Sinusoidal Alternating Current Ballast For Fluorescent Emergency Lighting

Abstract

A ballast for a lamp has a battery that provides direct current in response to a loss of power. The ballast has a conversion circuit that is configured to receive the direct current from the battery and convert it into alternating current that is provided to a lamp. The alternating current is substantially sinusoidal, and has a current crest factor of 1.7 or less. The conversion circuit can use a push-pull topology. The ballast can also include a cathode-heating circuit for pre-heating a cathode associated with the lamp. The cathode-heating circuit pre-heats a cathode of the lamp by increasing the current output of the conversion circuit. The ballast can also include circuitry for converting mains power to power appropriate to illuminate the lamp.

Description

TECHNICAL FIELD

The disclosure relates generally to apparatus and methods for providing emergency power to linear fluorescent lamps. More specifically, the disclosure relates to apparatus and methods for providing current to fluorescent lamps using an emergency battery such that the life of the lamp is not shortened unnecessarily.

BACKGROUND

Most modern buildings have the capability to provide emergency lighting in the event of an interruption to the main power supply. Emergency lighting not only improves safety, but is required by most building codes. Conventional emergency lighting systems work by identifying a failure in the main power supply and switching to a battery backup to supply power to some of the lamps in the building until such time as main power (sometimes referred to as mains power) is restored.

Although simple in concept, there are many issues that emergency lighting systems must resolve. One such issue relates to the fact that most buildings that have emergency lighting use linear fluorescent lamps to provide illumination. Fluorescent lamps are designed to run on alternating current. The batteries that provide emergency power provide direct current. Accordingly, the circuitry, or “ballast,” that provides current from the battery to the fluorescent lamp must convert the battery's direct current into alternating current capable of illuminating the fluorescent lamp.

Conventional emergency ballasts suffer from several limitations that can reduce their effectiveness. First, conventional emergency ballasts do not transform the battery's direct current into substantially sinusoidal alternating current. FIGS. 1a, 1b, and 1c show the output waveforms of several conventional emergency ballasts. As the waveforms illustrate, none of these emergency ballasts output a substantially sinusoidal signal. FIG. 1a is effectively a triangle wave. FIG. 1b has a strong direct current bias in the negative region. FIG. 1c similarly has a strong negative DC current bias. Although each of the signals shown in FIGS. 1a, 1b, and 1c may be capable of illuminating a linear fluorescent lamp, the shape of the waveforms these emergency ballasts provide can have serious deleterious effects on a linear fluorescent lamp.

These deleterious effects stem from the fact that wave patterns such as those shown in FIGS. 1a, 1b, and 1c accelerate the processes that shorten the life of fluorescent lamps. Conventional fluorescent lamps require a certain amount of mercury vapor to be present within the lamp to allow the lamp to start, or “strike.” Over the life of any fluorescent lamp, the mercury begins to deposit itself at one end of the lamp. Eventually, as more and more mercury is deposited, the mercury level within the lamp falls to the point of no longer being sufficient to allow the lamp to strike. When the balance between the positive and negative halves of the supply current are approximately equal—as they are in a substantially sinusoidal wave—this process occurs at the slowest possible pace. When the signal has a strong direct-current bias, the process of mercury deposit accelerates.

A conventional way of determining whether the balance between positive and negative current is appropriate is by graphing the current waveform and verifying symmetry with no DC bias. Conventionally, determining how sinusoidal a wave form is involves calculating the current crest factor. The current crest factor is half of the peak-to-peak value of the wave, divided by the root mean square (RMS) value for that wave. An ideal current crest factor (i.e., the current crest factor for a substantially sinusoidal wave) is approximately 1.414. The ANSI (American National Standards Institute) standard current crest factor for fluorescent lamps is 1.7 or less. When the signal applied to the lamp has a stronger direct current bias, such as the signals shown in FIGS. 1a, 1b, and 1c, it is also common for the current crest factor to exceed 1.7. The lamps with a current crest factor above 1.7 will exhibit cathode erosion which will cause “sputtering” and shorten the lamp life. DC biasing will cause mercury to migrate toward one end of the lamp or the other. This causes accelerated deposition of mercury on one end of the lamp, which can significantly shorten the life of a lamp. Referring again to FIGS. 1a, 1b, and 1c, the current crest factors of these waves are approximately 2.02, 2.21, and 1.95, respectively, and accordingly, have current crest factors that are too high for proper fluorescent operation.

