Dimmer for discharge lamp utilizing a pulse enabling circuit
United States Patent Kappenhagen DIMMER FOR DISCHARGE LAMP UTILIZING A PULSE ENABLING CIRCUIT Pittsburgh, Pa.  
 Apr. 1, 1975 Primary Examiner-Rudolph V. Rolinec Assistant Examiner-E. R. LaRoche Alrorney, Agent, or Firm-R. A. Stoltz [57] ABSTRACT A lighting control apparatus (dimmer) responsive to an electrical demand signal to control the portion of the voltage cycle during which voltage is supplied to [22] Filed: June 10, 1974 d h l f AC 1 I one or more rsc arge amps rom vo tage source, [21] Appl- 478l37 using a pulse enable circuit to provide pulsing of the firing circuit appropriate for discharge lamps. The ap- 52 us. CI 315/194, 315/199, 3 l5/29l, paraws comprises a Shaping Circuit responsive to a 3 5 3 7 3 5 4 mand signal, a timing circuit, a pulse enable circuit, a [51] Int. Cl H05b 41/16, GOSf 1/08 Single pulse per half-cycle firing mea and 3 mm! 581 Field of Search 315/194, I99, 200 R, 208, state switching The PulSe enable circuit P 315/209 R, 291, 307, 324, DIG. 4 vents activation of the firing means when the solid state switching means is in a conductive state (firing 5 References Cied pulses based on the voltage cycle could otherwise UNITED STATES PATENTS occur before the lagging current of the previous halt 44 9 7 k H5 [99 has fallen to zero). This combines the advantages of x33 gr ti g: 35x94 low power consumption of single pulse firing circuits 6/1974 &#34;5 X with a reliability of firing of discharge lamps obtained heretofore only in multiple pulse firing circuits.  
 3 Claims, 5 Drawing Figures CURRENT RECT&#39;F&#39;ER TRANSFORMER l DIMMER FOR DISCHARGE LAMP UTILIZING A PULSE ENABLING CIRCUIT CROSS-REFERENCE TO RELATED APPLICATION In copending application Ser. No. 478,138, filed by the same inventor and owned by the same assignee, is described a dimmer utilizing an output synthesis circuit which provides a waveform having the same wave shape and proportional in magnitude to the voltage which is transmitted to the load. The output synthesis circuit provides the same function as a feedback transformer, but at lower cost.  
 BACKGROUND OF THE INVENTION The instant invention relates to lighting systems which control the level of illumination of one or more lamps, such as in a stage lighting system, and more particularly to electronic dimmers which control the flow of power to discharge lamps.  
  Most electronic dimmers are designed for and used with an incandescent lamp load and use timing circuits referenced from the 60 cycle line (supply) voltage. One such dimmer is described in Ser. No. 370,134, filed June 13, 1973 by Kappenhagen and Tabor, now Pat. No. 3,821,601. While such dimmers can also control a load consisting of a discharge lamp in series with a ballast over a portion of the range, difficulties are encountered at demands for near full brightness. The ballast, discharge lamp combination is an inductive load and the current zero generally occurs considerably after the reversal of the (SO-cycle supply voltage. As an early single firing pulse (occurring after the voltage reversal but before the current reversal) will be ineffective, no power will be supplied during that half cycle. Thus, such a dimmer might work well at low power demands but cause flashing of the discharge lamps at demands for high lighting level (due to the firing of the solid state switching means only on alternate half cycles). One method of avoiding the foregoing difficulty is to provide for multiple firing pulses during each half cycle. Such multiple pulse firing circuits, however, require higher power rating devices in the firing circuits (which are therefore more expensive) and also result in essentially random variations in the timing of the firing. The firing is essentially random, as any attempted early firing, results in actual firing, not as soon as the solid state switching device stops conducting, but at some later time which is dependent on the frequency of pulsing and exactly where in the multiple pulse cycle the soild state switching means stops conducting. The random timing variations are especially troublesome in regulated circuits as the feedback of the regulated circuit is continually attempting to correct for these random variations and this correction may be in the wrong direction.  
