How do Light Dimmers Work?
A light dimmer works by essentially chopping parts out of the AC voltage. This
allows only parts of the waveform to pass to the lamp. The brightness of the
lamp is determined by the power transferred to it, so the more the waveform is
chopped, the more it dims.
Mains power is comprised of an alternating current that flows in one direction
and then in the other, along the cable, at the rate of 50 or 60 cycles per
second (known as Hertz). The value 50 or 60Hz is dependent on the countries
power system. The current alternates back and forth changing direction at the
zero point. If we were to look at this waveform it would appear as a stretched
S shape on its side ~. Draw a line through the middle and this is what is called
the zero crossing point. At this instant in time no current is flowing in either
direction. This is the point at which a dimmer is electronically synchronized
to turn the power ON or OFF. By chopping the waveform at the zero-crossing
point, smooth dimming can be achieved without the lamp flickering. This turning
on and off of the power device occurs every time the mains crossing point is
reached (half phase), 100 or 120 times per second.
Typically light dimmers are manufactured using a Triac or Thyristor as the
power control device. These electronic parts are semiconductors not dissimilar
to transistors. A Thyristor is a Uni.-directional device and hence, because
AC power flows in both directions, two are needed. A triac is a bidirectional
device and therefore only one is needed. An electronic circuit determines
the point in time at which they turn ON (conduct). The ON state continues
until the next zero-crossing point, at which point the device turns itself
OFF. The electronic circuit then provides a delay, which equates to the dimness
of the lamp, before turning the control device back on. The slight capacitance
of the load, filters the chopped waveform resulting in a smooth light output.
Some controllers use a microprocessor control with the above timing function
being handled by an analogue circuit. More sophisticated systems, called
digital dimmers, operate the switching direct from microprocessor. This has
the advantage of greater reliability, quieter operation, lower cost and smaller
controls.
Below is a typical picture of the mains sine wave, and a phase-controlled
waveform.
Read more on
What is a dimmer?
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