Rapid Starting of Gas Discharge Lamps

This invention relates generally to the starting of gas discharge lamps, and more particularly concerns overcoming problems relating to such starting.

Gas discharge lamps such as mercury vapor lamps have come into widespread use, two examples being sources of ultraviolet light employed in print reproduction, and street lamps. Although such lamps are clean and efficient devices, as compared with carbon arc lights, they have presented two long standing problems. First, they require objectionably long warm-up times, i.e up to about 5 minutes in certain cases; and secondly, there is the practical impossibility of re-starting a warmed-up mercury vapor lamp (should it be even momentarily turned off) until it cools. Therefore, for repetitive on-off duty, it has been the practice to leave the lamp on all the time, and resort to a mechanical shutter which is moved to control exposure. Such shutters, however, are subject to mechanical failure, and resultant prolonged human exposure to a mercury vapor lamp in ON condition can cause serious damage to the eyes and skin.

It is a major object of the invention to overcome both of the above problems, enabling powering of gas lamps only when needed and thereby additionally producing energy savings, extension of lamp life and elimination of physical damage to humans otherwise resulting from prolonged exposure to lamp rays. Further, mercury vapor lamps used as street lights can be restarted immediately after power interruptions (as for example may be due to lightning, and short power failures), enhancing traffic and pedestrian safety.

Basically, the starting device comprises means to apply to the lamp one or more transient voltage pulses or spikes which initially change in amplitude in a polarity direction relatively in opposition to the polarity of the main voltage simultaneously supplied to the electrodes; further, each pulse thereafter is typically caused to change in amplitude in a polarity direction in aid of the polarity of the main voltage then being applied to the electrodes. As a result, a rapid start of the lamp is achieved.

It is another object of the invention to provide circuitry to achieve the above; with pulse oscillation frequency much greater than the main AC voltage frequency. Such circuitry may typically include series connected inductance and capacitance, and control means electrically connected with the capacitance to effect capacitor discharge and pulse production in response to predetermined increase or decrease in the amplitude level of the main AC voltage; accordingly, pulse production may be achieved when the main AC voltage is near maximum absolute value, to aid the start; also, when the main AC voltage drops below that level, the command of pulse production is terminated.

It is a further object of the invention to provide second means to produce ionization of the gas within the lamp during application of voltage pulses to the lamp electrodes, such second means typically including an ion gun, and other circuitry including inductance and capacitance to apply an auxiliary voltage pulse to the gun in synchronism with pulse application to the lamp electrodes.

It is an additional object of the invention to provide unusually effective ballast circuitry between the main AC source and the lamp, such ballast shortening the time from start to full light out, without drawing excessive current from the mains and without applying excessive power to the lamp.

These and other objects and advantages of the invention, as well as the details of illustrative embodiments, will be more fully understood from the following description and drawings, in which:

FIG. 1 is a circuit diagram;

FIGS. 2 and 3 are wave forms related to the operation of the FIG. 1 circuit;

FIG. 4 is a circuit diagram;

FIGS. 5 and 6 are wave forms related to the operation of the FIG. 4 circuit; and

FIG. 7 is a circuit diagram.

Referring first to FIG. 1, it illustrated one preferred form of starting device for a discharge lamp 10 to which main AC voltage is supplied via input terminals 11 and 12, ballast 13, and lamp electrodes 10a and 10b. The main AC voltage waveform appears at 15 in FIGS. 2 and 3.

In accordance with the invention, first means is electrically connected with at least one lamp electrode to apply to the lamp a transient voltage pulse which initially changes in amplitude in a polarity direction relatively in opposition to the polarity of the main voltage 15 simultaneously supplied to the electrodes. See for example the sharp initial amplitude change at 16a of pulse 16 in FIG. 2, the pulse being applied at 20 to electrode 10b in FIG. 1. Further, the pulse is thereafter caused to change in amplitude in a polarity direction in aid of the polarity of the main voltage 15. See for example the sharp amplitude change at 16b of pulse 16 in FIG. 2, it being clear that the pulse oscillation frequency is much greater than the frequency of the main AC voltage. One or more such pulses are typically applied, as for example at 16, 17 and 18 during one half cycle of the main voltage wave; further, pulses 19, 20 and 21 are shown as applied during the next half cycle, the latter pulses being reversed in initial amplitude change direction relative to pulses 16-18. A time axis expanded version of pulse 19 is shown in FIG. 3, the shape of the pulse being altered as shown by the broken lines 19a upon the occurrence of lamp conduction.