 SUMMARY OF THE INVENTION It has been discovered that the foregoing disadvantages of the prior art can be avoided by using a pulse enable circuit which prevents the activation of the firing means when the solid state switching means is in a conductive state.  
  The lighting control apparatus of the instant invention is responsive to an electric command signal to control the portion of the voltage cycle during which voltage from an AC voltage source is supplied to a dis charge lamp load (the load comprising one or more discharge lamps and one or more ballasts). This apparatus comprises a solid state switching means, a single pulse per half cycle firing means, a pulse enable circuit, a timing circuit, and a shaping circuit. The solid state switching means has gating means and two power terminals, these power terminals being connected in series with the load across the AC voltage source. The single pulse per half cycle firing means has an output connected to the gating means of the solid state switching means and the firing means renders the solid state switching means conductive when the firing means is initiated. The pulse enable circuit has an output connected to the firing means and the pulse enable circuit senses the status of (preferably from the voltage across) the solid state switching means and prevents the activation of the firing means when the solid state switching means is in a conductive state. The timing circuit has an output connected to the pulse enable circuit and the timing circuit generates the signal, which, when coupled to the firing means (by the pulse enable circuit) initiates the firing means to render the solid state switching means conductive during the remainder of the half cycle (that is, at least until the AC voltage source falls to zero and generally somewhat longer, the current not falling to zero until later in the inductive load). The shaping circuit has an output electrically connected to the timing circuit and the shaping circuit is adapted to receive a demand signal and to generate an output signal which is a function of the demand signal.  
 BRIEF DESCRIPTION OF THE DRAWINGS The invention can be best understood by reference to the following drawings, in which:  
  FIG. 1 is a block diagram showing the interrelationship between the essential elements of the instant invention;  
  FIG. 2 is a block diagram of a preferred configuration, showing the relationship of elements of one type of pulse enable circuit;  
  FIG. 3 is a block diagram of a preferred configuration showing elements of an alternate type of pulse enable circuit;  
  FIG. 4 is a block diagram of a preferred configuration, showing additional elements which are preferably included; and  
  FIG. 5 is a detailed circuit diagram of a preferred configuration.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS The block diagram of FIG. 1 shows the basic elements of the lighting control apparatus for the control of a discharge lamp load. An external demand signal is introduced into the shaping circuit. The shaping circuit transforms the demand signal (which is typically directly proportional to the desired apparent luminous intensity) into a signal which is a function of the appropriate lamp votage to produce that luminous intensity. Typically, this shaping is done to compensate both the eye response (approximately a square root function) and for the relationship between load voltage and luminous intensity for the particular lamp type.  
  The shaped signal is sent to the timing circuit. The timing circuit provides the proper relationship between the firing pulse and the phase of the AC voltage source such that the solid state switching means will conduct for an appropriate fraction of the half cycle.  
  The pulse enable circuit senses the status of the solid state switching means (the solid state switching means is typically 2 SCRs or a triac) and prevents firing of the firing means when the solid state switching means is in a conductive state. The pulse enable circuit can be of a type which senses the current through the solid state switching means, as current through the solid state switching means indicates that the solid state switching means is in a conductive state. The pulse enable circuit can also be of a type which analyzes the magnitude of the voltage across the solid state switching means. It will be noted that neither zero source voltage nor the presence of voltage across the solid state switching means is an absolute indicia of a non-conductive state of the solid state switching means. With an inductive load such as the ballast and discharge lamp load combination, zero voltage of the voltage source does not necessarily mean that the solid state switching means is non-conductive. In addition, because of the forward drop, some voltage will normally appear across a conducting solid state switching means. Thus a voltage sensing type pulse enable circuit cannot rely on source voltage or the mere presence of voltage across the solid state switching means, but must analyze the magnitude of voltage across the solid state switching means (a relative large voltage indicating that the solid state switching means is non-conductive).  
