US3482142A - Regulating system for arc discharge devices having means to compensate for supply voltage and load variations - Google Patents

Description

Dec. 2. 1969 CLUETT ET AL 3,482,142

REGULATING SYSTEM FOR ARc DISCHARGE DEVICES HAVING MEANS TO COMPENSATE FOR SUPPLY VOLTAGE AND LOAD VARIATIONS Filed D60. 29, 1967 r .Zkz ezziw/ flexed; Fm r50 ..F e

M :W' A A United States Patent US. Cl. 315105 1 Claim ABSTRACT OF THE DISCLOSURE A semi-conductor circuit for starting and controlling current drawn by a fluorescent 'lamp includes a voltage pulse discharge valve triggered by another electronic valve which breaks down once each half cycle of applied alternating current, the time of breakdown limiting the degree of ionization of the lamp by the voltage pulse and hence the current conducted each half cycle. T compensate for line current variations a photoconductor in the time constant circuit of the breakdown valve senses emission from an exciter lamp inductively coupled to the alternating current terminals by a filament transformer, and also connected in series with the fluorescent lamp to compensate for lamp current variations.

The starting and operation of arc discharge devices such as fluorescent lamps and high pressure mercury lamps presents two major problems. First, these and similar devices require a relatively high voltage to ignite an arc across the lamp as compared with the voltage needed to maintain the arc once it is ignited. Secondly, once thearc is ignited it has a negative resistance characteristic causing it to tend to draw increasingly more current until it reaches a run-away condition. Both problems have been solved by the use of an inductive ballast in series with the lamp and the line terminals supplying power to the'lamp. The voltage applied across the lamp by the ballast is adequate to maintain an ignited arc. The ballast steps up or gives an inductive kick to the voltage, producing peak voltages above the line peak voltage and adequate to strike the arc. The reactance of the ballast'then limits the current through the lamp to its rated value. The objections to the use of ballasts are that they are heavy and bulky by reason of the large amount of copper winding and iron core in their construction. They are expensive to make and draw substantial power not useful for lighting. They heat the environment of the lamp, and unless carefully constructed generate acoustic and electromagnetic noise.

A semi-conductor circuit lacking all the disadvantages of a reactance ballast has been developed, which circuit applies a voltage pulse to the lamp once each AC half cycle to ionize the lamp slightly less than necessary for conduction through successive half cycles, so that current through the lamp is limited by a tendency to extinguish each half cycle.

It is the object of the present invention to improve such a semi-conductor system for are discharge devices by providing automatic compensation for variations in the AC line voltage and in current drawn by the device.

According to the invention an electrical system for continuously controlling operation of a negative resistance arc discharge device comprises discharge terminals for connection to each end of the device, power terminals for connection to a supply of alternating voltage less than said predetermined are maintaining level, a power circuit connecting said power terminals to said discharge terminals, a voltage pulse generating circuit connected "ice in parallel with said discharge terminals, said pulse generating circuit including storage means and a triggered pulse discharge valve, a trigger circuit including a switching device connected between a power terminal and said valve and actuated by a predetermined voltage to trigger said valves and time constant means controlling the time in said alternating voltage cycle at which said predetermined voltage actuates the switching device, an ohmic radiation emitter inductively coupled to said power terminals to emit in proportion to variations in said alternating current supply, and said emitter also being connected in series with said discharge device to emit in proportion to changes in current drawn by the discharge device, and said time constant means including a device sensitive to radiation from said emitter to change said time constant in a sense to compensate for changes n said supply current and lamp current.

For the purposes of illustration a typical embodiment of the invention is shown in the figure which is a schematic diagram of a fluorescent lamp control system.

