Among electrical load devices, a gas discharge lamp and its associated ballast form one of the most recalcitrant systems to control and the present invention provides specific advantages in this regard. Accordingly, the present invention will basically be described in connection with its use in such a system to illustrate the control capabilities of the invention. However, it will be understood that the invention is applicable to other lamp systems, e.g., incandescent, and to other electrical load devices.
A gas discharge lamp and the light output therefrom are difficult to control due to the phenomena associated with the conduction of electricity through gas. Fundamentally, such a lamp requires at least an electron emitter and an electron collector, i.e., a cathode and an anode (the lamp electrodes), and a suitable gas ion population contained within the lamp envelope. When a sufficiently high instantaneous voltage differential exists between the electrodes, electrons will flow from the cathode to the anode through the gas ion column. In so doing, the electrons collide with the gas ions, ultimately causing photons to be emitted. The wavelengths of these photons depend on the molecular structure of the gas. In some gas discharge lamps these arc generated photons are used directly for illumination. In the case of phosphor excited lamps, the arc generated photons are primarily used to excite the phosphor molecules coated on the inside of the glass envelope of the lamp. The excited phosphors in turn emit longer wavelength photons in the visual spectrum band. This process is sometimes called fluorescence.
There are at least three first order problems that contribute to making a gas discharge lamp difficult to control. First, when the gas is electrically conducting in the so-called arc discharge region (as opposed to other current magnitude-defined regions of conduction) the lamp exhibits a negative volt-ampere characteristic, meaning that the voltage drop across the lamp decreases as the arc current increases. This volt-ampere characteristic of the gas lamp is the opposite of that of an incandescent lamp or of other resistance types of electrical loads. For this reason, a gas discharge lamp must be driven from a current limited source in that, unless limited, the current will increase to a disastrous level. One way of limiting fluorescent lamp current is through the use of a current-limiting magnetic ballast.
The second major problem concerns the fact that the gas conducts only after arc ignition, and this only occurs during the higher amplitude portion of the voltage sine wave. This factor rules out voltage control except for a relatively narrow range because the arc drops out of conduction at around 75% of the rated line voltage.
The third major problem is that the lamp cathodes must be properly heated. In particular, the cathodes must be heated so that electrons are readily available as current carriers for the arc to conduct. Normally, this heating is accomplished by the arc itself and/or by the ballast transformer heater windings. It is important to note that if the cathodes are not kept at the required thermionic emission temperature, the useful lamp life can be substantially shortened. In the case of the widely used F-40 rapid start fluorescent lamp, the American National Standards Institute (ANSI) specifies that, after arc ignition, at least 2.5 volts is required at each cathode. This voltage, together with the arc heating, will maintain the cathode at suitable emitting temperature.
The cathode heating voltage of rapid start lamps is provided by voltage taps on the secondary winding of the rapid start ballast. The ballast also provides the voltage transformation and inductance necessary to strike and limit the arc current, respectively.
With the advent of the electronic switches such as thyristors, i.e., SCRs, and TRIACs, control techniques were developed that could limit the "one-time" of the arc current within each half wave of the power line AC voltage sine wave. This technique provides an apparent dimming effect. However, if the arc current "on-time" is limited while also employing a standard rapid start ballast, the cathode heating time is also limited and as a consequence the cathodes are not properly heated. For this reason, thyristor dimming ballasts were developed which include independent cathode heating windings. With a dimming ballast, the thyristor only controls the ballast winding associated with the lamp arc. This type of control can be characterized as being "off" at the beginning of each voltage half wave and being turned on at some point in time during the voltage half wave. The thyristor then remains on until near the end of the voltage half wave (zero crossover) when there is insufficient holding current to keep the thyristor turned on.
To overcome the need to use a relatively expensive dimming ballast, I have spent the past seventeen years developing fluorescent lamp control systems of different types. Some of the techniques I have developed are described in pending applications directed to the Energy Conserving Automatic Light Output (ECALO) system wherein the current flowing in the primary of the ballast is uniquely controlled within the time period of each half wave of the AC voltage sine wave. These pending applications include Ser. Nos. 945,842, of Sept. 26, 1978 and Ser. No. 51,136, filed on June 22, 1979. In the systems disclosed in these patents, a control transistor is saturated full-on and as the voltage rises, the transistor control circuit is designed to limit the transistor ballast current when a preset minimum level is reached. Therefore, when the current exceeds the preset value, the transistor is switched from the saturated full-on state to the active or limiting region of transistor operation for part of the remaining time period of the voltage half wave. If the minimum current is all that is required, then the transistor remains in the active region until the excess voltage declines to that required by the arc. At this time the transistor again is saturated full-on until the voltage declines to zero. The process is repeated in the next half cycle. If more average current is required, and this is preferably related to the level of light output, the transistor is then switched full-on before the end of the period of active current limiting transistor operation. Hence, the time of active transistor operation can be varied within each voltage half wave and thus the light output can be varied from a minimum to maximum level. However, during the period of time that the transistor is operating in the active region of each voltage half wave, the transistor must "absorb" some of A.C. line voltage as seen at the ballast transformer winding voltage. Thus, the product of the voltage appearing across the collector and emitter of the transistor and the minimum or preset current flowing in the transistor emitter is power which must be dissipated by this transistor. The exact amount of dissipated energy will, of course, vary, depending on the time period within each half wave that the transistor is operating in the active region. The control provided by the ECALO system can therefore be described as a dissipative system which provides at least a minimum-on current in the earlier part of the time period of each voltage half wave which may be followed with a time controlled, full-on load compliance current during the latter portion of the voltage half wave.