  The output of the pulse enable circuit goes to the firing means. When the timing circuit has called for a firing pulse and the pulse enable circuit has determined that the solid state switching means is non-conductive, the firing means is initiated and a firing pulse is sent to the solid state switching means, rendering the solid state switching means conductive for at least the remainder of that half cycle of the AC voltage source (again, because of the lagging current in the inductive circuit, the solid state switching means will remain conductive slightly longer than the remainder of that particular half cycle of voltage As the initiation of the firing means requires both the proper indication from the pulse enable circuit and the signal from the timing circuit, the single pulse per half cycle firing means will be initiated when the second of these circuits (the timing circuit and the pulse enable circuit) indicate that the firing is appropriate.  
  FIG. 2 is a block diagram showing one configuration of pulse enable circuit. In this configuration a current transformer senses the current to the solid state switching means. The output signal from the current transformer is rectified and used to control a switching device such that no signal is coupled from the timing circuit to the firing means until the current to the solid state switching means has dropped to zero (it will be noted that some circuits require an initial pulse to start operating). One alternate method to produce the current signal is to integrate the voltage across the series inductor.  
  FIG. 3 is a block diagram of an alternate configuration of pulse enable circuit. This configuration measures the voltage across the solid state switching means and analyzes the magnitude of the voltage to determine whether or not the solid state switching means is conductive. A transformer is connected with its primary across the power terminals of the solid state switching means and its secondary to the input of a rectifier (preferably a rectifier bridge). The rectifier output is analyzed and, if above a predetermined minimum, the signal terminal of a switching device is initiated. The input terminal of the switching device is connected to the timing circuit and output terminal of the switching device provides the output terminal of the pulse enable circuit. The voltage drop across the solid state switching means when the solid state switching means is conductive is generally less than 10 volts while the voltage across the solid state switching means immediately after current reversal is typically quite high (when the current lags by 30, for example, the voltage across the solid state switching means immediately after the current has gone to zero and the solid state switching means has become nonconductive is approximately 150 volts). Thus the minimum voltage to indicate nonconducting can be set anywhere between about 10 volts and about volts. In such a circuit, a capacitor can be used to allow firing late in the half cycle by de laying the time at which the voltage being compared drops below the minimum (below which the pulse enable circuit would block the timing circuit signal). It should be noted, however, that a discharge lamp will not operate satisfactorily at very low powers and that there is no need to have the pulse enable circuit conduct a firing pulse during the last l0 or l5 of the voltage half cycle.  
  FIG. 4 is a block diagram showing additional elements which can be used in a lighting control apparatus for a discharge lamp load. In this circuit there is also shown a feedback circuit which measures some load parameter (such as current, but more typically voltage) which parameter is fed back to a comparator circuit which generates a signal to the timing circuit based on a comparison between the shaping circuit output and the feedback circuit output. A reset circuit having an output connected to the timing circuit is also provided. The reset circuit periodically initializes conditions in the timing circuit. Preferably the timing circuit generates a ramp of voltage (the slope of the ramp being adjusted by the output of the comparator circuit) and the reset circuit initializes the ramp to a predetermined value, possibly when the solid state switching means is fired, but preferably when the AC voltage source passes through zero. The operation of the reset and comparator circuits is similar to that described in the aforementioned Pat. No. 3,82l,60l.  
