stage lighting controller

7/27/2019 Stage Lighting Controller

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STAGE DIMMER -A comprehensive unit designed to handle up to 20A per channel with emphasis uponease of construction and versatility in operation

SINCE THE EARLY DAYS of theheatre the need for Iighting has

been all-important J u s t as Important

has been the need for control of that

ghting.Thisranges from very crudenitially to very sophisticated todayoftemwith a computerdoing theontrollingin the creation of special

moods andeffe c ts.

Th e first types ofdimmer used of

which there are still some examples

i n older theatres, was a variable

resistance type which used either a

variable orswitched power resistor in

series with the load With small loads

a wire wound resistor or a carbon pile

was usedwhile larger loads used a

tank of saline solution with a central

electrode which was raised or

lowered in the Iiquid,effectively

changing the resistance This type of

dimming, while reasonably effective,

dissipated a lot of power which made

life uncomfortably hot for the

operator, since to minimise

mechanical linkages the dimmers

themselves were often i n the control

room.


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correction, is used to ensure accuratesynchronization. The control curve isalso modified to give a subjectivelymore linear response and it has theability to drive a fluorescent loadwithout requiring a ballast resistor.Both the maximum and minimumlight levels are adjustable withoutinteraction giving reliable andpredictable output. This is especiallynecessary if a dimmer fails for some

reason and is replaced by a spareunit.

The Protection racketThe protection of SCRs and Triacs,especially Triacs, is usually difficultas they tend to fuse faster than thefuse purportedly protecting them.The use of a cheap Triac whichrequires an expensive fuse to protectit is false economy. We have used alarge rugged Triac (40 A device forthe 20 A dimmer) which allowseconomical fuses to be used,

especially for the 10 A version.On the control side we will bedescribing a panel with two sets oflong sliders per dimmer with twomaster controls which allow the nextscene to be set up then faded inwhen required. A igital memory

awhich can prereco d scenes andrecall them on demand may bepublished later.

Dimmer Module - ConstructionAssemble the boards with the aid of

the overlay. The heatsink should bedrilled and tapped for the triac toallow easy replacement if evernecessary. Note that the mounting ofthe fuse is different for the 10 and20 A dimmers.

The choke is bolted onto the PCBusing the long clamping bolts,preferably using rubber grommets inthe holes in the board (they may haveto be drilled out to do this). The leadsfrom the choke should be bent suchthat they go into the holes providedwithout going near the mounting

bolts which are at earth potential.The leads can now be soldered (bothsides on the 20 A unit).

The pulse transformer can now beadded according to Table 1 Becareful when winding thistransformer not to damage theinsulation on the wire as there is240 V between windings. We alsorecommend some epoxy between thetransformer and the board.

The printed circuit boards for thetwo versions of the dimmer board areidentical in layout and differ only inthat the connector end of the 20 Aboard is double sided to present agreater area of contact with theconnectors.

Controller-Construction?The component numbering system

used on the controller drawings isdesigned to indicate which channel aparticular component is part of. Theprinted circuit board drawing for thedimmer board is too large to publishin the magazine at full size; however,the pattern is available from ouroffices for the cost of an SAE - alarge SAE!

If the dimmer modules are notrequired to be connected throughsockets, the total cost can be reducedby connecting directly to the modulesand mounting them in a box. In the

20 A unit the heavy wires should bebolted on to the appropriate pads toensure contact to both sides of theboard.

One more modification to thecontrol desk is the addition of ablack-out switch which allows alllights to be blacked out withoutmoving the master control. This issimply done by switching the supplyvoltage on the master potentiometersfrom the 8 V supply as set by RV3 toOV. RV3 should be adjusted suchthat with one master at maximum,the second at minimum and one

individual control at maximum thatits output voltage should be + 10volts.

Setting upWith the dimmer module the trimpotentiometer has to be adjusted sothat the output pulse from IC7 occursat the very end of each half cycle.This is easiest set using anoscilloscope although anapproximate setting can be madewithout one.

If the dimmer is connected up to areasonably heavy load and adjustedfor about 1 / 3 level it will probablybe found that with RV3 at one endthe light level is not stable and tendsto flash. This is caused by the syncpulse occurring after the end of thehalf cycle and the trigger pulses fromthe previous half cycle triggering thenext. The trim potentiometer RV3should be turned back about turnfrom the position at which this effect

stops.

