Note: Descriptions are shown in the official language in which they were submitted.
PHN. 8062,
l~lZ95 LOOP/WJM/
10-6-1976.
"Device provided with a low-pressure sodium vapour dis-
charge lamp".
.
The invention relates to a device provided with
a low-pressure sodium vapour discharge lamp and with a cur-
rent stabilizing ballast element connected in series there-
.~,
with, the lamp containing a discharge tube which contains
! 5 except sodium also a rare gas, and in which the lamp is
provided with a coating which envelops the discharge tube
for the main part and which reflects _ fra red radiation,
the effective temperature T of the wall of the discharge
¦ 1;ube belng between 245 C and 270 C in the operating con-
,~, 10 dition of the lamp.
¦ In this respect the effective temperature of
the wall of the discharge tube must be understood to mean
that wall temperature which results in the same voltage-
current characteristic o~ the discharge tube as when the
discharge tube would have the same temperature all over.
In a known device of the type indicated in the
preamble bhe lamps is supplied with an electric current
whose instantaneous value is substantially sinusoidal as
a function of the time. A disadvantage of thls known device
is that the luminous efficacy of the lamp is relatively low.
It is an object of the invention to mitigate
this disadvantage but which must not result in a consider-
able reduction of the effective electric current through
j the lamp,
A device according to the invention provided with
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1071295 PHN. 8062.
10-6-1976.
a low-pressure sodium vapour discharge lamp and with a
current stabilizing ballast element connected in series
therewith, in which the lamp comprises a discharge tube
which contains except sodium also a rare gas, the lamp be-
ing provided with a coating which envelops the discharge
tube for the greater part and which reflects infra red
radiation, and in which the effective temperature T of the
wall of the discharge tube is between 245 C and 270 C in
the operating condition of the lamp, is characterized in
that - with the exception of any peak current having a
duration smaller than 0.5 msecond - the instantaneous cur-
rent density in the discharge tube is kept below the value
S = =65 + 1.3 (T _ 245)2 mA/cm2 and in which the effective
current density in -the discharge tube is kept abov~ the
value 0.8 S mA/cm2.
An advantage of a device according to the in-
vention is that the luminous efficacy of the lamp is relati-
vely large. Also the effective current through the lamp
need not be smaller than in the case of the known device
which shows a sinusoidal variation of the lamp current.
The invention is based on the recognition that
large current densities in the discharge tube which occur
in the known device at the instants around the tops of the
sinusoidal lamp current, should be avoided~ It has namely
appeared to the inventors that at the indicated large cur-
rent densities the sodium concentration near the axis of
the discharge is so low that the discharge is effected for
a large part with.the aid of the rare gas. However, this is
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107~295 P~N. 8062.
10-6-1976.
a discharge having a relatively small luminous efficacy.
In a device according to the invention a low-
pressure sodium vapour discharge lamp is therefore operated
with current densities which do not exceed given maximum
values S. This S which depends on the temperature of the
discharge tube is the value above which the efficiency of
the generation of light is relatively low. The requirement
~ that the effective current density in the discharge tube
! should be above the value 0.8 S is correlated with the fact
1 10 that to obtain a large luminous efficacy the effective cur-
¦ rent density should not be too far below the value S.
It should be noted that the requirements imposed
on the current density can be satisfied by means of a direct
current of a suitable value. ~eeding a low-pressure sodium
~ 15 vapour discharge lamp with direct current is indeed known
', E~ se, for example~from the article "The sodium lamp",
Philips Technical Review December 1937, pages 353 to 360
inclusive but there it does not relate to lamps which are
provided with an infra-red radiation reflecting coating.
The temperature interval between 245 C and 270 C
corresponds to sodium vapour pressures between approximately
1,24 x 10 3 Torr and 4x 10 3 Torr. This is a pressure inter-
val at which the light generation can be effected with the
highest efficiency.
A low-pressure sodium vapour discharge lamp in a
device according to the invention may have been provided,
or may not have been provided with a separate heating element.
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~ PHN, 8062,
1~71Z95 1 o-6-1976.
If no separate heating element is present, substantially
exclusively the generation of heat due to the discharge
current must result in that the wall of the discharge tube
assumes a temperature in the indicated interval of between
245 C and 270 C, If a heating element is present the com-
bination of the heat generation of the discharge current
and the heat generation of the heating element must result
in a temperature of the discharge tube wall in the indicated
interval. It is also conceivable that the device according
to the invention is, for example, arranged in a closed
luminair so that then the required temperature of the dis-
char.ge tube wall can be obtained with relatively low current
densities Further possibilities are found in the quality
of the heat insulation~ for example by enveloping the dis-
charge tube not only by one infra-red radiation reflecting
coating but by a plurality of such coatings.