  FIG. 5 is a detailed circuit diagram of a lighting control apparatus illustrative of the instant invention (the block diagram of FIG. 4 is generally applicable to FIG. 5). Table 1 is a component listing for this circuit:  
 TABLE l-Continued TABLE l-Continued Reference Component Reference Component Identification Value Identification Value n18 50k ohm pot 5 Tl Stancor P-8394 R19 82 ohm at V4 watt T2, T1 Stancor P-86l0 R20. R29 330 ohm at V. watt T4 Spra ue l1 Z 2100 l:l:l R22, R23, R28 4.7k at V4 watt Zl 1N7 6A R27, R36, R42 1.5k at A Watt 22 1N968B R26 74 ohm at V4 watt Z3 V4 M 4.7A 2 5 R40 47k at V4 watt l Z4 1N95 7B R30 43k at V4 watt Reactor 0.71 millihenry choke R31 1 meg at V4 watt R37, R38 lit at Va watt n I a I n C l ufd at 100 volts FIG. IS illustrative of a circuit in which the voltage C2 5.0 ufd at 50 volts th t t d t d t C3 0.15 ufd 100 volts 5 across e so I s a e swic mg means is use 0 e er- C4 0.33 ufd at 100 volts mine the status of the Solid state sw1tch1ng means. R27 5% gagkg g j gi is connected in series with one winding (the primary) Dl through D4 1N4l48 of T2 and this series combination is placed directly gfifgzgfig pg $2222 across the solid state switching means. Diodes D1 Q] though Q6 on single chip silicon General through D4 rectify the output signal from the secon- 012 m h on 2 2, ST G I dary of T2. This signal is divided by R39 and R40 and mug 3 c enem applied to the base of Q27. When the conduction volt- 0 Ihmugh Q1 l Single p Siliwn General age of Z4 is reached, Z3 clamps the base of 027 to less 0&#39;9 020 $3355 than its rated base-emitter voltage. 028 is conductive 02 I. 022 2N71l when the timing circuit is in a firing initiating condition. 1% T3 5:32: 8 As soon as both Q27 and 028 are in the conductive Tl Sprague 11 Z 2:00 state, the firing pulse is initiated. C7 is used to broaden g mgggg the time which Q27 is conductive, such that the firing Z3 M Z 5 pulse can be initiated during the latter portion of the g 2 h k 30 voltage half cycle.  
  C o e While the invention has been explained by describing particular embodiments thereof, it will be apparent to those skilled in the art that modifications may be made TABLE 1 without departing from the scope of the invention.  
  What is claimed is: fi &#39;f CmnPonem l. A lighting control apparatus responsive to an elecentification Value trlcal demand signal to control the portion of each half 51.335 R33 {25 cycle during which voltage is supplied to a discharge 3 L8 2; 5:23,, lamp load from an AC voltage source, said apparatus R5 620k at Mt Watt 40 compnsmg: R6. R2l 8.2k at Va watt R7 560k ,Awan a. sohd state switching means having gating means R8 470k at A watt and two power terminals, said power terminals R43 being adapted to be connected in series with said Rll), R35 20k at V4 watt Rll L6 me at v. watt load across said AC voltage source; 5:? R24 gg f g fi 45 b. a single pulse per half cycle firing means having an R14 620 ohm 1 watt output connected to said gating means of said solid 3:; 1 R41 335 state switching means, which firing means renders R18 3 fi m 33 said solid state switching means conductive when g g R29 goon? at iaywan said firing means 15 initiated;  
  o m at 4 watt R23 R28 Mk at wan c. a pulse enable circuit means having an output con- R27, R36, R42 l.5k at watt nected to said firing means, for sensing the status r38 zgkogtm gnwan of said solid state switching means and preventing R30 43k at A watt the activation of said firing means when said solid 53% R38 &#39;l g f 55 state switching means is in a conductive state;  
  4 I [5k d. a timmg circuit having an output connected to said 8 g 5 volts pulse enable circuit means, said timing circuit gen- C3 z sg erating a signal, which, when coupled through to E? 12353 ufgd atslm volts the firing means by said pulse enable clrcult means,  
  u at V0 is C7 MP 0056 200 will Init ate the firing means to render the solid Dl through D4 1N414B state switching means conductive; and gi mgfi gg {S23}: e. a shaping circuit having an output electrically con- 0] fi 06 on single chip silicon Genera] nected to said timing circuit, said shaping circuit 563523 being adapted to receive said demand signal and to 012 through 017 on single chip Silicon Genera] t t t 1 h. h f f 563323 genera e an on pu signa w IC is a unction 0 07 through 01 l on single chip Silicon General said demand signal, O18 O19 O20 gfigggg 2. The apparatus of claim 1, wherein said pulse enable circuit means comprises:  
 cuit, the rectified signal from said transformer being effective to turn said switching device on when there is voltage across said solid state switching means indicative of said solid state switching means being non-conductive.  
  3. The apparatus of claim 2, wherein a reset circuit having an output connected to said timing circuit is included, said reset circuit monitoring the voltage of said AC voltage source and initializing said timing circuit each time said voltage passes through zero.  
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