Dimmer
From Free net encyclopedia
Dimmers are devices used to vary the control the brightness of a light. By decreasing or increasing the RMS voltage and hence the mean power to the lamp it is possible to vary the intensity of the light output. Although variable-voltage devices are used for various purposes, the term dimmer is generally reserved for those intended to control lighting.
Dimmers range in size from small units the size of a normal lightswitch used for domestic lighting to high power units used in large theatre or architectural lighting installations. Small domestic dimmers are generally directly controlled although remote control systems (such as X10) are available. Modern professional dimmers are generally controlled by a digital control system like DMX.
In the professional lighting industry changes in intensity are called “fades” and can be “fade ups” or “fade downs”. Dimmers with direct manual control had a limit on the speed they could be varied at but this issue is pretty much gone with modern digital units (although very fast changes in brightness may still be avoided for other reasons like bulb life).
Contents |
History
Early dimmers were directly controlled through the manual manipulation of large dimmer panels, but this meant that all power had to come through the lighting control location, which could be inconvenient and potentially dangerous, especially with systems that had a large number of channels, high power lights or both (such as a stage disco or other similar venue).
When thyristor dimmers came into use, analog remote control systems (often 0-10V) became feasible. The wire for the control systems was much smaller (with low current and lower danger) than the heavy power cables of previous lighting systems. Each dimmer had its own control wires which meant a huge number of wires leaving the lighting control location and running to each individual dimmer. Modern systems use a digital control protocol such as DMX to control a large number of dimmers (and other stage equipment) through a single cable.
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 (up to 25% for resistive loads, more for temperature dependent loads like lamps) 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.
Variable autotransformers (often referred to as variacs) 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. Whilst variacs have fallen out of use for lighting they are still used in other applications such as under/overvoltage testing of equipment due to the fact they deliver a reasonably pure sine wave output and produce no radio frequency noise.
Image:Dimmer CD80.jpg 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. 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. This development also made it possible to make dimmers small enough to be used in place of normal domestic light switches.
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
Non domestic 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, and 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 (see picture) next 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 can be made to have whatever curve the manufacturer desires and may have a choice between a linear relationship and selection of 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 (implement)
- Ganging and derating
- Heat dissipation
- Heat sink
- Incandescent
- Potentiometer
- Triac
External links