Neon lamp power supply for producing a bubble-free discharge without promoting mercury migration or premature core saturation

A power supply for a gas-discharge lamp includes a voltage source and drive circuit that produces an asymmetric output, a step-up transformer for stepping up the voltage to an appropriate level for driving the lamp, a blocking capacitor connected in series with the transformer and the lamp for preventing DC current from flowing through the transformer and the lamp to avoid core saturation of the transformer. A DC voltage is established across the lamp that prevents the formation of "bubbles" or "beads" in the gas discharge. An inverter is used to periodically reverse the polarity of the DC voltage to prevent mercury migration in mercury-containing lamps.

BACKGROUND 
The present invention relates to power supplies for use with gas-discharge 
display lamps. More particularly, the present invention relates to power 
supplies for use with inert gas lamps such as neon lamps, for example, and 
lamps containing mercury and an inert gas. 
Historically, in the early generations of neon signs, neon lamps or tubes 
forming the neon signs were powered with "core and coil" transformers 
operating at a low AC frequency such as the frequency of the public 
utility, for example. These transformers, however, were generally 
cumbersome to use because of their size and weight. It should be 
understood that the term "neon lamp" is used herein to refer to all 
gas-discharge lamps that use an inert gas and is not limited to lamps that 
contain only neon gas. 
Later generations of neon lamps were powered with more compact power 
supplies operating at higher AC frequencies, typically in the kilohertz 
range and above. 
One problem that occurs with the use of high frequency power supplies, 
however, is the generation of "bubbles" or "beads" in the gas discharge. 
The bubbles form a nodal pattern of alternating high and low intensity 
regions that resembles a string of beads. This nodal pattern is caused by 
standing waves that are present within the neon tube and which are 
produced by high frequency excitation of the gas. The pattern may move 
along the length of the neon tube depending on the excitation frequency of 
the power supply and the particular geometry or shape of the neon tube. In 
addition, the presence of bends and splices, for example, will affect the 
frequency at which bubbling occurs. Neon tubes are often formed into a 
complex assembly of letters or artistic shapes and designs, thus 
increasing the likelihood of bubbling. Therefore, it may not be 
technically feasible to select an operating frequency at which bubbling 
does not occur throughout the various neon tube lengths that are present 
in a complex neon tube assembly. In many cases, more than one nodal 
pattern will occur in a single neon tube, and each nodal pattern may move 
at different velocities and in different directions. 
One way to eliminate bubbling is to add a DC component to the AC input 
power. FIG. 1 illustrates a conventional circuit for generating a DC 
component in a power supply for a neon tube. Voltage output from the high 
frequency AC voltage source 10 passes through the inductor 12, which 
limits the amount of current drawn by the neon tube 14. The input voltage 
is stepped up by an output transformer 16 to an appropriate level for 
driving the neon tube 14. An automatic bias circuit 18, consisting of a 
capacitor 20 and a diode 22 connected in parallel, allows current to flow 
in one direction from the anode 24 to the cathode 26 of the diode 22. 
Current flow in the opposite direction acts to back bias the diode 22, 
thus allowing the capacitor 20 to charge up and to produce the DC voltage 
component. 
Mercury vapor is often used in neon tubes to alter the color of the light 
that is produced. Also, mercury vapor is commonly used in phosphor-coated 
neon tubes as a medium for exciting the phosphor to produce a luminous 
glow therefrom. Radiation produced in the mercury gas discharge is an 
effective excitation source for the phosphor coating. 
Neon signs often have segments of different colors that are produced by 
using various phosphors and/or gases that discharge those different 
colors, and it is desirable to have a single power supply for the entire 
assembly. 
When mercury-containing tubes are powered by a power supply having a DC 
component, such as that described above, mercury atoms tend to migrate 
toward the cathode or the negative end of the neon tube. This migration 
causes a deficiency of mercury near the positive end, which results in the 
undesirable effect of the negative end glowing brighter than the positive 
end. As discussed above, however, a DC component is necessary to prevent 
bubbling in neon tubes and therefore cannot be completely eliminated from 
power supplies used for mercury-containing lamps. 
One method for reducing the effects of mercury migration is proposed in 
U.S. Pat. No. 5,189,343 and U.S. Pat. No. 5,367,225, both assigned to 
Everbrite, Inc. The Everbrite method consists of alternating the polarity 
of the DC current flowing through the neon tube by using high-voltage 
semiconductor switches connected to the secondary windings of the output 
transformer. An alternative method proposed by Everbrite is to apply an 
asymmetrical waveform to the neon tube, which acts in conjunction with the 
geometry of the neon tube to produce a DC offset current therethrough. 
