Dimmers

Dimmers are devices used to vary the brightness of luminaires. By varying the voltage and hence the available power to the lamp it is possible to vary the intensity of the light output. Modern professional dimmer systems respond to signals sent to them from the lighting control system using a standard digital protocol DMX512. The processes used to provide this varying voltage to the load vary from model to model and each has its own distinct advantages in specific applications.

Thyristor dimmers switch the power on and off at specific points in the mains cycle to deliver varying output levels. Simply put, lamp intensity is controlled by cutting the waveform at a selected point thus limiting the amount of energy flowing to the filament.

Triacs are commonly used to execute this switching process as they are cost effective and reliable. For heavy duty applications where higher fault current tolerances are required, back to back SCR’s (Silicon Controlled Rectifiers) are used to perform this switching role.

While elegant in its simplicity this process generates a number of by-products. Each time the current is switched it rises from zero to the set level in around 2 microseconds causing a sharp current rise that creates significant wide spectrum radio interference as well as a sudden shock to the lamp filament causing it to resonate or “sing”. To control these problems dimmers use a large inductor or “choke” in series with the load that has enough inertia to slow the rate at which the current rises each time it is switched on. These inductors are classified in terms of the time it takes for the switched voltage to rise from 10% to 90% of its set level at a 90 degree firing angle. This is referred to as the rise time of the dimming circuit. Generally the higher the value the quieter the dimmer will be.

The advent of new switching devices has opened up new possibilities for dimming technology. The IGBT, or “insulated gate bipolar transistor,” is a semi conductor capable of switching high currents on and off many times per half cycle. Using robust, high speed IGBTs it is now possible to cut the incoming mains into a large number of pieces to create a sine wave output of continuously variable amplitude. By retaining the waveform, the sine wave dimmer effectively eliminates the problems of filament noise and mains harmonics, offering a clean power source with a linear characteristic.

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