The exemplary system of the figure comprises a pair of high output fluorescent lamps X, Type 48T12, having terminals l and 1 connected to filaments F. The pair of lamps may be considered as one lamp having a rated starting voltage of at least 240 volts peak and an arc maintaining voltage of 120 volts RMS. Arc current is supplied to one lamp terminal I from one volt, 60 cycle alternating current line terminal A through an autotransformer having a primary winding L1 and a secondary winding L2 with a turns ratio of 1 to 2. The other lamp terminal I is connected to the other line terminal C through a 2.2 ohm resistor R. The system, so far described, comprises the power circuit for the lamp. A filament transformer has a primary winding T1 connected across the line terminals A and C, and secondary windings T2 and T3, the latter being connected to terminals 1 and f for the lamp filaments F to supply heating current thereto.

Connected in parallel with the lamp is a voltage pulse generating circuit comprising a primary voltage pulse storage capacitor C1 (8 microfarads) and a bilateral controlled electron valve V1, known as a triac (RCA TA2893). A triac is triggered into avalanche conduction in either direction between its primary electrodes when a voltage of either polarity is applied to its gate electrode g. An equivalent network is two silicon controlled rectifiers connected in parallel in opposite polarity. Other bidirectional electron valves triggered externally or internally may be substituted. The triac is triggered by a diac D1 (Type TI-43), similar to the triac but lacking agate electrode. A diac breaks down to avalanche conduction when the voltage across it exceeds a predetermined value. A diac may be replaced by two avalanche diodes connected in parallel. The. diac D1 is connected between the triac gate g and the junction j in a time constant network connected between the power terminals A and C in parallel with the lamps X.

The time constant network comprises, in series, storage capacitors C2 (0.1 microfarad) and C3 (0.05 microfarad), and a photoconductor Rt (Sylvania PL 466E). During each half cycle of alternating current at the power terminals A and C capacitors C2 and C3 charge at a rate dependent on the RC values in the time constant network until the voltage at the junction j reaches the breakdown value of the diac D1. Typically breakdown occurs at about 70 volts or at about 25 of the half cycle. Breakdown of the diac applies a voltage through a 1 kilohm resistor to the gate electrode g of the triac which abruptly is triggered into avalanche conduction, allowing the.primary capacitor C1, charged from a previous cycle, to discharge and reverse its charge to the instantaneous line voltage through the triac. The capacitor thereby discharges a voltage pulse or oscillatory pulse train through the autotransformer primary L1. This discharge voltage is stepped up in the transformer secondary L2, and the stepped up voltage pulse or train is applied to the lamp terminals 1. At this instant a limited number of ions are established in the lamp depending on the amplitude and duration of the pulse. Shortly thereafter the lamp fully ignites and conducts line current for part or all of the remaining half cycle. At a time either before or shortly after the half cycle when the line voltage again passes through zero, the arc almost extinguishes since the limited ionization cannot maintain the arc until the line voltage rises in the subsequent half cycle to are maintaining voltage level.

With a conventional ballast, once the arc is ignited by one or more initial inductive kicks and the lamp attains its negative resistance condition, the excess ionization by the ballasts allows the line voltage to carry the lamp through successive half cycles of conduction without applying higher are starting voltage. In fact the ballast is necessary to prevent run-away conduction. In contrast the present pulse generating circuit injects an ionizing voltage pulse each half cycle. Failure of the pulse generating circuit to re-ionize the lamp each half cycle would immediately or quickly result in extinction of the arc. Thus, 3

with rare exception, the generating circuit applies an are starting voltage to the lamp once each half cycle of line voltage. On the other hand, the values of the generating circuit components are selected to limit the degree of ionization to that just necessary to support about one half cycle of conduction at line voltage. By thus limiting the amount of ions available to conduct current at the lower line voltage the lamp tends to, and almost does, extinguish each half cycle as the alternating line voltage passes through zero. With this ionization control the lamp is prevented from progressing to run-away condition. The pulse generating circuit thus eliminates the conventional heavy ballast, but with a new mode of operation provides the are starting and current limiting functions of such a ballast. The prior heavy ballast (e.g. ten pounds) is replaced by a significantly lighter autotransformer (e.g. three pounds) Whose much lower inductance draws a small fraction of the power drawn by a conventional ballast.