In the circuitry example as shown in FIG. 1, the means to produce the pulses typically includes series connected capacitance C.sub.3 and inductance T.sub.1 characterized as causing the pulse to have the illustrated transient wave form. Further, control means is connected with such circuitry to effect pulse production in response to predetermined change in amplitude of the main AC voltage, as during each half cycle of the latter. Such control means may with unusual advantage include Zener diode means as at 120 and 121 connected back-to-back and in series with the ballast 13, as via leads 22 and 23. The Zener diodes are also connected in series with capacitance 24, via diode 25, so that the capacitance 24 will become charged when the Zeners conduct in response to increase of the main voltage to predetermined level, on each half cycle. When the capacitance charges to predetermined voltage level, the diode 25 conducts, firing the triac Q.sub.1.

Upon conduction of the latter via path 26 between main power heads 23 and 23a, the capacitor C.sub.1 is discharged through the primary of the pulse transformer T.sub.1. The latter may typically produce a medium voltage pulse (7-15 KV, for example) applied, as described, across the lamp electrodes (C.sub.3 being low impedance to the pulse). The energy of one pulse is normally insufficient to establish full conduction in the arc region of the lamp, reliance being placed on a succession of such places (as at 16-18) superposed on the main voltage to alter the ionization state of the gas within the lamp, and free charges, thereby to initiate lamp conduction. When the lamp conducts, the voltage at lead 22 falls, disabling the pulse generator A that includes elements 120, 121, 24 and 25 previously described. Should the lamp ever stop conducting while main power is applied, the starting cycle will automatically repeat.

With respect to the connection of T.sub.1 to oppose the main voltage, it would normally appear that such a connection is incorrect. It is found, in this regard, that after T.sub.1 delivers an initial pulse, conduction ceases momentarily but, within the lamp, due to the reverse conduction in oppsitions to main voltage, a condition is brought into existance which aids the main voltage. While the exact nature of such condition is not understood, it is thought that the reverse current within the lamp creates a cloud of electrons produced by secondary emission at the electrode which then becomes the cathode for the main current flow.

Also provided is what may be referred to as second means to produce ionization of gas within the lamp during application of the voltage pulse to the lamp electrode by transformer T.sub.1. Such second means may with unusual advantage include an ion gun 126, and other circuitry to apply an auxiliary voltage pulse to the gun. For example, that circuitry may include a transformer T.sub.2 whose end taps are connected between the gun and lead 23a, T.sub.2 having an intermediate tap 27 connected via a capacitor C.sub.2 and resistor R with lead 23. When the triac Q.sub.1 is fired as described, the capacitor C.sub.2 is discharged through the primary of T.sub.2, which causes T.sub.2 to produce a relatively very high voltage (i.e., 15-20 KV) pulse at relatively low current applied to the gun. This produced ionization of the gas in the Townsend region in the gun; however, the ionization level is not sufficient (if the lamp is hot and the mercury is vaporized, as in a Sylvania type H377 lamp, for example) to enable the main voltage to cause conduction. On the other hand, such ionization assists the lower voltage from T.sub.1 to cause reverse current flow, allowing the relatively very low voltage from the mains, plus the second cycle of T.sub.1 output, to cause forward current to bring the lamp into full conduction. Typically, the ionization level will be sufficient for T.sub.1 voltage to cause high reverse current (0.5 to 2 amps) to flow for a short time (5-20.mu. second, for example) in the lamp.

If T.sub.2 is not employed, the voltage level from T.sub.1 must be higher in order to first ionized the gas and then to produce the reverse current, as described above. It should also be noted that if T.sub.1 is connected with conventional polarity so as to aid the main voltage, and/or if T.sub.1 does not produce a second oscillatory pulse after the initial pulse (i.e. a second pulse such as 17 after initial pulse 16, for example), very much more energy is required from T.sub.1 to bring the lamp into main conduction.

