A number of types of dimmers have been used in theatre, but today almost all dimmers are electronic dimmers. The vast majority are Silicon Control Rectifiers, or SCRs. Electronic dimmers work by chopping off a portion of the AC sine wave. If half of the wave is cut off, the lights burn at half intensity. If 3/4 of the wave is cut off, 1/4 is left and the light burns at 25% intensity. Electronic dimmers are smaller and lighter than the older types, but their biggest advantage is that they are remote-controllable.

One effect of chopping the AC sine wave is to add electronic "noise" to the electric circuits. This can show up as a hum or buzz in the sound system. This noise can be partially controlled by making sure that the lighting supply and the sound supply are completely separate and have separate grounding systems to limit cross-talk of this hum.

Dimmers come in various sizes according to the wattage they can handle. A 10 amp dimmer can handle 1200 watts (10 a. X 120 v.), a 20 a. dimmer handles 2400 watts, and a 50 a. dimmer holds 6000 w. To figure how many instruments of a given size will fit on a dimmer, simply divide the wattage of their lamps into the dimmer's watt capacity.

Alternately, divide the total load wattage by 120 v. to find the required circuit amperage.

One thing you cannot do is control a motor with an SCR dimmer. You will burn up the motor and may damage the dimmer.


At one time, dimmers were quite expensive, and a large number of circuits were controlled by a relatively few large dimmers. Hard-wired patch panels were used to connect circuits to dimmers. To control them, dimmers were controlled directly, or they were controlled using "preset boards". These had several parallel banks or "scenes" of controllers for the dimmers. The levels for several cues could be set in advance, then activated one after another as a show progressed.

Today dimmers are fairly cheap, and one dimmer-per-circuit is now the norm. At the same time, computer control boards have replaced preset boards. Instead of a physical preset controller, there is now a virtual control channel. The hard patch panel's connection between circuit and dimmer has been replaced by the assignment of dimmers to the virtual control channels. This is called a "soft patch", since it happens in the control board's software.

Soft patch has several advantages over hard patch. Softpatch can be done more quickly and accurately; it can be changed and corrected more easily right from the control board. Soft patch allows more flexibility; a patch panel limits the number of circuits in a dimmer to the capacity of the dimmer, while a soft patch allows everything from one channel-per-dimmer to every dimmer in the system to be assigned to a single channel. And a soft patch can be backed up to a computer disk and reloaded later in moments, even mid-show, while a hard patch must be redone manually.

Portable dimmer systems use a modern version of a somewhat older approach. Essentially you are creating a complete lighting system from one location and then removing it and reinstalling it in each subsequent venue. Instead of a patch panel or hard wired connections, road racks have plugs into which cable to the instruments is plugged in directly. To make this process easier, large road shows use multi-circuit cable or "mults" which allow 6 or 12 circuits to be connected at a time. At the batten end is a device called a breakout or fanout which connects to the multicable and separates the circuits out into individual circuit cables. The insulation on the fanout cords can be thinner than standard cable but are limited to a maximum of 10 ft. for each circuit cable before termination. The soft patch remains the same, but the wiring of circuit to dimmer is more flexible.

The biggest advantage for computer boards comes in running cues. Manual and preset boards limit the number of cues to how many can physically be set in the time allowed. With computer boards the cues are set and recorded in advance, and complex cues can be played back literally as fast as they can be triggered. Cues can even trigger other cues, and several be run simultaneously.

Control boards typically have several "modes". Setup mode allows the system to be modified and a show to be recorded or read from disk.Patch mode is where soft patch is set. Stage mode shows what levels are active & live on stage, while Blind mode shows recorded levels without displaying them live. Recording a cue in Stage mode records the cue as seen at that moment. Recording in Blind records a cue that is not seen until played back later. This can be useful to correct problems during a performance before the audience can see them.

Some boards work on a preset model. Each cue has every active level recorded into it, even if most show no change from the previous cue. A board may also work in tracking mode. Only those channels whose levels change are recorded. Any dimmer whose levels are unchanged is ignored, and simply carries through. This can be especially useful when changing a cue in the middle of a sequence. Many boards allow a choice; you can choose whichever is most appropriate.

Concert boards use a different model. A preset board can be thought of as a single stack of cues arranged in order. The board picks up the top one and runs it before moving on to the next. Concert boards need more flexibility as concert play orders can vary literally on a whim from night to night. Concert boards use a "page stack" model instead. A number or cues are set side by side and all can be accessed and run in any order. The another page containing yet more cues can be called up and any cue on that page can be run. Cues can be accessed in any order or any number of times just by calling up its page and running its controller.

