Typical fluorescent lamps contain electrodes that, when operated at the lamp's rated discharge current, are heated to cause thermionic emission of electrons. Such lamps typically contain two electrodes. Each electrode serves as an anode during a first half-cycle of the alternating current (AC) provided to the electrodes, while the other electrode serves as cathode. During the subsequent half-cycle, the electrodes swap roles. Thus, each electrode serves as cathode and anode on alternating half-cycles.
Such fluorescent lamps may be dimmed by reducing the current supplied to the electrodes. Reducing the current, however, also reduces the electrode temperature. If the cathode, for example, is not sufficiently hot, it may not have sufficient thermionic emission to maintain the discharge as a thermionic arc. Rather, it may be forced to operate in a “cold-cathode” (i.e., high cathode-fall, high sputtering) mode. The resulting sputtering damage may cause the cathode, and thus the lamp, to fail within a few hours. Less destructive, but equally fatal in the long run, is the fact that a not-quite-hot-enough cathode will have a higher-than-normal cathode fall even though the cathode continues to operate in the thermionic-arc mode. If the cathode fall exceeds the so-called “disintegration voltage” or “sputter voltage,” incoming mercury ions bombard the cathode with sufficient energy to dislodge (i.e., “sputter”) surface atoms, bringing about increased rate-of-loss of cathode coating and short life.
In order to avoid these effects in fluorescent lamp dimming, an auxiliary electrode heating current may be supplied to the electrode filament to heat the filament sufficiently, by Ohmic heating, to cause thermionic emission. At low dimming currents with minimal heating of the cathode from the discharge current, the auxiliary supply may be the only heat source available to maintain cathode temperature. The auxiliary supply may be a low voltage, typically <6 volts, AC supply connected to the two ends of the filament structure holding the emissive coating. Resistive heating in the filament then furnishes the necessary heating power to maintain the cathode temperature at a desired level. The heating level and corresponding cathode temperature may be controlled by adjustments in the voltage or current furnished by the auxiliary heating power supply.
Obviously, the lower the dimming level at which the lamp is operated, the higher the auxiliary electrode heating current that will be required. If the voltage of the auxiliary heat supply is too low, then the cathode is too cold, the thermionic-arc cathode fall is too high, sputtering occurs with accelerated loss of cathode coating, and short lamp life results. Accordingly, it would be desirable to define a lower limit for the auxiliary electrode heating current in order to keep the lamp life within a reasonable range. On the other hand, if the voltage of the auxiliary heat supply is too high, the cathode temperature is too high, and excessive evaporation of cathode coating leads to short lamp life, even though the cathode fall is maintained well below the disintegration voltage. Steering a course between the Scylla and Charybdis perils represented by sputtering or excess evaporation is therefore desirable in selecting an appropriate cathode-heating-voltage profile as a function of discharge current. Such considerations may be particularly useful in the design of dimming ballasts.
By techniques known in the art, cathode temperature may be measured with an optical pyrometer, provided special lamps with phosphor wiped away from the ends are used to render the cathode visible. Alternatively, average cathode temperature may be determined in lamps without wiped ends by measuring the ratio of hot-to-cold-resistance of the cathode tungsten coil.
A technique is known for determining heater current as a function of discharge current in one particular lamp type of one particular wattage (see F. S. Ligthart, H. Ter Heyden, and L. Kastelein (Paper 17L, 5th International Symposium on Science and Technology of Light Sources, York, England 1989). This technique, however, involves life-testing a number of lamps at various values of heater current and discharge current for a long period of time. Examination of the resulting discolorations on the ends of the lamps could discriminate between sputtering, which may lead to so-called “end band” discoloration, excess evaporation, which may lead to so-called “cathode spotting” discoloration, and satisfactory heater voltage, which exhibits little or no discoloration.
FIG. 1 provides typical sputter and vaporization curves obtained using this technique. Heater-current versus lamp-current points where excessive evaporation was found are identified with a □; points where excessive sputtering was found are identified with a o; points where neither excessive evaporation nor excessive sputtering were found are marked with an x. Points lying between the two curves may be considered acceptable. Points lying below the sputter curve may lead to sputtering. Points lying above the vaporization curve may lead to excessive evaporation.
Though such a technique may provide information that is useful in determining the correct heater-current profile versus dimming current, it is far too cumbersome for an electronic ballast manufacturer to employ efficiently. To provide comprehensive results, it would have to be repeated for every different wattage of every different lamp type. In addition, cathode designs employed by different lamp manufacturers for the same lamp type are different, requiring testing of a number of different versions of the same lamp type.
FIG. 2 provides a plot of cathode fall as a function of phase angle for a typical fluorescent lamp. Specifically, FIG. 2 shows cathode fall as a function of phase angle in a typical T12 Rapid Start fluorescent lamp operating at rated current and heater voltage. Measurements of cathode fall were made using special lamps equipped with so-called “Langmuir Probes” (see John F. Waymouth, “Electric Discharge Lamps,” MIT Press 1971, Chapter IV and Appendix B). It should be understood that, as the data provided in FIG. 2 is on an absolute, rather than relative, basis, the plot of FIG. 2 provides a standard against which other methods for measuring cathode fall may be compared.
FIG. 3A provides a plot of cathode and anode falls for a typical fluorescent lamp. FIG. 3B provides a plot of arc current supplied to produce the cathode and anode falls plotted in FIG. 3A. Cathode fall in 60-Hz AC operated fluorescent lamps was measured via a method, attributable to Hammer, et al., wherein a capacitive probe, which may be a foil sheet, for example, is wrapped around a portion of the lamp that contains the electrode (see, for example, E. E. Hammer, “Comparative Starting Characteristics in Typical F40 Systems”, Preprint, IESNA Conference Minneapolis Minn. 1988, and its published version, JIES Winter 1989, p 64; and E. E. Hammer, “Cathode Fall Relationships in Fluorescent Lamps”, Preprint #69, IESNA Conference, Miami Beach Fla., 1994, and its published version, JIES, Winter 1995, p116). The probe picks up fluctuations of plasma potential in proximity to the electrode, and presents them to an oscilloscope for detection. As the negative glow in front of the cathode is approximately an equipotential blob of high-density plasma at a potential (positive relative to the cathode) that is equal to the cathode fall, fluctuations of plasma potential on the cathode half cycle may be interpreted as the signature of fluctuations of cathode fall during the half cycle.
Positive swings of potential are attributed to cathode fall (negative glow plasma that is positive with respect to the electrode) while negative swings are identified with anode fall (negative glow plasma that is negative with respect to the electrode). When allowance is made for the difference in lamp-current waveform, the shape of the curve agrees well with that shown in FIG. 2. The jagged fluctuations seen in FIG. 3A on the anode half cycle are so-called “anode oscillations.”
Because capacitive coupling causes a loss of DC reference, the Hammer method provides no information as to the value of the zero of potential. Thus, although the Hammer method may provide qualitative information about the shape of the cathode and anode fall waveforms, and about the peak-to-peak difference between peak cathode fall and peak anode fall, it does not provide the magnitude of either. Further, fluctuating potential signals from the cathode heater voltage may be picked up. Also, a very high input impedance is required for the measuring oscilloscope, which precludes the use of the Hammer method on small-diameter, compact fluorescent lamps. Additionally, capacitance between the signal lead and the shielded cable shunts the signal to ground, which reduces the apparent amplitude of the cathode fall variation. It would be desirable, therefore, if apparatus and methods were available for making capacitive measurements of the magnitude of cathode fall in fluorescent lamps, without the limitations exhibited by the Hammer method.