The generation of the DC current by use of an asymmetrical waveform may be 
understood by considering the voltage-current characteristics of a neon 
tube. When operated at a high frequency, the neon tube has voltage-current 
characteristics similar to a pair of Zener diodes D.sub.a, D.sub.b 
connected back to back and in series with a resistor R, as schematically 
shown in the equivalent circuit of FIG. 2. Little current will flow below 
the breakdown voltage of the diodes D.sub.a, D.sub.b, and above the 
breakdown voltage the current through the equivalent circuit, and thus 
through the neon lamp, is limited by the impedance of the external circuit 
connected thereto, such as by the impedance of the inductor 12 of FIG. 1. 
The resistor R of the equivalent circuit of FIG. 2 is not effective in 
limiting current because its impedance is, in general, low compared with 
the impedance of the inductor 12. Also, the neon tube can have bi-stable 
operating points, in which a single operating voltage can give rise to two 
different operating currents, and therefore the internal resistance of the 
neon tube (or R in FIG. 2) is not a predictable means for limiting 
current. 
According to FIG. 3, if the high-frequency AC voltage source 30 has an 
asymmetrical output waveform, such as that shown in FIG. 4, a 
corresponding asymmetrical current is produced and supplied to the 
inductor 32 and then to the output transformer 36 of FIG. 3. This 
asymmetrical current flows from the secondary windings 38 of the output 
transformer 36 to the neon tube 34. 
In theory, if the neon tube 34 is replaced with a purely resistive load, 
the asymmetrical current through the resistive load would resemble the 
waveform through the secondary windings 38. Specifically, as shown in FIG. 
4, the average current over a complete current cycle would be equal to 
zero but the peak current would have a magnitude that depends on its 
polarity. In other words, the peak current during one polarity of the 
current cycle would be larger than the peak current during the other 
polarity of the current cycle, with the overall average current being zero 
over the complete current cycle. 
In practice, the resistive load discussed above cannot adequately represent 
the neon tube 34 because the symmetrical nature of the neon tube 34 does 
not allow it to follow the asymmetrical current as faithfully as the 
resistive load would. Although the average voltage across the secondary 
windings 38 and the neon tube 34 is zero over a complete voltage cycle, 
the average current through the secondary windings 38 and the neon tube 34 
is not zero. Instead, a DC offset current is established that acts to 
compensate for the asymmetrical current supplied to the neon tube 34. This 
DC offset current produced by the asymmetrical voltage source 30 serves to 
prevent bubble formation in the neon tube 34 in a manner similar to that 
in which the DC component produced by the automatic bias circuit 18 of 
FIG. 1 serves to prevent bubble formation. 
An undesirable effect of establishing a DC offset current through the 
secondary windings 38 of the output transformer 36 is that the DC offset 
current can result in a DC offset flux produced by the transformer 36, 
which can result in premature core saturation. An air gap set up in the 
flux path may be used to prevent DC offset current-induced core 
saturation, however, such an air gap would lead to excessive losses in the 
transformer 36 due to stray flux emanating from the air gap. 
OBJECTS AND SUMMARY OF THE INVENTION 
In view of the above-mentioned deficiencies in existing neon lamp power 
supplies, it is an object of the present invention to provide an improved 
neon lamp power supply that powers neon lamps or tubes to produce a 
bubble-free gas discharge without promoting mercury migration, and that 
does not suffer from premature core saturation caused by a DC offset 
current. 
According to an aspect of the present invention, a neon lamp power supply 
includes a high-frequency voltage source for producing an asymmetrical 
voltage, a high-voltage transformer for stepping up the voltage to an 
appropriate level for driving a neon tube, and a blocking capacitor 
connected in series with the transformer and the neon tube for preventing 
DC current from flowing through the transformer and the neon tube, thus 
preventing core saturation. A DC offset voltage is established across the 
neon tube that prevents the formation of bubbles. A timer periodically 
reverses the polarity of the DC voltage to prevent mercury migration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Preferred embodiments of an improved neon lamp power supply according to 
the present invention are described below with reference to the 
accompanying drawings, in which like reference numerals represent the same 
or similar elements. 
FIG. 5 is a circuit diagram for a neon lamp power supply 50 according to a 
first embodiment of the present invention. The power supply 50 is 
comprised of a DC voltage source 52 connected to a half-bridge inverter 54 
which, in turn, is connected with a step-up transformer 56 that steps up 
the voltage output from the voltage source 52, and an inductor 58 that 
acts to limit the flow of current to the primary winding 60 of the 
transformer 56. The transformer 56 is, in turn, connected in series with a 
blocking capacitor 62 and a neon tube 64. The half-bridge inverter 54 is 
of a conventional type and is comprised of first and second transistors 
66, 68 connected with first and second capacitors 70, 72 in a half-bridge 
configuration. The first and second transistors 66, 68 are respectively 
connected to first and second switches 74, 76 of a drive circuit 90. Each 
of the first and second switches 74, 76 has an "A" state and a "B" state, 
and both the first and second switches 74, 76 are connected to a timer 78. 
