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Each dimmer had its own control wires, resulting in many wires leaving the lighting control location. Dimmers based on rheostats were inefficient since they would dissipate a significant portion of the power rating of the load as heat. They were large and required plenty of cooling air.

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.

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Finally, as they relied on mechanical control they were slow and it was difficult to change many channels at a time. Early examples of a rheostat dimmer include a salt water dimmer or liquid rheostat ; the liquid between a movable and fixed contact provided a variable resistance.


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The closer the contacts to each other, the more voltage was available for the light. Salt water dimmers required regular addition of water and maintenance due to corrosion; exposed parts were energized during operation, presenting a shock hazard. The coil-rotation transformer used a fixed-position electromagnet coil in conjunction with a variable-position coil to vary the voltage in the line by varying the alignment of the two coils.

Rotated 90 degrees apart, the secondary coil is affected by two equal but opposite fields from the primary, which effectively cancel each other out and produce no voltage in the secondary. These coils resembled the standard rotor and stator as used in an electric motor, except that the rotor was held against rotation using brakes and was moved to specific positions using high-torque gearing.

Because the rotor did not ever turn a complete revolution, a commutator was not required and long flexible cables could be used on the rotor instead. Variable autotransformers trade name " Variac " were then introduced. While they are still nearly as large as rheostat dimmers, which they closely resemble, they are relatively efficient devices. Their voltage output, and so their dimming effect, is largely 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 were equipped with motor drives that could slowly and steadily reduce or increase the brightness of the attached lamps.

Light dimmer circuit using triac and diac (Practical)

Autotransformers have fallen out of use for lighting but are used for other applications. Solid-state or semiconductor dimmers were introduced to solve some of these problems. Semiconductor dimmers switch on at an adjustable time phase angle after the start of each alternating current half-cycle, thereby altering the voltage waveform applied to lamps and so changing its RMS effective value.

Because they switch instead of absorbing part of the voltage supplied, there is very little wasted power. Dimming can be almost instantaneous and is easily controlled by remote electronics. This development also made it possible to make dimmers small enough to be used in place within the pattress of normal domestic light switches. The switches generate some heat during switching and can also cause radio-frequency interference.

The suppression circuitry might be insufficient to prevent buzzing to be heard on sensitive audio and radio equipment that shares the mains supply with the lighting loads. In this case, special steps must be taken to prevent this interference. In the electrical schematic shown, a typical silicon-controlled rectifier SCR based light dimmer dims the light through phase-angle control.

This unit is wired in series with the load. R1 and C1 form a circuit with a time constant. As the voltage increases from zero at the start of every halfwave C1 will charge up. The SCR will shut off when the current falls to zero and the supply voltage drops at the end of the half cycle, ready for the circuit to start work on the next half cycle. Leading edge dimmers work well with incandescent lamps, but not with all types of LED lamps.

An even newer, but still expensive technology is sine-wave dimming , which is implemented as high power switched-mode power supplies followed by a filter. 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 10 V.

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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. In the late 70s, serial analogue protocols were developed.

These multiplexed a series of analogue levels onto a single wire, with embedded clocking signal similar to a composite video signal in the case of Strand Lighting's European D54 standard, handling dimmers or separate clocking signal in the case of the US standard AMX Digital protocols, such as DMX have proved to be the answer since the late 80s. In early implementations a digital signal was sent from the controller to a demultiplexer , which sat next to the dimmers. 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. Some dimmers in residential applications are also equipped with a radio receiver to be used as wireless light switches which can be remotely controlled by a radio transmitter. Patching is the physical "hard patch" or virtual "soft patch" assignment to a circuit or channel for the purpose of control.

Dimmers are usually arranged together in racks, where they can be accessed easily, and then power is run to the instruments being controlled. In architectural installations electricity is run straight from the dimmers to the lights via permanent wiring this is called a circuit. They are hard run and cannot be changed.


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However venues such as theatres demand more flexibility. To allow for changes for each show, and occasionally during shows, theatres sometimes install circuits run permanently to sockets around the theatre. Instead of these circuits going directly to the dimmer they are connected to a patch bay.

Step 2: Switching - NPN Transistor

A patch bay usually sits next to the dimmers enabling the dimmers to be connected to specific circuits via a patch cable. The patch bay may also enable many circuits to be connected to one dimmer and even series connection for low-voltage lamps. Also in some theatres individual cables are run directly from the light to dimmer. The assigned connections between the circuits either at the patch bay or in the form of individual cables and the dimmers is known as the mains or hard patch.

This is most common in older theatres, and on a tour where dimmers will be brought in by the touring company. Most modern fixed installations do not have patch bays, instead they have a dimmer-per-circuit and patch dimmers into channels using a computerised control consoles Soft Patch. 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 slowly at the top.

The shape of the curve resembled that of the third quarter of a sine wave. Different dimmers produced different dimmer curves, and different applications typically demanded different responses.

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Show Categories. This important and easy to read title covers the history of electrical and electronic dimming, how dimmers work, current dimmer types from around the world, planning of a dimming system, looking at new sine wave dimming technology and distributed dimming. Integration of dimming into different performance venues as well as the necessary supporting electrical systems are fully detailed. Significant levels of information are provided on the many different forms and costs of potential solutions as well as how to plan specific solutions.

Architectural dimming for the likes of hotels, museums and shopping centres are included. Practical Dimming is a companion book to Practical DMX and is designed for all involved in the use, operation and design of dimming systems. The product has been added to your basket. Entertainment Technology Press publishes a rapidly expanding range of books covering the technical aspects of entertainment technology. Books currently available include titles on lighting, audio, rigging, production, stage engineering, TV, safety, standards, biography and history. Toggle navigation.

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