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'Cold
Cathode' Sign Tubes
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As has been shown in the article '
inductor
resonance experiments', long discharge tubes provide an
effective way to demonstrate the electric field patterns around coils
and radio antennas. Clear glass is also preferable to phosphor-coated
glass; because it allows any discharge density variation to be seen and
gives greater contrast for the location of nodes. The problem however
is that the least-expensive transparent tubes are of the silica
mercury-in-argon germicidal type, these being designed to maximise the
hard-UV output at 254nm. Such lamps can be tamed by fitting a
Lee 226 UV filter sleeve,
but the need for an extra step in making the tube harmless adds
temptation for the neglect of health-and-safety measures. Also, since
the largest spectral component of the lamp output is not wanted, the
tube is inefficient at converting RF energy into visible light.
Thus an interest in
cold-cathode sign tubes; as a release from the evil basilisk glare of
the low-pressure germicidal lamp, and as a move to favour tubes that
are actually designed to be looked-at.
The cold-cathode
tubes used in advertising signs and hoardings are commonly called "neon
tubes", although only the clear-glass red-light types actually contain
neon. The most widely-used fill gas is the same as in ordinary
fluorescent and germicidal lamps, Hg in Ar, used either directly for
its cyan colour (generally called "clear blue"); or used in tubes with
internal phosphor coatings to produce a wide range of colours. The
difference between the germicidal tube and the clear Hg-Ar sign tube
however, is that the pressure in the sign tube is somewhat higher
(causing a reduction in the hard-UV emission) and the glass is UV
opaque.
In order to experiment with sign
tubes, both as field indicators and out of general interest, the author
had two straight tubes made by the UK signmaker
Neon
Creations Ltd..
These were both constructed on 60cm lengths
of 15mm OD clear-glass tubing, with pre-made straight electrode
assemblies giving a total inter-electrode distance of 64 cm. The
electrodes used were rated for running at up to 90mA. One tube was
filled with pure neon gas at a pressure of 11 mbar (8.25 Torr). The
other was dosed with a few tiny drops of liquid mercury, to produce
mercury vapour at saturated-vapour-pressure (SVP), and filled with
Argon to 12 mbar (9 Torr). These filling details, incidentally, were
decided by the tube-designer with a view to providing the best light
output. The two tubes have very different strike voltages, leading to
different electrical properties.
Sign tubes are
normally powered using something approximating a constant-current
source. They are also operated with a reasonably symmetric
(non-polarised) AC waveform in order to prevent unequal electrode
erosion. In early neon signs, the supply was a shunt-transformer
operating at power-line frequency. This is a transformer providing
several kV off-load; but engineered to have very high leakage
inductance, so that it behaves as a constant-voltage source in series
with a ballast inductor. Nowadays, the usual supply is an
electronic
constant-current inverter, operating at several tens of kHz,
and giving
several kV off-load.
An interesting point of
relevance here is that electronic ballasts for fluorescent lamps do not
use
cathode pre-heating to get the tube to start. Instead they strike the
tube by generating a high off-load voltage. This initiates a glow
discharge that heats the thermionic cathodes and causes the starting
process to complete. Thus the typical switch-on behaviour of an
electronically-energised FL is that it gives a feeble steady light
initially (no flickering) and then switches to full brightness about 1s
later. The high-voltage starting facility means that an electronic FL
ballast will start an Hg-Ar sign tube, provided that the tube is not
excessively long.
The straight Hg-Ar sign tube is shown below, operating from an
emergency-exit-light
inverter (Type S484 - 6V DC input, push-pull transistor
oscillator, ca. 30 kHz, with HV winding and 1 nF capacitor ballast).

Inverter input: 6 V, 2.0 A.
Running freq: 30.3 kHz.