Max and MinWhen adjusting the maximum andminimum levels the minimum shouldbe adjusted first. Note that thecontrol potentiometer must beslightly up off zero to get any lightand minimum should be adjusted atthis point. The maximum should beadjusted with both the master andindividual control at maximum andset to the point where the light levelis just starting to drop. F

Shown above is a completed dimmer module


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POWER SUPPLY

T2

F i g 1 0 . R e l a t i o n s h i p b e t w e e nthe end of half cycle and the

synculse.

Fig 9. Waveforms shown are:Syncrdse(outputC7), curvegenerator (output IC5). mixeroutput (output IC2). oscillatoroutput (IC4). transformer drive,

(Q1).

Fig 11. An expanded view of thedrive waveform showing Q1

HOW ITDIMMER

WORKS

To help explain the operation the circuit canoe broken into seven sections.

1. Power supplyThis is a simple full wave rectifier whichgives about 18 V after being filtered by Cl6and C17. Using 3 terminal regulators this isreduced to 12 volts which is needed for thecircuitry.

2. Control tone filter and sync generatorAs the name implies this removes the control

tones that the supply authority superimposeson the mains voltage. These are normallyabout 1050 Hz and can cause problems byupsettmg synchronization ofdimmers.Thefilter is a low pass type comprising IC6 andassociated components. As filters alwaysalter the phase relationship this is correctedusing phase shift networks. Cll/R21 andC12/R22. Potentiometer RV3 is used to ens-ure the phase shift is zero (at 50 Hz) withnormal component variations. If the outputof IC6 is between +0.6 volts and -0.6 voltsneither D5 nor D6 will be forward biasedsufficiently to change the input voltages to1C7 so its output will be -10 volts. As theoutput voltage of 1C6 is a clean 50 Hz sinewave of about 6 volts amplitude this will onlyoccur at a small region about the zero cros-sing point. At all other times the output ofIC7 will be + 10 volts. The result is a negative

pulse, about 250 S wide at the zero crossingpoint of the 50 Hz.3. Curve generatorThis produces the output shown in Fig. 6.When the sync pulse occurs, transistors Q2and Q3 discharge capacitors C8 and Cl0.Immediately on release of the sync pulse theoutput of IC5 begins to ramp up slowly due toR16 charging ClO. However, while initiallythe voltage across R14 is zero and thereforedoes not affect the charging of ClO. as C8beg ins to cha rge due to R15 its effect be-comes more and more dramatic. A curve isnecessary as it gives a better input/outputvoltage relationship but the curve must bereproduceable hence the circuit used.

4. Input bufferThis serves two purposes; firstly, it allows a

MODULEmegohm input impedance and secondly itdetects when the input voltage falls below 0.1volt and turns the dimmer output completelyoff. This allows the minimum light control tobe turned up to give a better control range, iewith the filaments just glowing, yet havethem off if the control voltage is reduced tozero.

If the voltage is aboveO.l volt, the diode Dlwill lift the voltage on pins of 1C1 to equalthat of the input on pin 3. However if thevoltage falls below this level, the voltage n

!?pin 2 will remain at about 0.1 volt due to Rand the output of IC1 will go to about -10volts.

5. Mixer-comparatorIC 2 mixes the input voltage, the output of thecurve generator the sync pulse and theminimum adjustment potentiometers. Thisgives the waveform shown in Fig. 2 with theinput voltage and the minimum adjustmentonly moving the curve up and down withoutaltering the shape. When the output of IC2falls below zero volts the output of IC 3 goesfrom -10V to +10 volt with D3 and R8/9providing about 1 volt of positive feedback.The voltage has to rise to above 1V to forcethe output back to -10 volts. The diode isnecessary to ensure that the voltage at theinput of the oscillator IC4 remains within thesupply voltage of the IC (+ 12 V, 0 V).

6. Oscillator/ tria riveA CMOS osci

4l#C4 is used to drive Q1

which supplies energy for the pulsetransformer Tl. The oscillator will onlyoperate when the control inputs (pins 1 and13) are + 10 V. The frequency is controlled byC5 and is set at about 150 kHz. Resistor R13provides current limiting for the pulse transformer while R12 prevents the reverse vol-tage damagingQl if the load on the secon-dary load (the triac) becomes disconnected.

7. Power stageThis is simply a triac with a choke in series topr even t both RF1 and filament rattle and afuse to protect agains t shor t circuitsCapacitors are also used as bypasses to helpprevent RFl.


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stage lighting controller

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