In the mode of operation described above the average current drawn by the lamp each half cycle depends on the amount of ionization injected in the lamp by the primary storage capacitor C1. This in turn depends on the amplitude to which the line voltage has risen at the time the valve V1 is triggered by breakdown of the switch device D1. The earlier the breakdown and triggering, the lower the instantaneous line voltage, and the less the ionization and average current in the lamp. In the circuit of FIG. 1 the number of ions, and the line voltage, control the average current through the lamp. These factors can be chosen to represent the proper lamp circuit. However, current drawn by the lamp, and hence its light emission, exhibit undesired variations as the alternating current supply voltage changes and as the lamp impedance varies, particularly during warm up.

Supply voltage changes may be compensated by varying the charging time of the time constant circuit. For this purpose a 6 volt incandescent lamp B, such as Sylvania Type 8760, is connected in series with an additional secondary 1 volt winding T2 of the filament transformer, whose primary is winding T1. The function of the lamp is to emit light radiation to a CdSe or CdS photoconductor Rt such as Sylvania Type PL466E. The resistance of the photoconductor Rt, being in the time constant network, controls the charging time of the network and hence the firing time of the diac D1. For example, as the peak supply voltage rises from normal amplitude the light emission of the lamp E increases causing a reduction in the resistance of the photoconductor Rt and of the charging time of the time constant network. Consequently the diac breakdown voltage at junction j is reached earlier in the AC half cycle thereby compensating for the higher supply voltage. Conversely a line voltage drop will result in later triggering of triac V1 at atime in the AC half cycle when the voltage has reached the desired level for starting pulse discharge.

According to the present invention not only supply voltage variations but also variations in current drawn by the fluorescent lamps X due to impedance or other charges are compensated by connecting the exciter lamp E in series with the fluorescent lamp X. So that the exciter lamp E will not carry the full current of the fluorescent lamps, it is shunted by the low, 2.2 ohm resistance R. Increases or decreases in lamp current cause corresponding increases or decreases in exciter lamp emission and decreases or increases in the. resistance of the photoconductor Rt, compensating for the fluorescent lamp current change simultaneously and in the same way as supply voltage changes are compensated.

A system as shownand described weighs 3.75 pounds as compared with ll'pounds for a conventional ballast, yet is less noisy, more efilcient, and operates from 20 to C. with unobservable flicker from to volts of alternating current.

While one desirable embodiment of the invention has herein been disclosed by Way of example, it is to be understood that the invention is broadly inclusive of any and all modifications falling within the terms of the appended claim.

We claim:

1. An electrical system for continuously controlling operation of a negative resistance ionized arc discharge device comprising discharge terminals for connection to each end of the device,

power terminals for connection to a supply of alternating voltage less than a predetermined arc maintaining level,

a power circuit connecting said power terminals to said discharge terminals,

a. voltage pulse generating circuit connected in parallel with said discharge terminals,

said pulse generating circuit including storage means and a triggered pulse discharge valve,

a trigger circuit including a switching device connected between a power terminal and said valve and actuated by a predetermined voltage to trigger said valves, and time constant means controlling the time in said alternating voltage cycle at which said predetermined voltage actuates the switching device,

an ohmic radiation emitter inductively coupled to said power terminals to emit in proportion to variations in said alternating current supply, and said emitter also being connected in series with said discharge device to emit in proportion to changes in current drawn by the discharge device,

and said time constant means including a device sensitive to radiation from said emitter to change said time constant in a sense to compensate for changes in said supply and lamp current.

References Cited UNITED STATES PATENTS 2/1967 Hutson 315-l0l 3/1967 Howell 31S207 X 9/1967 Nuckolls 3l5158 X 12/ 1967 Deelman 32321 US. Cl. X.R.

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