Above is a close-up of one of the
tube electrodes. The term 'cold cathode' is something of a misnomer
because the inside of the electrode cup is coated with thermionic
emitting oxides, the surface being heated by ion bombardment in the
same way as the glow-start cathode in a fluorescent tube. The term
'non-preheatable' might be more accurate, but is unlikely to catch-on.
The electrode assemblies are purchased by the sign-maker pre-made,
complete with glass pinch and stub tube, ready to be welded to the sign
tube. The metal shell is nickel-plated iron. The ceramic guard disk
crimped into the end of the cup prevents erosion of the lip (a
field-emission breakdown point). The electron-emissive material coating
the inside of the cup is in carbonate form when the electrode is
supplied, but is converted into an oxide mixture by heating the
electrode to red-heat during a part of the manufacturing process known
as 'bombardment'.
In the photograph
above, the black deposits and occasional shiny droplets on the inside
of the tube are the mercury reservoir.
The Hg-Ar mixture has a relatively
low striking voltage (about 1 kV peak for the tube discussed above). A
tube filled with pure neon however is a little more difficult to
illuminate. For a pressure of 8.25 Torr (mm Hg) and an electrode
separation of 64cm, reference to the
Paschen
curve gives a breakdown voltage of about 2.1 kV
(neglecting non-uniformity of the electric field). This is too
high for starting with a small FL inverter; but the running voltage is
only about 1 kV. Thus we might run the tube from an electronic FL
ballast, but (apparently) we can't start it.
A simple solution to
the starting problem is to add an external priming electrode. This will
work with a high-frequency source because an electrode attached to the
outside of the glass can perform the same function as an internal
electrode in series with a small capacitance. Such an electrode is used
in the pictures below. It consists of a thin wire, connected to the
electrode at one end, then wound around the glass in a wide-spaced
helix and brought close to the electrode at the other end. It is held
in place temporarily by a pair of nitrile-rubber O-rings. The wire
shown has the somewhat gargantuan diameter of 0.3 mm, but there is no
reason why it can't be practically invisible.
The sequence below
shows what happens as the input voltage to the FL inverter is varied.

Data below are input voltage, input current and
running frequency:
A: 4.0 V, 95 mA, 46.2 kHz ; A glow discharge
starts between the wire and the outer electrode shell.
B: 3.7 V, 125 mA, 45.7 kHz ; Increasing the
voltage starts a discharge between the wire and the electrode interior.
The voltage needed to sustain this discharge is less than the voltage
required to start the initial glow.
C: 4.4 V, 0.19 A, 45.1 kHz ;
D: 4.9 V, 0.24 A, 44.7 kHz ;
E: 5.6 V, 0.31 A, 44.5 kHz ;
F: 6.0 V, 1.8 A, 36.9 kHz ; Both electrodes
produce thermionic electrons, and the transition from glow discharge to
arc occurs. Notice the enormous jump in the inverter input current from
E to
F.
To get a crude idea of the inverter output voltage, note that the
inverter uses a pair of BCU83 NPN transistors in push-pull
configuratuin. These have a nominal saturation voltage of 0.5V, so
the voltage switched across the transformer primary will be
approximately V
in - 0.5. Therefore, the output
will
be approximately:
V
out = V
max (V
in
- 0.5) / (6 - 0.5)
V
max is about 1 kV peak (for 6V DC in).
Neon Sign
making at Neon
Creations.

On the left in the
picture is the vacuum
pumping system (NeonPro NVS2000) with gas dosing manifold and cylinders
of neon and
argon. In the process of being bent, and in the box in the foreground,
are tubes with internal phosphor coating. These fluoresce when
irradiated with UV light from the Hg-Ar discharge and produce colours
other than cyan-blue and red.
(Photo DWK, 2013 Sept 13th)
Pumped, bombarded,
filled and sealed. In the background the tubes are
illuminated for 'ageing in'. In the forground are tubes with block-out
paint applied to create dark regions (between letters,
etc.).
(Photo
DWK, 2013 Sept 13th)
Final
test and dispatch (Photo DWK 2013 Sept 13th)
© D. W. Knight, 2013, 2021
Last edited: 2021 Aug 1st .