Dimmer - Definition
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Dimmer - Description :
Dimmers are devices used to vary the voltage running to a light. By decreasing or increasing the voltage to the lamp it is possible to vary the intensity of the lamp. Changes in intensity are called “fades” and can be “fade ups” or “fade downs.” Although variable-voltage devices are used for various purposes, the term dimmer is generally associated with lighting control. Small dimmers are seen more and more in the home. These have traditionally been actuated by direct control. The advent of X-10 technology is making remote control of home dimmers possible. A digital signal is imposed on the house wiring and remote dimmers decode the signal and respond accordingly. Large dimmer systems are used in settings with high wattage lighting instruments, or with a large number of instruments that must be controlled independently such as a stage disco or other similar venue. 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. When thyrister 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.
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.
Variable autotransformer dimmer
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Variable autotransformer dimmer
Variable autotransformers (often reffered 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 sinewave output and produce no radio frequency noise.
Thyristor dimmer rack
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Thyristor dimmer rack
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 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 lightswitches.
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. |
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