A device according to the invention may, for
example~ consist of a resistor arranged in series with a
; low-pressure sodium vapour discharge lamp, this series
circuit being connected to a dc voltage source whose output
voltage is carefully kept constant.
~. In a preferred embodiment of a device according
i to the invention which is destined for connection to an ac
voltage source a rectifier bridge is present and the series
arrangement of the lamp and the ballast element intercon-
nects two output terminals of that rectifier bridge, those
output terminals also being interconnected via a capacitor.
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107~Z95 PHN, 8062.
10-6-1976.
An advantage of this preferred embodiment is that
; the lamp may be supplied with a current which results in a
high luminous efficacy and that the device may also be con-
nected to an ac voltage mains available in many places. The
capacitor then serves to smooth ripples in the dc current
supply of the lamp.
It is conceivable that if the effective value of
the supply voltage is subjected to variations a compensation
therefor can be found in the manner indicated hereinafter.
For this purpose an auxiliary device should be connected
across the lamp in such a way that when the lamp current
exceeds a given threshold value the magnitude of the re-
sistance of the ballast connected in series with the lamp
is changed. This would then be a control in which at an
increase of the amplitude of the input ac voltage the bal-
last resistance is increased. It is also conceivable that
such a compensation device does not act on the voltage ~
across the lamp but on the intensity of the current through
the lamp.
In a further preferred embodiment of the device
according to the invention, intended for connection to an
electric supply source which supplies a sinusoidal ac
voltage the lamp is shunted by an auxiliary branch which
comprises a switching element and a resistor, the switching
element being periodically closed at instants at which the
instantaneous current density in the discharge tube threatens
to exceed the value S.
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PHN. 8062.
~ 1~7~295 0-6-1976,
`~ It is an advantage of this device that it can be
connected to an ac voltage mains without necessitating the
use of a rectifier arrangement. The method underlying this
preferred embodiment is that the tops of the sinusoidal cur-
rent are cut off, namely by making a conductive shunting
branch across the lamp at the moment that such a current
threatens to flow through the lamp. The result is that then
the maximum instantaneous current density in the lamp re-
malns below the S ~alue.
The switching element in the branch which shunts
the lamp may, for example, be a controlled semi-conductor
switching element, for example a switching element having a
bidirectional thyristor characteristic ("Triac"). It mi~.ht,
for example, also consist of two thyristors connected in
anti-parallel.
In a further improvement of said last preferred
embodiment of a device according to the invention the switch-
ing element is a light-sensitive switching elemcnt and the
. ballast element in series with the lamp is a coil~ an auxi-
liary lamp associated with the light-sensitive switching
element being connected between two taps of the coil.
An advantage of this further improvement is that
the shunting branch of the lamp becomes conductive at a
proper instant in a very simple manner. For~ if the instan-
taneous current stre~gth through the relevant windings of
the coil would exceed a g~ven value the auxiliary lamp
will radiate so much llght onto the light~sensitive switching
107~Z95 PHN- 8062.
10-6-1976.
element that the latter can become strongly conductive so
that the shunting branch starts carrying a current. Then
the total mains current no longer passes through the lamp.
If then, a short moment later, the voltage between the input
terminals of the device has fallen also the current strength
in the ballast coil will decrease again and consequently
also the voltage to which the au~iliary lamp is connected.
This causes less light to fall on the light-sensi-tive switch-
ing element and the shunting branch of the lamp will again
assume a high resistance. This means that the current which
is taken off from the mains will then again flow substanti-
ally completely through the lamp.
The invention will be further explained with refe-
rence to the drawing in which:
Figure 1 shows a first device according to the
invention,
Figure 2 shows a second device according to the
invention.
In Figure 1 references 1 and 2 are terminals in-
tended for connection to an ac voltage mains. Terminal 1
is connected to a rectifier 3. A rectifier 4 is also con-
nected to ~his therminal 1. Two rectifiers have also been
connected to the terminal 2, namely '5 and 6 respectively.
The rectifiers 3 to 6 inclusive constitute a rectifier
bridge, The output terminals of this bridge are intercon-
nected on the one hand by means of a series arrangement of
a resistor 7 and a low-pressure sodium vapour discharge lamp
107~295 PHN. 8062.
10-6-1976.
8 and on the other hand by means of a capacitor 9. The
~amp 8 has a U-shaped discharge tube 10 which is enveloped
by an outer bulb 11. The discharge tube 10 comprises~
besides sodium, a rare gas consisting of 5,5 Torr Neon to
which 1~ Argon has been added, Reference 12 indicates a
lamp cap. The inner side of the outer bulb 11 is provided
with an infra-red reflecting coating consisting of indium
oxide. This coating is indicated by means of a dashed line.