An ion gun of the type referred to is described in my prior application Ser. No. 371,396, filed June 19, 1973, now U.S. Pat. No. 3,870,924.

Referring now to FIG. 4, improved ballast circuitry is shown in combination with lamp 10, which is of the medium to high pressure type, such as the Sylvania Model previously referred to. Such ballast shortens the time (i.e. from 3-5 minutes as in conventional to only a few seconds, i.e. 15 seconds for example) from start to full light out without drawing excessive current from the mains and without applying excessive power to the lamp.

The circuit essentially comprises an autotransformer T.sub.3 and a gate controlled bilaterally conductive device such as triac Q.sub.2 connected in series between the mains 11 and 12; an inhibit circuit 51 connected between leads 23 and 23a; and a pulse generator 52 responsive to the output at 53 of the inhibit circuit to control firing of the triac, as shown. Note that the pulse generator output is connected to the triac gate at 54.

In operation, when power is applied to the cold lamp 10 which is non-conductive, the voltages V.sub.1 and V.sub.2 as indicated are essentially the same as at the mains, (i.e. 240 VAC, for example) as is clear from FIG. 5, and there is no voltage change across T.sub.3. Therefore, V.sub.2 across Q2 has the same phase and amplitude as V.sub.1. The latter commands the inhibit circuit which prevents the pulse generator from operation when its V.sub.2 command has the same phase as V.sub.3. Therefore, Q.sub.2 does not conduct, and full supply voltage remains applied to the lamp to assist starting.

When the cold lamp conducts via the exciting inductance L.sub.2, (typically at about 70MH), V.sub.1 becomes low due to the small voltage drop across the lamp (i.e. 20 volts, for example), and there is then a large voltage gradient along T.sub.3. This causes V.sub.2 to tend to be of opposite phase from V.sub.1. The inhibit circuit then no longer prevents the pulse generator from operating, and Q.sub.2 conducts. T.sub.3 can now operate as an autotransformer, delivering high currents (e.g. 30 AMPS, for example) to the lamp while drawing much less current (e.g. 14 amps) from the main. The ratio of these two currents is determined by the position of the tap 55 (e.g. 40% for example) and the magnitude determined by the inductance of L.sub.1 (e.g. 8 MH) and the voltage drop across the lamp. As the lamp warms, voltage across the lamp increases leaving less voltage across L.sub.2. Thus, the lamp current decreases.

When the voltage V.sub.1 increases sufficiently, Q.sub.2 will not conduct at the beginning of each half cycle of lamp current; voltage V.sub.1 lags the main voltage since, when O.sub.2 is off, current is supplied by the exciting inductance L.sub.2 and the lamp voltage is in phase with this lamp current. Thus, voltage V.sub.2 will be of the same polarity as V.sub.1 and Q.sub.2 not triggered until the main voltage is at high enough value during the half cycle to drive V.sub.2 to reverse polarity. Then triac Q.sub.2 conducts for the remainder of the half cycle. This action continues until Q.sub.2 no longer conducts at any time, and the lamp is powered directly from the mains via the leakage inductance L.sub.2.

Benefits of this system over prior ballasts include rapid warm-up without excessive mains current; approximate constant power to the lamp during warm-up: since lamp current and lamp voltage are inversely related, this prevents thermal shock or other damage to the lamp due to excessive power input; independence of changes in lamp parameters or line voltage; since circuit action demands that sufficient voltage (up to the maximum available from the mains) is applied to the lamp to maintain conduction, no adjustments need be made for lamp aging, installation of new lamps, normal changes in mains voltage, etc.; and reversible action; i.e. should the lamp for some reason cool, appropriate higher current will automatically be provided to return lamp quickly to full output conditions.

FIG. 7 combines the circuits of FIGS. 1 and 4, with all the benefits of each, into a superior lamp starting circuit. Note that L.sub.1 of FIG. 4 has been included into T.sub.1 of FIG. 1 by adjusting T.sub.1 for the desired exciting inductance.

The invention is applicable to mercury and/or metal additive mercury lamps, an example of the latter being Sylvania Type MP2000. In this case, the T.sub.1 voltage applicable to the lamp electrodes would be in the range 15KV to 30KV, to hot re-strike, the ion gun also being usable.