Sodium discharge lamps have a similar action to mercury lamps, but the filling used is
sodium instead of mercury. Lamps designated SOX and SLI work at low pressure and
their luminance is low. They therefore have to be very long to give a good light output; in
order to reduce the overall length the tube of an SOX lamp is bent into the shape of a U
and the resulting construction is shown in Figure 7.8. The SLI lamp has a straight tube
and is therefore double ended.
To withstand the sodium vapour the inner tube is made of ordinary glass with a thin
coating of special glass fused onto its inner surface. The inner tube is enclosed in a
double-walled vacuum flask. Each electrode consists of a coated spiral whose ends are
twisted together; there is no flow of heating current through the electrode as there is in a
mercury lamp. Neon is contained within the inner tube with the sodium and starts the
discharge. When the lamp is cold the sodium condenses and exists as small globules
along the length of the tube. It is important that they should be fairly evenly distributed
along the tube, and, therefore, the lamp must be kept nearly horizontal. At the same time,
the sodium must not be allowed to condense on the electrodes. To satisfy this
requirement luminaires for sodium lamps are arranged to hold the lamp tilted slightly
above the horizontal.
The operating pressure is very low, being in the region of 1mm mercury, although the
vapour pressure of the sodium alone is of the order of 0.001mm mercury, the rest being
due to the neon. To start an arc through the neon when the lamp is cold requires a voltage
higher than normal mains voltage (about 450V). The necessary striking voltage is
obtained from an autotransformer which is specially designed to have poor regulation,
that is to say the voltage when current flows drops greatly below the no-load voltage.
Consequently, as soon as the discharge starts the voltage drops to that required to keep
the discharge going. The transformer thus performs the functions of the ballast and no
separate choke is required. A capacitor for power factor correction is, however, needed.
The discharge which starts in the neon is of a red colour. This warms the tube and
gradually vaporizes the sodium. After about twenty minutes, the sodium is fully
vaporized and gives its characteristic yellow colour. The sodium discharge lamp is the
most efficient means so far known of converting electrical into light energy, but because
of its peculiar colour the low pressure sodium lamp is limited to street lighting and
similar applications. The control gear, consisting of autotransformer and power factor
capacitor, is usually accommodated within the column which supports the luminaire.
Alternatively, it can be housed within the luminaire and this is done in floodlights and
other luminaires intended for mounting at low level.
High-pressure sodium lamps give a rather sunny yellow light. For this reason when
first introduced they were called solarcolour lamps, but they are now more generally
referred to by the designation SON. They are suitable for factories and warehouses and
are now also widely used for street lighting and floodlighting. The pressure within the arc
tube when the lamp is fully warmed up is between 30 and 60kPa. The lamp runs at a
temperature of 1300°C and to withstand the corrosive properties of sodium at this
temperature alumina ceramic is used in the manufacture of the tube.
As with the low-pressure sodium lamp, the SON lamp does not have heated cathodes
or auxiliary electrodes, but starts cold with a high voltage pulse. A typical circuit is
shown in Figure 7.9. The circuit comprises a
Figure 7.10 Performance of SON lamp
thyristor starting circuit, which pulses a high voltage across the lamp, once the lamp
strikes the pulse generator is shut down and the current through the lamp is restricted by
the ballast.
High pressure sodium lamps can work either vertically or horizontally. They may be
mounted at any angle below 20° above the horizontal. The tube does not have to be as
long as the low-pressure sodium tube and the construction and shape are similar to those
of the mercury discharge lamp.
High pressure sodium lamps have a very high efficiency and a long life. Most highpressure
sodium lamps have rated lives of more than 24,000 hours. Both light output and
the power consumed increase very rapidly with an increase in the supply voltage and the
design of the ballast has to be such as to limit variation in the applied voltage in order to
preserve lamp life. Figure 7.10 shows the variation of light output with voltage.
sodium instead of mercury. Lamps designated SOX and SLI work at low pressure and
their luminance is low. They therefore have to be very long to give a good light output; in
order to reduce the overall length the tube of an SOX lamp is bent into the shape of a U
and the resulting construction is shown in Figure 7.8. The SLI lamp has a straight tube
and is therefore double ended.
To withstand the sodium vapour the inner tube is made of ordinary glass with a thin
coating of special glass fused onto its inner surface. The inner tube is enclosed in a
double-walled vacuum flask. Each electrode consists of a coated spiral whose ends are
twisted together; there is no flow of heating current through the electrode as there is in a
mercury lamp. Neon is contained within the inner tube with the sodium and starts the
discharge. When the lamp is cold the sodium condenses and exists as small globules
along the length of the tube. It is important that they should be fairly evenly distributed
along the tube, and, therefore, the lamp must be kept nearly horizontal. At the same time,
the sodium must not be allowed to condense on the electrodes. To satisfy this
requirement luminaires for sodium lamps are arranged to hold the lamp tilted slightly
above the horizontal.
The operating pressure is very low, being in the region of 1mm mercury, although the
vapour pressure of the sodium alone is of the order of 0.001mm mercury, the rest being
due to the neon. To start an arc through the neon when the lamp is cold requires a voltage
higher than normal mains voltage (about 450V). The necessary striking voltage is
obtained from an autotransformer which is specially designed to have poor regulation,
that is to say the voltage when current flows drops greatly below the no-load voltage.
Consequently, as soon as the discharge starts the voltage drops to that required to keep
the discharge going. The transformer thus performs the functions of the ballast and no
separate choke is required. A capacitor for power factor correction is, however, needed.
The discharge which starts in the neon is of a red colour. This warms the tube and
gradually vaporizes the sodium. After about twenty minutes, the sodium is fully
vaporized and gives its characteristic yellow colour. The sodium discharge lamp is the
most efficient means so far known of converting electrical into light energy, but because
of its peculiar colour the low pressure sodium lamp is limited to street lighting and
similar applications. The control gear, consisting of autotransformer and power factor
capacitor, is usually accommodated within the column which supports the luminaire.
Alternatively, it can be housed within the luminaire and this is done in floodlights and
other luminaires intended for mounting at low level.
High-pressure sodium lamps give a rather sunny yellow light. For this reason when
first introduced they were called solarcolour lamps, but they are now more generally
referred to by the designation SON. They are suitable for factories and warehouses and
are now also widely used for street lighting and floodlighting. The pressure within the arc
tube when the lamp is fully warmed up is between 30 and 60kPa. The lamp runs at a
temperature of 1300°C and to withstand the corrosive properties of sodium at this
temperature alumina ceramic is used in the manufacture of the tube.
As with the low-pressure sodium lamp, the SON lamp does not have heated cathodes
or auxiliary electrodes, but starts cold with a high voltage pulse. A typical circuit is
shown in Figure 7.9. The circuit comprises a
Figure 7.10 Performance of SON lamp
thyristor starting circuit, which pulses a high voltage across the lamp, once the lamp
strikes the pulse generator is shut down and the current through the lamp is restricted by
the ballast.
High pressure sodium lamps can work either vertically or horizontally. They may be
mounted at any angle below 20° above the horizontal. The tube does not have to be as
long as the low-pressure sodium tube and the construction and shape are similar to those
of the mercury discharge lamp.
High pressure sodium lamps have a very high efficiency and a long life. Most highpressure
sodium lamps have rated lives of more than 24,000 hours. Both light output and
the power consumed increase very rapidly with an increase in the supply voltage and the
design of the ballast has to be such as to limit variation in the applied voltage in order to
preserve lamp life. Figure 7.10 shows the variation of light output with voltage.
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