Dimmer
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Dimmers are used in stage lighting to regulate the amount of power supplied to a lighting instrument. They are usually controlled remotely by a lighting control console, via a control protocol such as DMX.
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Types of dimmer
Early examples of a dimmer include a salt water dimmer. In a salt water dimmer, there were two metal contacts in a glass beaker. One contact was on the bottom, while the other was able to move up and down. The closer the contacts to each other, the higher the level of the light. Using salt water dimmers was a tedious and precarious task that included filling the beakers with water, checking the concentration of the salt, and raising or lowering the top contact. Salt water dimmers were not efficient due to the evaporation of water and the corrosion of the many metal pieces. These dimmers were colloquially known as "piss pots", for obvious reasons. Many old theatre electricians still recount stories of how they were initiated into the art by being requested to "top up a pot" and receiving a shock, as unbeknownst to them the pot was live...
Dimmers were also often based on rheostats. These were inefficient; when set to the middle brightness levels, they could dissipate as heat a significant portion of the power rating of the load so they were physically large and required plenty of cooling air. Also, because their dimming effect depended a great deal on the total load applied to each rheostat, the load needed to be matched fairly carefully to the power rating of the rheostat. Finally, as they relied on mechanical control they were slow and it was difficult to change many channels at a time.
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Variable autotransformers were then introduced. While they were still nearly as large as rheostat dimmers, they were highly efficient devices and their dimming effect was independent of the load applied so it was far easier to design the lighting that would be attached to each autotransformer channel. Remote control of the dimmers was still impractical, although some dimmers (typically, for "house light" use) were equipped with motor drives that could slowly and steadily reduce or increase the brightness of the attached lamps. |
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Variable autotransformer dimmer |
Thyristor (and briefly, thyratron) dimmers were introduced to solve some of these problems. Because they use switching techniques instead of potential division there is almost no wasted power, dimming can be almost instantanious and is easily controlled by remote electronics. Triacs are usually used instead of SCR thyristors. dimmer rack (http://www.hstech.org/howto/electric/etcrack.jpg) The switches generate some heat during switching, and can cause interference. Large inductors are used as part of the circuitry to suppress this interference. When the dimmer is at 50% power the switches are switching their highest voltage(>300V in Europe) and the sudden surge of power causes the coils on the inductor to move, creating buzzing sound associated with some types of dimmer; this same effect can be heard in the filaments of the incandescent lamps as "singing". The suppression circuitry adds a lot of weight to the dimmer, and is often insufficient to prevent buzzing to be heard on audio systems that share the mains supply with the lighting loads.
Sine-wave dimming promises to solve the weight and interference issues that afflict thyristor dimmers. These are effectively high power switched-mode power supplies. They rely on a new generation of insulated gate bipolar transistors (IGBTs) which are still relatively expensive.
Control
Dimmers are usually controlled remotely by means of various protocols. Analogue dimmers usually require a separate wire for each channel of dimming carrying a voltage between 0 and 10V. Some analogue circuitry then derives a control signal from this and the mains supply for the switches. As more channels are added to the system more wires are needed between the lighting controller and the dimmers. Digital protocols, such as DMX512 have proved to be the answer. In early implementations a digital signal was sent from the controller to a demultiplexer, which sat next to the dimmers. This converted the digital signal into a collection of 0-10V signals which could be connected to the individual analogue control circuits.
Modern dimmer designs use microprocessors to convert the digital signal directly into a control signal for the switches. This has many advantages, giving closer control over the dimming, and giving the opportunity for diagnostic feedback to be sent digitally back to the lighting controller.
Patching
Dimmers are usually arranged together in racks, where they can be accessed easily, then cables are run to the instruments being controlled. In architectural installations cables are run straight from the dimmers to the lights. However venues such as theatres demand more flexibility. The lighting rig may change dramatically for each show, and occasionally during shows. Many theatres have cables run permanently to sockets(called circuits) around the theatre, however not all the sockets are needed for each show, so there will be fewer dimmers then there are circuits. A patch bay Patch Bay Picturesnext (http://www.hstech.org/howto/electric/dimmers2.htm) to the dimmers enables the dimmers to be connected to specific circuits. The patch bay may also enable many circuits to be connected to one dimmer, and even series connection for low-voltage lamps. This patch bay is known as the mains or hard patch. Analogue dimmers may also have a soft patch to match output channels from the lighting controller to control selected dimmers.
Dimming Curves
The design of most analogue dimmers meant that the output of the dimmer was not directly proportional to the input. Instead, as the operator brought up a fader the dimmer would dim slowly at first, then quickly in the middle, then quickly at the top. The shape of the curve resembled that of the third quarter of a sine wave. Different dimmers produced different dimmer curves. Digital dimmers are able to control the output much more exactly, giving a linear relationship between the control signal and power output. Many digital dimmers can be switched to different curves, so that they can be used with older analogue dimmers.
Preheat
Incandescent (filament) lamps should not be switched to full power from cold, and doing so can shorten their life dramatically owing to the large inrush current that occurs. To soften the blow to the lamps slightly, dimmers may have a preheat function. This sets a minimum level, usually around 5-10%, which is not obvious to the audience, but stops the lamp from cooling down too much. This also speeds up the instrument's reaction to sudden bursts of power which operators of rock'n'roll style shows appreciate. The opposite of this function is sometimes called top-set. This limits the maximum power supplied to an instrument, which can also extend its life.
The Digital Revolution
Modern digital desks can emulate preheat and dimmer curves, and allow a soft patch to be done in memory, and this is often preferred as it means that the dimmer rack can be exchanged for another one without having to transfer complicated settings.
See also
- Choke
- Fluorescent
- Fan
- Ganging and derating
- Heat dissipation
- Heat sink
- Incandescent
- Multilocation
- Potentiometer
- Triac