The first transistor 66 is connected to a supply terminal 80 of the 
voltage source 52, and the second transistor 68 is connected to a return 
terminal 82 of the voltage source 52. The blocking capacitor 62 serves to 
block any DC offset current produced by the power supply 50. 
During operation, the first and second transistors 66, 68 are respectively 
driven by corresponding drive waveforms shown in FIGS. 7A and 7B. 
When the drive waveform for the second transistor 68 is in a "high-A" 
state, indicated by "T.sub.on-2 " in FIG. 7B, and the second transistor 68 
is in an "on" state, the corresponding drive waveform for the first 
transistor 66 is in a "low" state, as shown in FIG. 7A, and the first 
transistor 66 is in an "off" state. The voltage V.sub.Q, which is either 
the supply voltage V.sub.S or the return voltage V.sub.R, takes the value 
V.sub.R. This establishes a current flow from the junction of the first 
and second capacitors 70, 72 in the direction of I.sub.2. An existing 
current in the inductor 58 remaining from the preceding half-cycle of 
operation is discharged before the current flow in the direction of 
I.sub.2 is established. 
When the drive waveform for the second transistor 68 is in a "low" state, 
the drive waveform for the first transistor 66 is in a "high-A" state, 
indicated by "T.sub.on-1 " in FIG. 7A, and the voltage V.sub.Q takes the 
value of the V.sub.S. This establishes a current flow from V.sub.S through 
the first transistor 66, through the inductor 58, through the primary 
windings 60 of the transformer 56 to the junction of the first and second 
capacitors 70, 72 in the direction of I.sub.1. An existing current in the 
inductor 58 remaining from the preceding half-cycle of operation is 
discharged before the current flow in the direction of current I.sub.1 in 
FIG. 5 is established. 
Under equilibrium conditions the net change in charge on the first and 
second capacitors 70, 72 is zero, and a DC offset voltage V.sub.C adjusts 
itself until equilibrium is achieved. Since for the "A" waveforms or pulse 
trains the duty cycle of the first switch 74 is less than the duty cycle 
of the second switch 76, V.sub.C is less than half of V.sub.S. Therefore, 
the combination of asymmetric duty cycles for the first and second 
transistors 66, 68 which produces the DC offset voltage V.sub.C prevents 
the formation of bubbles or beads in the gas discharge of the neon tube 
64. 
In order to prevent mercury migration in neon lamps containing mercury, the 
polarity of the DC offset voltage V.sub.C is reversed by periodically 
interchanging the duty cycles of the first and second transistors 66, 68. 
Specifically, the first and second switches 74, 76 are periodically and 
alternately switched between the "A" state and the "B" state by use of the 
timer 78. 
The timer 78 may be comprised of a free-running multivibrator-type circuit 
or a switch that operates at the frequency of the public utility or a 
subharmonic thereof. The duty cycle of the "A" state and the "B" state 
must be 50% for each state in order to prevent mercury migration in 
mercury-containing neon tubes. According to a preferred embodiment, the 
timer 78 operates at a frequency that is below the audible range of 
frequencies in order to avoid generating acoustic noise in the power 
supply 50. Preferably, the timer 78 has a counting circuit that operates 
at the public utility frequency or at a related frequency, and the duty 
cycles are toggled once every several minutes. 
Immediately after a change in the duty cycle to reverse the DC polarity, 
the power supply enters a transient state in which the net charge on the 
first and second capacitors 70, 72 adjusts to compensate for the new duty 
cycle. Therefore, it is preferable to minimize this transient by 
minimizing the capacitance values for the first and second capacitors 70, 
72 by using the lowest values that are large enough to sustain normal 
operation of the half-bridge inverter 54. According to a preferred 
embodiment, capacitance values of about 2 microfarads are sufficient for 
power levels of about 200 watts. Optionally, because the first and second 
capacitors 70, 72 have low capacitance values, as discussed above, they 
may be replaced with a single capacitor in the position of either the 
first capacitor 70 or the second capacitor 72 to simplify the construction 
of the power supply 50. 
An optional smoothing circuit 96 may be connected between the DC voltage 
source 52 and the half-bridge inverter 54 to smooth the voltage supplied 
to the half-bridge inverter 54. 
The inductor 58 may be omitted if the transformer 56 has a leakage 
inductance that is sufficient to impede the flow of current to the neon 
tube 64. If the leakage inductance of the transformer 56 is not sufficient 
for limiting the current flow to the neon tube 64, however, the blocking 
capacitor 62 may be used to limit the current flow, in which case the 
transformer 56 must have a sufficiently low bandwidth so that a nearly 
sinusoidal waveform is produced. 
