These include both neon advertising lights and lamps for illumination used in large
stores, cinemas and similar areas.
If a sufficiently large potential difference is applied between the electrodes of a
discharge tube, the arc can be struck and maintained without any heating of the cathode.
With a hot cathode, the volt drop across the electrode is small, but with a cold cathode, it
is higher. A long tube helps to keep a larger part of the total applied voltage drop across
the arc and a smaller part across the electrode, and cold cathode lamps are, therefore, made longer than the hot cathode lamps described previously. The greater length, in fact,
increases the efficiency and hence also the light output. It is because of this that they are
used in stores and cinemas and under the projecting canopies which are now so popular at
the entrances to commercial buildings, including hotels.
The high voltage required is provided by transformers, and in order to keep the
amount of high voltage wiring to a minimum the lights are supplied with transformers in
self-contained units suitable for direct connection to the mains. A common arrangement
has three 2.75m-long tubes physically parallel to each other, but electrically connected in
series. The circuit diagram of such a luminaire is shown in Figure 7.11. The two
transformers and all the high voltage wiring are sealed within the luminaire and are not
accessible.
The two primaries are in parallel across the mains supply, and each has a power factor
correcting capacitor. The high-voltage windings each have one end connected to earth
and are so arranged that the voltage between their two other ends is double the maximum
voltage to earth. The transformers give a voltage high enough to strike the arc. As in the
case of the transformers used with the hot cathode sodium lamps, they are designed to
Figure 7.11 Cold cathode lamps
have poor
regulation so that once the arc has been struck, the voltage drops to that
required to maintain the arc. For a typical three-tube luminaire the striking voltage is
3600V and the running voltage 2000V.
The great advantage of the cold cathode tube is its very long life, which is in the
region of 15000 hours as against about 5000 hours for an ordinary fluorescent tube. It
also maintains its output better throughout its life, and starts instantly. Its life is not
reduced by frequent switching. When the tube is used for lighting it is filled with mercury vapour and has a suitable fluorescent coating on its inside surface. As with ordinary
fluorescent tubes a variety of colours is available.
The length of cold cathode tubes makes them suitable for bending into special shapes.
This makes them useful where special decorative effects are wanted and also, of course,
for advertising purposes. For advertising use many different colours can be obtained by
the use of different types of glass, which may be coloured or have fluorescent coatings.
The earliest gas used was neon, and ‘neon light’ has stuck as the popular generic name
for all advertising lights of this type.
The tubes can be bent into the shape of letters or other symbols and successive tubes
can be connected in series, an example of such an arrangement being shown in Figure
7.12. For letters like H or T the tube must be bent back on itself.
stores, cinemas and similar areas.
If a sufficiently large potential difference is applied between the electrodes of a
discharge tube, the arc can be struck and maintained without any heating of the cathode.
With a hot cathode, the volt drop across the electrode is small, but with a cold cathode, it
is higher. A long tube helps to keep a larger part of the total applied voltage drop across
the arc and a smaller part across the electrode, and cold cathode lamps are, therefore, made longer than the hot cathode lamps described previously. The greater length, in fact,
increases the efficiency and hence also the light output. It is because of this that they are
used in stores and cinemas and under the projecting canopies which are now so popular at
the entrances to commercial buildings, including hotels.
The high voltage required is provided by transformers, and in order to keep the
amount of high voltage wiring to a minimum the lights are supplied with transformers in
self-contained units suitable for direct connection to the mains. A common arrangement
has three 2.75m-long tubes physically parallel to each other, but electrically connected in
series. The circuit diagram of such a luminaire is shown in Figure 7.11. The two
transformers and all the high voltage wiring are sealed within the luminaire and are not
accessible.
The two primaries are in parallel across the mains supply, and each has a power factor
correcting capacitor. The high-voltage windings each have one end connected to earth
and are so arranged that the voltage between their two other ends is double the maximum
voltage to earth. The transformers give a voltage high enough to strike the arc. As in the
case of the transformers used with the hot cathode sodium lamps, they are designed to
Figure 7.11 Cold cathode lamps
have poor
regulation so that once the arc has been struck, the voltage drops to that
required to maintain the arc. For a typical three-tube luminaire the striking voltage is
3600V and the running voltage 2000V.
The great advantage of the cold cathode tube is its very long life, which is in the
region of 15000 hours as against about 5000 hours for an ordinary fluorescent tube. It
also maintains its output better throughout its life, and starts instantly. Its life is not
reduced by frequent switching. When the tube is used for lighting it is filled with mercury vapour and has a suitable fluorescent coating on its inside surface. As with ordinary
fluorescent tubes a variety of colours is available.
The length of cold cathode tubes makes them suitable for bending into special shapes.
This makes them useful where special decorative effects are wanted and also, of course,
for advertising purposes. For advertising use many different colours can be obtained by
the use of different types of glass, which may be coloured or have fluorescent coatings.
The earliest gas used was neon, and ‘neon light’ has stuck as the popular generic name
for all advertising lights of this type.
The tubes can be bent into the shape of letters or other symbols and successive tubes
can be connected in series, an example of such an arrangement being shown in Figure
7.12. For letters like H or T the tube must be bent back on itself.
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