In practice, an emergency ballast that provides a current with a heavy DC bias to a linear fluorescent lamp can result in significantly increased expenses. For example, a linear fluorescent lamp that would normally have a lifetime measured in years could fail in as little as, by way of example, 90 minutes of emergency operation. Accordingly, even a single power outage could result in the failure of conventional linear fluorescent lamps connected to the emergency ballast. Further, for environmental reasons, fluorescent lighting technology is moving toward the use of less and less mercury in linear fluorescent lamps. Accordingly, with less mercury in the lamps to begin with, the problem is exacerbated, resulting in even shorter lamp life when under emergency operation.

A further problem with conventional emergency ballasts is that they do not provide cathode-heating features that are needed to maintain a reasonable life cycle for reduced-mercury lamps. As one of skill in the art would recognize, fluorescent lamps operate by heating up a cathode and causing it to emit electrons. The electrons ionize noble gas atoms in the lamp, causing the atoms to emit a photon that strikes the phosphors on the glass, causing light. To ionize properly, the gasses have to be heated to a certain temperature, by way of example only, 25-30 degrees centigrade for a standard T8 lamp. If a lamp were to be cold-started, the initial voltage required to cause ionization would be very high. Mercury is used in fluorescent lamps to allow the noble gasses to ionize at lower voltages.

Mercury, however, is highly toxic, and efforts are continuously made to reduce, if not eliminate, the mercury in fluorescent lamps. Because conventional emergency ballasts operate at a lower voltage, they may not provide sufficient energy to strike a low-mercury lamp. Further, even if a conventional emergency ballast did provide sufficient energy to strike a low-mercury lamp, the extra energy required can cause the cathode in the lamp to age prematurely, unnecessarily reducing the life of the lamp.

SUMMARY

The present invention provides an emergency ballast that provides power to a lamp that can minimize unnecessary aging of a lamp. In one exemplary embodiment, an emergency ballast can receive direct current from a battery in response to a loss of mains power. A conversion circuit can convert the direct current into alternating current. The alternating current is substantially sinusoidal, and can have a current crest factor of 1.7 or less. The conversion circuit can use a push-pull topology to create the alternating current.

The ballast can also include a cathode-heating circuit that can pre-heat the cathode of a lamp. The cathode-heating circuit can increase the current output of the conversion circuit. The ballast can also include circuitry for converting mains power to power that is appropriate for powering the lamp.

The present invention also provides a method for providing emergency power to a lamp. First, it is determined whether an interruption in mains power has occurred. In response to determining that an interruption in mains power has occurred, direct current from a battery is provided to a conversion circuit. The direct current is then converted to substantially sinusoidal alternating current, which can then be provided to a lamp. The alternating current can have a current crest factor of 1.7 or less. The lamp can be a fluorescent lamp. The method can also heat a cathode of the lamp. The method can heat the cathode by increasing the current that is supplied to the cathode of the lamp. The method can also convert mains power to power that is appropriate for lighting the lamp in response to determining that mains power has not been interrupted.

The present invention also provides a circuit for providing current to a fluorescent lamp. The circuit can include a battery for delivering direct current in response to an interruption in mains power. The circuit can also include a conversion circuit for converting the direct current to alternating current. The conversion circuit can employ a push-pull topology. The alternating current can have a substantially sinusoidal waveform with a current crest factor of 1.7 or less. The alternating current can then be supplied to a fluorescent lamp. The circuit can also include a cathode heating circuit for pre-heating a cathode of the lamp. The circuit can also include a ballast for converting mains power to current appropriate for powering a fluorescent lamp.

These and other aspects, features, and embodiments of the invention will become apparent to a person of ordinary skill in the art upon consideration of the following detailed description of illustrated embodiments exemplifying the best mode for carrying out the invention as presently perceived.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the exemplary embodiments of the present invention and the advantages thereof, reference is now made to the following description in conjunction with the accompanying drawings in which:

FIG. 1 a is a waveform that demonstrates the output of a first conventional emergency ballast;

FIG. 1 b is a waveform that demonstrates the output of a second conventional emergency ballast;

FIG. 1 c is a waveform that demonstrates the output of a third conventional emergency ballast;

FIG. 2 is a flow chart describing an exemplary method for providing substantially sinusoidal output from an emergency fluorescent ballast according to one exemplary embodiment;

FIG. 3 is a circuit diagram illustrating the circuits in an exemplary emergency ballast that provides substantially sinusoidal output from an emergency fluorescent ballast according to one exemplary embodiment;

FIG. 4 is a circuit diagram illustrating a magnified view of an exemplary circuit that implements the method of FIG. 2 for providing substantially sinusoidal output from an emergency fluorescent ballast according to one exemplary embodiment;

FIG. 5 is a waveform that demonstrates the output of the exemplary circuit of FIG. 4;

FIG. 6 is a flow chart describing an exemplary method for providing cathode heating from an emergency fluorescent ballast according to one exemplary embodiment;

FIG. 7 is a circuit diagram illustrating an exemplary circuit for providing cathode heating according to one exemplary embodiment.