FIG. 6 is a circuit diagram for a neon lamp power supply 51 according to a 
second embodiment of the present invention, which is an AC analog of the 
circuit of FIG. 5. The power supply 51 is comprised of an AC voltage 
source 92 connected to a rectifier 94 which, in turn, is connected to a 
half-bridge inverter 54. Other than the AC voltage source 92 and the 
rectifier 94, the elements of the second embodiment are similar to those 
of the first embodiment shown in FIG. 5. 
The output of the power supply 51 may be controlled to achieve current or 
voltage regulation by varying the pulse widths while maintaining the 
desired asymmetry. Conventional pulse-width modulation techniques may be 
used to vary the pulse widths. 
Alternatively, the output of the power supply 51 may be controlled by 
producing a resonance so that the frequency of the waveform or pulse train 
may be adjusted toward or away from the resonance in order to adjust the 
output. The resonance may be produced by adding parallel capacitance to 
the secondary winding 61 of the transformer 56 or by using existing stray 
capacitance present in the power supply 51 and combining the stray 
capacitance with the inductor 58. Preferably, the resonance frequency has 
a value similar to the operating frequency of the AC voltage source 92. 
An optional smoothing circuit 96 may be connected between the AC voltage 
source 52 and the half-bridge inverter 54 to smooth the voltage supplied 
to the half-bridge inverter 54. 
According to a preferred embodiment, the AC voltage source 52 operates at a 
higher frequency than the frequency of the public utility. 
FIG. 8 is a circuit diagram for a neon lamp power supply 100 according to a 
third embodiment of the present invention. The power supply 100 is 
comprised of a DC voltage source 102 connected to an inverter 104 which, 
in turn, is connected to a step-up transformer 106. The transformer 106 is 
connected in series with a blocking capacitor 108 and a neon tube 110. A 
drive circuit 130 connected to a switch 120 and a timer 118 is used to 
produce an asymmetrical output waveform. The inverter 104 is comprised of 
first and second MOSFET switches 112, 114 each with a duty cycle that 
alternates between a finite value and zero. The MOSFET switches 112, 114 
alternately behave as a single transistor inverter. An inductor is not 
used to impede the flow of current to the neon tube 110 because it is 
assumed that the transformer 106 has a sufficient leakage inductance for 
that purpose. The transformer 106 must be one that can withstand the DC 
offset current produced by the inverter 104. 
The blocking capacitor 108 serves to prevent the DC offset current from 
reaching the neon tube 110 so that only the DC offset voltage acts to 
prevent bubble or bead formation in the gas discharge of the neon tube 
110. The blocking capacitor 108 does not affect the flux levels within the 
transformer 106. If the leakage inductance of the transformer 106 is not 
sufficient for limiting the flow of current to the neon tube 110, the 
blocking capacitor 108 may be used to limit the current flow and the 
transformer 106 must have a sufficiently low bandwidth so that a nearly 
sinusoidal waveform is produced. 
An example of a drive waveform produced by the drive circuit 130 is shown 
in FIG. 10. 
The timer 118 is used to periodically change the polarity of the DC offset 
voltage to prevent mercury migration in mercury-containing neon tubes. The 
timer 118 periodically reverses the asymmetry by reversing the duty cycle 
of the voltage supplied to the transformer 106. That is, the output 
waveform from the drive circuit 130 is applied alternately to the first 
and second MOSFET switches 112, 114 in accordance with the output from the 
timer 118. The timer 118 may be omitted if the power supply 100 is to be 
used with tubes containing only neon gas. 
An optional smoothing circuit 140 may be connected between the DC voltage 
source 102 and the half-bridge inverter 104 to smooth the voltage supplied 
to the half-bridge inverter 104. 
FIG. 9 is a circuit diagram for a neon lamp power supply 101 according to a 
fourth embodiment of the present invention. The power supply is an AC 
analog of the circuit of FIG. 8. The power supply 101 is comprised of an 
AC voltage source 150 connected to a rectifier 152 which, in turn, is 
connected to an inverter 104. Other than the AC voltage source 150 and the 
rectifier 152, the elements of the fourth embodiment are similar those of 
the third embodiment shown in FIG. 8. 
An optional smoothing circuit 140 may be connected between the AC voltage 
source 150 and the half-bridge inverter 104 to smooth the voltage supplied 
to the half-bridge inverter 104. 
According to a preferred embodiment, the AC voltage source 150 operates at 
a higher frequency than the frequency of the public utility. 
The embodiments described above are illustrative examples of the present 
invention and it should not be construed that the present invention is 
limited to those particular embodiments. Various changes and modifications 
may be effected by one skilled in the art without departing from the 
spirit or scope of the invention as defined in the appended claims. For 
example, an integrated oscillator/driver circuit may be used instead of 
the switches 74, 76 in the drive circuit 90 of FIGS. 5 and 6. Also, either 
mechanical switches or electronic switches, or a combination of both, may 
be used in the present invention.