Device for alternating bulb polarities of a DC lighting system

A device for alternating the polarity of an illuminated DC fluorescent light bulb to eliminate cathodic degeneration while avoiding the visible effects of flicker to maintain a constant lighting effect.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates to DC lighting systems, and more 
particularly, to a device for periodically alternating the polarity of a 
DC power signal used to drive a DC light bulb. 
2. Description of the Background 
Fluorescent lighting systems are in wide use due to the high-efficiency 
cool light which is generated. Conventional fluorescent lighting systems 
may be one of two types including those operated from AC power, or those 
operated from DC power. AC-powered systems are the most common in houses, 
factories and public places. However, AC-powered systems are prone to 
flicker, which may result in harm to the eyes. In contrast, the continuous 
DC signal applied to DC systems solves the problem of flicker because the 
bulb is always biased with a signal of the same polarity. Unfortunately, 
the advantage of uniform polarity does not come without cost. When a 
constant power signal is applied to a fluorescent bulb, severe aging and 
eventual degradation of the cathode will result. 
It would be greatly advantageous to provide a solution to the 
above-described problem, and co-pending U.S. Patent Application Ser. No. 
07/575,425 attempts such a solution with a device for periodically 
alternating bulb polarities of a DC fluorescent lighting system. Disclosed 
therein is a means for randomly setting the bulb polarity of a fluorescent 
light bulb prior to application of power. When power is applied, either 
bulb terminal has an equal chance of becoming the cathode. This results in 
uniform degradation of the cathode over time. 
It would be equally advantageous if the bulb polarity could be alternated 
more frequently than at each power-up. However, for this a solution must 
be provided for the problem of flicker which is inherent in AC fluorescent 
lighting systems. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a device 
for alternating the polarity of an illuminated DC fluorescent light bulb 
while maintaining a steady lighting effect. 
It is another object of the present invention to eliminate flicker due to 
the periodic reversal of polarity as in AC fluorescent lighting systems 
while avoiding the problem of unilateral degradation of bulb terminals 
created by fixed DC power.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a block circuit diagram of the preferred embodiment of 
the present invention is shown. 
DC power supply unit 101 may be any power supply capable of generating the 
requisite DC power for driving a fluorescent light. Preferably, power 
supply 101 provides direct rectification and filtering of power supplied 
from a conventional AC source. 
Polarity switch 102 is connected directly to the output terminals of power 
supply 101. Polarity switch 102 may be an electrodynamic-type relay or any 
other circuit capable of selectively switching the polarity of DC power 
input from power supply 101. 
Lighting apparatus 103 is connected to the output of polarity switch 102 
for receiving power of a selected polarity therefrom. Lighting apparatus 
103 may be a conventional fluorescent light assembly including a bulb 
holder, a fluorescent bulb carried by the bulb holder, a limiter, a power 
cord, and a cover. 
Polarity signal setting device 104 is connected between power supply 101 
and polarity switch 102 for selecting the polarity of DC power supplied to 
lighting apparatus 103 by controlling the position of switch 102, thereby 
switching the polarity of DC power input from power supply 101. 
Polarity signal setting device 104 comprises a random number generator 
capable of outputting a randomly selected HIGH or LOW signal and driving 
polarity switch 102 with the HIGH or LOW signal for predetermined length 
of time. 
In operation, power supply 101 provides a regulated DC (+) and (-) output 
from, for example, a single phase AC supply. The regulated DC power is 
supplied to polarity switch 102. Polarity switch 102 switches the 
regulated DC power through to lighting apparatus 103 in accordance with 
the HIGH or LOW control signal supplied by polarity signal setting device 
104. For example, if the polarity signal setting device 104 outputs a HIGH 
signal, a control signal, polarity switch 102 will switch the regulated DC 
power directly through to lighting apparatus 103. Conversely, if the 
polarity signal setting device 104 outputs a LOW signal, polarity switch 
102 will reverse the polarity of the regulated DC power supplied to 
lighting apparatus 103. 
Polarity signal setting device 104 determines the positive and negative 
polarities of the regulated DC power supplied to lighting apparatus 103 in 
accordance with an internal or external timing means (not shown). In the 
preferred embodiment, the timing means is internal to polarity signal 
setting device 104 and is simply a combination clock/timer which generates 
a higher frequency timing signal for a predetermined amount of time after 
power is applied to power supply 101. The higher frequency timing signal 
is supplied during the initial power-up of the lighting apparatus 103. 
This way, the terminals of the bulb alternately serve as cathode and 
anode, and degradation is greatly reduced in the same manner as an AC 
fluorescent lighting system. However, to eliminate the ensuing problem of 
flicker, the timing means internal to the polarity signal setting device 
104 reduces the frequency of the timing signal after initial power-up has 
occurred. The lower frequency timing signal is thereafter supplied as long 
as the system remains illuminated. As before, the terminals of the bulb 
alternately serve as cathode and anode, and degradation is reduced. In 
addition, the lower frequency of the timing signal eliminates the 
noticeable effect of flicker. 
Referring to FIG. 2, the output signals from the various circuit blocks of 
FIG. 1 are shown to illustrate the essential timing of the above-described 
device. The AC signal A is supplied from a conventional AC power source to 
power supply 101. Power supply 101 regulates and filters the AC power 
signal A, and outputs a regulated DC power signal B to polarity switch 
102. 
At the same time that the AC signal A is applied to power supply 101, 
polarity signal setting device 104 begins to supply timing signals (as 
shown in graph D) to polarity switch 102. 
As illustrated by signal C, each time a timing signal D is supplied from 
polarity signal setting device 104, polarity switch 102 reverses the 
polarity of its output signal which is applied to lighting apparatus 103. 
Initially, and for a short time after AC power signal A is first applied to 
power supply 101, timing signals D are supplied from polarity setting 
signal device 104 at a high frequency, for example, 5 KHZ. Hence, polarity 
switch 102 will reverse the polarity of the output signal applied to 
lighting apparatus 103 at a high rate during initial illumination, thereby 
reducing degeneration at the bulb terminals. 
After the lighting apparatus 103 has been fully illuminated, timing signals 
D are supplied from polarity setting signal device 104 at a lower 
frequency, for example, once every 180 seconds. Polarity switch 102 then 
continues to reverse the polarity of the output signal applied to lighting 
apparatus 103 at the lower rate, thereby eliminating visible flicker. 
It should be noted that other suitable frequencies may be selected, and 
frequencies may be gradually reduced after initial illumination. Moreover, 
the above-described device may be equally applicable in other types of 
lighting systems besides conventional fluorescent systems, such as opal 
lamps, mercury lamps, air discharge lights, arc lamps, etc. 
Having now fully set forth the preferred embodiments and certain 
modifications of the concept underlying the present invention, various 
other embodiments as well as certain variations and modifications of the 
embodiment herein shown and described will obviously occur to those 
skilled in the art upon becoming familiar with said underlying concept. It 
is to be understood, therefore, that within the scope of the appended 
claims, the invention may be practiced otherwise than as specifically set 
forth herein.