The drawings illustrate only exemplary embodiments of the invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments. The elements and features shown in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of exemplary embodiments of the present invention. Furthermore, electrical components shown in the drawings and figures represent exemplary circuits only. As one of skill in the art would understand, the electrical characteristics and response of certain components can often be replicated by the use of other components and/or combinations of components. In the drawings, reference numerals designate like or corresponding, but not necessarily identical, elements.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention is directed to electrical lighting devices. In particular, certain exemplary embodiments of this invention are directed to providing substantially sinusoidal current to fluorescent lamps in the event of a failure of main power. Certain exemplary embodiments include an emergency ballast that can be powered by a battery or other source of direct current, while still outputting substantially sinusoidal current. In certain embodiments, the sinusoidal current has a current crest factor of 1.7 or less. In certain other exemplary embodiments, the emergency ballast includes a cathode heating circuit that improves the emergency ballast's ability to illuminate a low-mercury fluorescent lamp.

The invention may be better understood by reading the following description of non-limiting, exemplary embodiments with reference to the attached drawings, wherein like or corresponding, but not necessarily identical, parts of each of the figures are identified by the same reference characters, and which are briefly described as follows. FIG. 2 is a flow chart describing an exemplary method 200 for providing a sinusoidal output from an emergency fluorescent ballast. In step 205, a fluorescent lamp is operated with the fluorescent AC ballast, which converts mains power to power that is appropriate for illuminating a fluorescent lamp.

In step 210, it is determined whether an interruption in AC power has occurred. In one exemplary embodiment, this determination is made by the relays in the system, which receive AC current across their windings, and switch from the Normally Closed position to the Normally Open position when power is cut off. If the determination in step 210 is negative, the NO branch is followed back to step 205, wherein operation of the lamp continues from the AC fluorescent ballast. If, however, the determination in step 210 is affirmative, the method 200 follows the YES branch to step 215, wherein power to the lamp is switched from the AC fluorescent ballast to the emergency ballast. The method 200 then proceeds to step 220, wherein the emergency ballast powers the lamp by converting DC power from a DC power source, such as a battery, to a substantially sinusoidal AC signal having a current crest factor of 1.7 or less.

The method 200 then proceeds to step 225, wherein it is determined whether AC power has been restored. If the determination in step 225 is affirmative, the method 200 follows the YES branch to step 205. If, on the other hand, the determination in step 225 is negative, the method 200 follows the NO branch to step 220.

FIG. 3 is an exemplary circuit diagram for an emergency ballast 300 that outputs a substantially sinusoidal AC signal having a current crest factor of 1.7 or less according to one exemplary embodiment. The emergency ballast 300 includes a battery 302. In an exemplary embodiment, the battery 302 is a nickel-cadmium rechargeable battery. Alternatively, the battery 302 is a nickel-metal hydride, lithium-ion, or any other rechargeable battery. In yet another alternative embodiment, the battery 302 is not rechargeable. The emergency ballast 300 also includes several inputs and outputs 304, 306, 308, 310, 312, 314. The emergency ballast 300 has an input 304 that is electrically coupled to the main AC power supply from the building (the “hot” wire, or “mains power”). The emergency ballast 300 also has inputs 306 and 310 that are electrically coupled to the lamp to be powered (not shown). The emergency ballast 300 also has inputs 308 and 312 that are electrically coupled to the AC fluorescent ballast. The emergency ballast 300 also has an output 314 that is electrically coupled to the lamp to be powered. The emergency ballast 300 also includes a circuit 400 that converts the DC output of the battery 302 to sinusoidal AC output. The circuit 400 will be discussed in further detail with respect to FIG. 4.

Turning now to FIG. 4, a circuit diagram illustrating a magnified view of an exemplary circuit 400 for providing a sinusoidal output from an emergency fluorescent ballast according to one exemplary embodiment is presented. Generally, the circuit 400 is electrically coupled to the positive and negative terminals of the battery 302 (FIG. 3), and includes components to invert the direct current received from the battery 302 (FIG. 3) into alternating current having a substantially sinusoidal output with a current crest factor of 1.7 or less. In the exemplary circuit 400 shown in FIG. 4, the circuit 400 employs discrete components and a push-pull topology to convert the battery 302 (FIG. 3) current into substantially sinusoidal alternating current. In alternative exemplary embodiments, however, the circuit 400 employs a half-bridge topology. In another alternative exemplary embodiment, rather than using discrete components, the circuit employs an integrated circuit configured to perform the same function.

