The present invention relates generally to high intensity discharge lamps, and more particularly, to ballast circuitry for igniting the same.
Efficiency, compatibility and longevity considerations have become ubiquitous within the artificial lighting industry and consumer base. To this end, manufacturers of incandescent, fluorescent and high-intensity (HID) light sources allocate substantial resources to improve operation of their mercury vapor, metal halide, high pressure sodium and low pressure sodium lamps. The relatively low power consumption and light color features associated with such sources have made HID lighting systems commonplace in factories, schools, retail stores, industrial buildings, studios, and malls and street settings.
Unlike conventional incandescent lamps that may be powered directly from a 120V/60 Hz utility source, HID lamps typically require a ballast for the ignition and subsequent operation of the lamp. Ballast circuitry regulates the flow of electrical current to a lamp in order to facilitate coordinated ignition and subsequent operation. In addition to various resistive and inductive components, circuitry associated with the ballast may include a transformer to provide a voltage to the lamp along with an ignition component configured to accelerate the state of the lamp to an ignited state. That is, a state exhibiting intense luminous output formed by the passage of electric current across a space between electrodes.
The ignited, or thermionic arc state of a lamp is typically preceded by two stages of operation that may be categorized as initial Ionization followed by glow discharge. During the ionization phase, a ballast supplies a continuous high voltage across the electrodes of the lamp. Once sufficient voltage is attained to achieve initial ionization the lamp enters the glow discharge stage. Glow discharge is a transitory stage continuing until the electrodes achieve thermionic emission. Ideally, the lamp transitions out of glow phase to the ignited state, and the ballast then reduces lamp voltage and increases lamp current. Thereafter, current is regulated and the lamp operates under steady state conditions.
Due to differing voltage requirements associated with the progressive stages of ignition, it is advantageous for the ballast circuitry to provide varying voltage protocols tailored to respective stages. For example, electrodes are cold in non-thermionic emission, prior to ignition. Consequently, a sufficiently strong electric field must be provided to lift electrons off of the surface of the electrodes. As such, conventional applications apply a very high voltage narrow spike embedded in a continuous waveform to ionize gas. In this manner, the energy conveyed in the narrow spike is proportional to the material that is ionized. In other word, the HID lamps go through several stages while being turned on, and as they reach their steady or ignited state of luminescence. At least one of these stages requires an application of high voltage in a narrow voltage spike that has several inherent disadvantages.
Despite the efficiencies and aesthetic advantages inherent to HID lamps, problems associated with ignition sequences may substantially degrade the performance and life expectancy of the lamp. Cold conditions of the electrodes at startup make them especially vulnerable to degradation from the high, spiking voltages of conventional applications, which routinely exceed five kilovolts in magnitude. Additionally, the hard spike of initial ionization stage and sporadic electron flow associated with glow discharge precipitates the occurrence of electrode sputtering.
Sputtering produces volatile particulate scattering of electrode surface material, such as tungsten. Over time, the tungsten may condense on and blacken the inner surface of the lamp. Transmission of light through the envelope decreases as the interior of the lamp blackens. Tungsten pieces may additionally absorb radiation and increase the temperature of the lamp casing to a critical level. This increased heat may decrease lamp life while requiring more power.
Thus, while the amount of energy in the voltage spike is proportional to the number of electrons excited, voltage levels additionally relate directly to the amount of damage sustained in the arc tube of HID lamps. That is, while the sudden and irregular nature of the voltage spike effectively excites electrons off of the electrodes, it also lifts neutral matter off of the electrodes resulting in darkening or blackening around the electrode. In this manner, material from the electrode is destroyed every time a high voltage pulse is applied to the electrode. Over time, the opacity of the arc tube will also increase to further frustrate lighting operation.
Another obstacle confronting conventional ignition applications concerns equipment ratings. For instance, the root-mean-square (RMS) value of wiring and sockets may prescribe a maximum voltage level and associated duration that the respective equipment can convey without incurring damage. An exemplary rating may, for instance, correspond to 5,000 kilovolts for one second. Exceeding those parameters can damage hardware and interrupt operations. While the spiking nature of prior art signals are typically short enough in duration to avoid violating such parameters, the magnitude of the associated voltage can still have severe effects on the electrodes, including the above discussed sputtering phenomena.
Consequently, some lighting manufacturers attempt to decrease the occurrence of spiking extremes by formatting or otherwise altering the waveform shapes associated with the power supplied to the lamp to achieve ignition. For instance, sinusoidal waveforms used in most applications may be supplanted with continuous square waves of longer duration and lower voltage levels. While such attempts demonstrate success in achieving a somewhat more controlled ignition, the large aggregate ionizing potentials associated with such voltage applications require special hardware, such as wires and sockets having unconventionally high power and RMS ratings. Such special requirements pose additional cost and compatibility obstacles to manufacturers. Furthermore, the continuous waves still accommodate extended periods of glow discharge phase and detrimental effect associated, therewith.
Consequently, what is needed is a manner of igniting a ballast and associated lamp without degrading the performance and overall life of the same. Further, such a method will ideally account for manufacturer specifications of hardware typically associated with HID lamps.
The invention addresses these and other problems associated with the prior art by providing an improved apparatus and method for operating a ballast to energize a lamp to an ignited state. Control circuitry associated with the ballast is operable to interrupt the ionizing potential once prior to the lamp""s reaching an ignited state. That is, an ignition cycle associated with the ionizing potential may be intermittent, having an interruption, or xe2x80x9coffxe2x80x9d period. For example, in one of numerous embodiments according to the invention, in a plurality of possibilities, a ten second ignition cycle will preferably include a one second xe2x80x9conxe2x80x9d period, followed by nine seconds of no ionizing potential. The ignition sequence and associated intermittent ionizing potential supply will repeat as necessary to achieve lamp ignition.
The intermittent nature of the ballast output enables a lamp to achieve an ignited state in a manner that obviates the need for the large, damaging voltage spike of the prior art. This feature mitigates loss of material from electrodes, the occurrence of tube blackening and the shortening of lamp life. Consequently, the efficiency of the lamp is improved over time, and lamp operation requires less applied power. Timing protocol of the invention additionally ensures compatibility with conventional RMS and other equipment ratings, reducing requirements for high-voltage wiring and sockets.
The invention further provides a mechanism for sensing various states of the lamp at its stages of ignition. For instance, a sensor may detect a voltage, current, radiated energy or some optical occurrence indicating the state of the lamp. In response to such detection, the control circuitry inhibits the cycling circuitry if the lamp has reached its ignited state or may repeat ionization cycles if it has not. As a result, the lamp transitions from ignition to normal steady state operation in a manner that minimizes tube blackening while maximizing lamp efficiency and longevity.
The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the following description.