamp from time to time, the maximum pressure of the rare gas being 6 Torr and the diameter of the discharge vessel being chosen to satisfy the condition that TE TD and wherein the ratio W/W+R is limited to the range of values between 0.35 and 0.65 and the duration of a period (W+R) of the lamp supply is limited between the values 27r.r,; and 21127 D in which:
1,; is the energy relaxation period (in seconds) of the electrons in the discharge vessel, and
1,, is the diffusion period (in seconds) of the electrons in the discharge vessel.
2. An arrangement as claimed in claim 1, characterized in that the pressure of the rare gas is approximately 2 to 3 Torr and the lamp diameter is approximately 35 to 40 mms.
3. An arrangement as claimed in claim 1 wherein said reversing means comprises a bridge circuit with at least two arms of the bridge each including a transistor and with the lamp connected across a pair of diagonal bridge terminals, and wherein said switching element comprises a further transistor connected in series with the bridge circuit.
4. An arrangement as claimed in claim 3 wherein the pressure of the rare gas is approximately 2 to 3 Torr and the diameter of the lamp is approximately 35 to 40 millimeters.
5. An arrangement as claimed in claim I wherein the switching frequency of said control device is approximately 50 kHz and the ratio W/W+R is chosen to be i l t