The exemplary circuit 400 includes an input 402 that is electrically coupled to and receives the positive output of the battery 302 (FIG. 3). The exemplary circuit 400 also includes input 404 that is electrically coupled to and receives current from the negative terminal of the battery 302 (FIG. 3). The positive output of the battery is passed through an inductor 406 that acts as a “choke” to limit the current being supplied to the transistors and remove DC bias. The current is then passed to the center tap of a transformer 408.

To create the substantially sinusoidal AC output, the negative terminal of the battery is alternatively electrically coupled to the first terminal 410 of the transformer 408, and then the other terminal 412 of the transformer 408. By repeatedly switching the terminal 410, 412 of the transformer 408 that is electrically coupled to the negative terminal of the battery 302 (FIG. 3), an alternating current is induced in the transformer 408.

The terminal switching is controlled by transistors 414 and 416 in conjunction with resistors 420, 422, and 424, and inductor 418. Resistors 420, 422, and 422, are tuned with inductor 418 to switch transistors 414 and 416 on and off such that the resulting output from transformer 408 is a substantially sinusoidal AC current with a current crest factor of 1.7 or less. By way of example only, the inductor 406 is a feed choke having an E1187 core, an inductance of 0.095 mH, and an RDC of 0.045 ohms. Transformer 408 has a primary winding with 7 turns, inductance of 0.004 mH, and RDC of 0.05 ohms. The secondary winding of transformer 408 has 1000 turns, inductance of 163 mH, and resistance of 44 ohms. Transistor 418 has 2 turns, an inductance of 0.00065 mH, and a resistance of 0.03 ohms. Resistor 424 is a ½ watt, 150 ohm resistor. Resistors 418 and 420 are ¼ watt, 1 ohm resistors. Capacitor 428 is a 600 volt AC, 330 pF film capacitor. As one of skill in the art would recognize, however, other combinations of resistors, inductors, and other components would result in a sinusoidal output from the emergency ballast.

As a result of the switching, transformer 408 produces a sinusoidal AC output at its output terminals 426 and 430. A first terminal 426 is electrically coupled to the AC fluorescent ballast. A second terminal 430 is electrically coupled to the cathode side of the lamp. In the exemplary embodiment, a capacitor 428 can be coupled to the cathode terminal to further smooth the output waveform.

Turning now to FIG. 5, a waveform demonstrating the output of the circuit 400 of FIG. 4 is shown. As shown in FIG. 5, the output has a current crest factor of 1.38, which is very close to the ideal current crest factor of 1.414. The current crest factor shown in FIG. 5 is also well under the ANSI recommended maximum current crest factor of 1.7. Because the output of FIG. 5 has a current crest factor so close to the ideal, a fluorescent lamp coupled to the emergency ballast will not experience unnecessary ageing, due to high current crest factor (cathode sputtering) or DC bias (mercury migration) as would likely occur with any of the emergency ballast outputs shown in FIGS. 1a, 1b, and 1c.

Turning now to FIG. 6, a flow chart describing an exemplary method 600 for providing cathode heating from an emergency fluorescent ballast according to one exemplary embodiment is provided. In step 605, current is received from an emergency ballast. In one exemplary embodiment, the current in step 605 is the current output of an emergency ballast such as emergency ballast 300 of FIG. 3. In step 610, it is determined whether cathode heating will be used. If the decision in step 610 is affirmative, the method 600 follows the “Yes” branch and proceeds to step 615, in which the level of current is increased. In an exemplary embodiment, the current is increased according to the specification of a lamp. By way of example only, for a T8 lamp, the current is increased by 350-400 milliamps. The method 600 then proceeds to step 620, wherein the increased current is supplied to the lamp.

Turning again to step 610, if the determination is negative, the method 600 follows the NO branch and proceeds to step 620, wherein a non-amplified current is supplied to the lamp. The method 600 then proceeds to step 625, wherein it is determined whether AC power has been restored. If the decision in step 625 is negative, the NO branch is followed and the method 600 returns to step 605. If the decision in step 625 is affirmative, the YES branch is followed and the method 600 ends.