Levels on a control board can be set several ways. There is usually a key pad where channel numbers and levels can be entered. There may be a wheel or touchpad which can adjust selected channels manually while you watch the results. Most boards also have submasters. A few channels or even whole cues can be recorded into a submaster and brought up individually or in groups to create a "look" for recording as a cue.

Moving light consoles grew out of the concert world, so moving lights often use a similar model which involves having what are essentially several parallel preset cue stacks arranged side by side. You can run down several cue stacks or move randomly between them like the page system described above. There may also be submasters which can be used like those on theatre boards. Today most theatre boards have added a moving light module to the regular cue system. Light attributes are treated and recorded as levels like any other dimmer in the regular light cues.

Control Signals and Wiring

Remote control boards "talk" to the dimmers via low voltage signals through fine wires. In the analog system used in preset boards and early computer boards, each controller sent a control signal continuously through a unique wire to its dimmer. This meant that for 60 dimmers, there were 61 wires to the rack (one per dimmer plus a common neutral). The signal was a continuum from Off to Full. The need for so many wires made it difficult to set up adaptive arrangements of equipment. Each installation was almost unique, a custom installation, so changing a hookup was as much work as the building it in the first place.

With the advent of computer boards, control moved from analog (continuous) to digital (discrete values represented by a specific number). The computer divides the range of zero to 100% into steps, then records the exact step a dimmer is set at during rehearsals. In playback the recorded level is sent to the dimmer, where it is converted into the analog signal needed to set the SCRs. Initially this was done on the same multiple wires that the analog control had used. But then schemes to do so with only a few wires were developed, using "multiplexing". Under multiplexing, instead of sending signals to the dimmers down many wires continuously, the levels of each of the dimmers are sent one after another down the same two wires. The signal goes to all the dimmers, and circuitry keeps count of how many levels have gone by. When a dimmer gets to its own number, it notes the level and ignores all the other levels. It then "remembers" its level until the cycle repeats and it gets to its own number again. How often the cycle repeats per second is called the refresh rate.

There was some work with Analog Multiplexing (AMX) but it really came into its own under Digital Multiplexing (DMX). Initially every manufacturer had their own version of DMX, but in the mid-1980's a common standard was adopted, called DMX-512. Created by a committee set up by USITT, DMX-512 is an 8-bit protocol, which can control up to 512 dimmers or device attributes per signal channel in 256 steps per device. It operates at 250 Kbytes per second. DMX-512 uses only two wires for its signal, but the USITT standard calls for four wires in the signal cable, surrounded by a sheath or shield to prevent interference, for a total of 5 conductors. The extra two wires are spares, and the connector used is supposed to be a five-pin XLR. Unfortunately, the standard is not always followed, especially by moving light manufacturers.

DMX-512 was specifically designed to run dimmers, but was soon adapted to run other devices as well, color scrollers and then moving lights, which it was never meant to handle. This has created certain problems. In the first place, with only 256 steps, DMX doesn't give ENOUGH steps to accurately control rotation and pan of a moving light. For instance, if you divide a full circle into 256 steps, then each step is slightly larger than a degree, and you might miss your mark if the light is set to pin spot. Also, moving lights have a number of what are called attributes, such as pan, tilt, color, iris, fader, gobo, etc. Each attribute uses at least one and often two DMX channels for each attribute, and there may be as many as 32 attributes in some of the newer more capable moving lights. It doesn't take too many moving lights to burn up 512 channels of DMX.

Moving light companies have been agitating for a new standard for some time. They want a 32-bit protocol instead of an 8-bit protocol to make their lights more finely controllable. This is like comparing DOS (8-bit) to Windows 2000 (32-bit) in a PC computer. Unfortunately, they have not yet developed a common protocol all can accept. Each company has their own protocol, like in the early days of dimmers, and if you use, for example, a Vari-light on a Vari-light control board, you have much more control than using the same light under DMX. However, this limits you to one company's equipment; a High End Cyberlight won't run on a Varilight console. Moving light companies are planning on using Ethernet instead of DMX as a carrier, but it will be awhile yet before a common "language" is accepted. Most modern control boards CAN output via Ethernet as well as via DMX cable, but they use DMX-over-Ethernet as the protocol.

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Copyright © 2002 Mick Alderson