The length of the lamp is approximately 110 cm and the dia-
meter approximately 6.5 cm.
In an embodiment the effective voltage between the
terminals 1 and 2 is approximately 220 Volts~ 50 Hertz. The
resistor 7 is approximately ~2 Ohms. The capacitor 9 has a
capacitance of approximately 250 mikro Farad. Reference 8
is a sodium lamp of approximately 180 Watt, The diameter
of one leg of the discharge tube 10 is approximately 19 mm.
j This means that the cross-sectional area of such a leg i~
approximately 2.8 cm2. In the embodiment shown the lamp
voltage is approximately 280 Volts and the lamp current
strength approximately o.65 A. The effective temperature
of the wall of the discharge tube 10 is approximately 254 C.
I The lamp delivers a luminous flux of approximately 37800
; lumen. This implies a luminous efficacy of approximate~y
210 lumen/Watt.
When the low-pressure sodium vapour discharge tube
indicated in Figure 1 by reference 8 is fed by means of a
sinusoidal alternating current, so when it is fed not
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1071Z95 PHN. 8062.
10-6-1976.
according to the invention, the effective current strength
being the same as in the case described with reference to
Figure 1 the luminous efficacy is lower than in the case
of the invention. In the case of the invention the luminous
efficacy is approximately 15 ~ higher than for the supply
with the sinusoidal alternating current.
If the effective input voltage between the ter-
minals 1 and 2 of the device of Figure 1 is insufficiently
constant use may, for example, be made of the dotted auxi-
liary device 13 by means of which the magnitude of the
ballast can be changed by means of a transistor, in depend-
ancy on the voltage across the lamp.
References 30 and 31 in Figure 2 indicate input
terminals intended for connection to a supply source which
delivers a sinusoidal ac voltage of 380 Volts, 50 Hertz.
The terminal 30 is connected to a stabilizing ballast 32
which is constructed as a coil.The other side of the coil
32 is connected to a low-pressure sodium vapour discharge
; lamp 33. The other side of the lamp 33 is connected to the
terminal 31. The lamp 33 is shunted by a series arrangement
of a resistor 34 with two light-sensitive transistors 35
and 36 which are connected in anti-parallel. The transistors
35 and 36 are incorporated in a light-tight compartment 38
together with an auxiliary lamp 37. The auxiliary lamp 37
is connected between two taps 40 and 41 of the coil 32 via
a bidirectional-breakdown element 39. The breakdown element
39 is a "diac".
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1071Z95 PHN. 8062.
10-6-1976.
Tho lamp 33 comprises a discharge tube 42 which
is enveloped by an outer bulb 43. The innerside of the outer
bulb 43 comprises an infrared-radiation reflecting coating
which is indicated by means of a dashed line. This coa-ting
consists of indium oxide. References 44 and 45 indicate end
parts of the lamp 33.
The device shown in Figure 2 operates as follows.
If the terminals 30 and 31 are connected to the source of
the sinusoidal current, a current flows through the lamp 33
via the ballast 32. If, however, the instantaneous current
strength increases to above a given value, a voltage is
found between the terminals 40 and 41 which exceeds the
breakdown voltage of the element 39. Thereupon the lamp 37
starts to give light. It illuminates the two transistors
35 and 36 which consequently become conductive, Depending
on the polarity of the mains supply one of the transistors
will realize a current-carrying shunt branch across the
lamp 33 via the resistor 34. This means that when the in-
stantaneous current strength through the lamp 33 threatens
to exceed a given maximum value, an auxiliary current starts
flowing through the shunting branch 34, 35 or 36, 34 res-
pectively. The device is so adjusted that this occurs if
the maximum instantaneous current density in accordance with
the requirements indicated threatens to be exceeded. In an
embodiment the lamp 33 is a lamp for approximately 175 I~'att,
the impedance of the coil 32 is approximately o.96 Henry.
The effective temperature of the discharge tube 42 of the
lamp 33 is again approximately 254 ~ in the operating con-
dition.
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1~71295 PHN. 8062.
10-6-1976.
In the case of Figure 1 the maximum instantaneous
current density through the discharge tube is approximately
230 mA/cm2. This is below the value S =165 + 1.3(T _ 245)2
= 270 mA/cm2. The effective current density is also appro-
ximately 230 mA/cm2. This is more than 0.8 S = 216 mA/cm2.
In the case of Figure 2 the maximum instantaneous
current density through the discharge tube is adjusted to
260 mA/cm2. This is below the value S = 165 + 1.3(T _ 245)2
= 270 mA/cm2. The effective current density through the
discharge tube 34 is approximately 230 mA/cm2. This is more
than 0,ô 5 = 216 mA/om~.
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