Turning now to FIG. 7, a circuit diagram illustrating an exemplary circuit 700 for providing cathode heating according to one exemplary embodiment is shown. As shown, the exemplary circuit 700 is provided as a modification of the substantially sinusoidal AC output circuit 400 provided in FIG. 4. As one of ordinary skill in the art would understand, however, the circuit 700 for cathode heating can be applied to any implementation of a fluorescent ballast, including other existing emergency ballast implementations that do not provide sinusoidal AC output, such as those referenced in FIGS. 1a, 1b, and 1c. To the extent that the exemplary embodiment of a cathode heating circuit 700 modifies the circuit 400 described in FIG. 4, components already described in FIG. 4 will be described using the same reference numerals used in FIG. 4.

In an exemplary embodiment, the outputs 426 and 430 of the circuit 400 are electrically coupled to windings 706 and 708. The exemplary windings 706 and 708 are inductively coupled to the transformer 408 and are tuned to drive an increased current to the lamp. In one exemplary embodiment, the output of the windings 706 and 708 is passed through smoothing capacitors 710 and 712 to remove unwanted noise from the signal. Capacitors 710 and 712 adjust the filament current, and are used to fine-tune the cathode current output from the windings 706 and 708. By way of example only, windings 706 and 708 have an inductance of 0.01 mH and 8 turns. Capacitors 710 and 712 are 63 volt, 220 nF film capacitors.

The windings 706 and 708 increase the current applied to the lamp without requiring a corresponding increase in the output of the power source, for example, the battery 302 (FIG. 3). The increased current heats the cathodes, thus reducing the amount of voltage necessary to successfully strike the lamp, which allows the emergency ballast 300 to operate low mercury fluorescent lamps. Although the exemplary embodiment provides additional windings 706 and 708 to increase the current output from the emergency ballast, one of skill in the art would understand that other combinations of components could achieve the same result.

The emergency ballast 300 providing substantially sinusoidal output can also be combined with a standard AC fluorescent ballast to provide a complete fluorescent ballast solution. When combined with a standard fluorescent ballast, the emergency ballast 300 may further be modified with the cathode heating circuit 700.

Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons of ordinary skill in the art upon reference to the description of the invention. It should be appreciated by those of ordinary skill in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or methods for carrying out the same purposes of the invention. It should also be realized by those of ordinary skill in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the scope of the invention.

Claims

1. A ballast for a luminaire, comprising: a battery that provides direct current in response to a loss of mains power to the luminaire;a conversion circuit, wherein the circuit receives the direct current from the battery, converts the direct current into substantially sinusoidal alternating current, and provides the alternating current to a lamp in the luminaire.

2. The ballast of claim 1, wherein the substantially sinusoidal alternating current has a current crest factor of 1.7 or less.

3. The ballast of claim 1, wherein the loss of power comprises a loss of a mains power.

4. The ballast of claim 1, wherein the conversion circuit comprises a push-pull topology.

5. The ballast of claim 1, further comprising a cathode-heating circuit configured to pre-heat a cathode of the lamp.

6. The ballast of claim 6, wherein the cathode-heating circuit increases the current output of the conversion circuit.

7. The ballast of claim 1, wherein the lamp is a fluorescent lamp.

8. The ballast of claim 1, further comprising circuitry for converting mains power to power appropriate to illuminate the lamp.

9. A method for providing emergency power to a lamp, comprising: determining whether mains power has been interrupted;providing direct current to a conversion circuit in response to a determination that mains power has been interrupted;converting the direct current to alternating current, wherein the alternating current is substantially sinusoidal; andproviding the alternating current to the lamp.

10. The method of claim 9, wherein the alternating current comprises a current crest factor of 1.7 or less.

11. The method of claim 9, wherein the lamp comprises a fluorescent lamp.

12. The method of claim 9, wherein the conversion circuit comprises a push-pull topology.

13. The method of claim 9, wherein the direct current is supplied by a battery electrically coupled to the conversion circuit.

14. The method of claim 9, further comprising the step of heating a cathode of the lamp.

15. The method of claim 14, wherein heating the cathode comprises increasing the current to a cathode of the lamp.

16. The method of claim 9, further comprising converting mains power to power appropriate for lighting the lamp in response to determining that mains power has not been interrupted.

17. A circuit for providing current to a fluorescent lamp, comprising: a battery configured to deliver direct current in response to an interruption in mains power; anda conversion circuit configured to convert the direct current to alternating current, wherein the alternating current comprises a substantially sinusoidal waveform having a current crest factor of 1.7 or less, and to provide the alternating current to a fluorescent lamp.

18. The circuit of claim 16, wherein the conversion circuit comprises a push-pull topology.

19. The circuit of claim 16, further comprising a cathode heating circuit for pre-heating a cathode of the lamp.

20. The circuit of claim 16, further comprising a ballast for converting mains power to current appropriate for powering a fluorescent lamp.