page_title: Dimmer
section_title: Solid-state dimmer
hierarchical_section_title: Dimmer / Types of dimmer / Solid-state dimmer
caption_attribution_description: English: Voltage waveform of a thyristor dimmer adjusted to 60 volts RMS output with 120 V input. The load is incandescent lamp bulbs.
context_page_description: Dimmers are devices connected to a light fixture and used to lower the brightness of light. By changing the voltage waveform applied to the lamp, it is possible to lower 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 light output from resistive incandescent, halogen, and compact fluorescent lights and light-emitting diodes. More specialized equipment is needed to dim fluorescent, mercury vapor, solid-state, and other arc lighting.
  Dimmers range in size from small units the size of domestic light switches to high-power units used in large theatrical or architectural lighting installations. Small domestic dimmers are generally directly controlled, although remote control systems are available. Modern professional dimmers are generally controlled by a digital control system like DMX or DALI. In newer systems, these protocols are often used in conjunction with Ethernet.
  In the professional lighting industry, changes in intensity are called "fades" and can be "fade up" or "fade down".
context_section_description: 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.  Inductors or chokes 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 (>325 V in Europe) and the sudden surge of power causes the coils on the inductor to move, creating a 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 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.  European dimmers must comply with relevant EMC legislation requirements; this involves suppressing the emissions described above to limits described in EN55104.
  
  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. Diodes (D2, D3, D4 and D5) form a bridge, which generates pulsed DC. 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. When C1 is able to make Zener diode D6 conduct and inject current into the SCR, the SCR will fire. When the SCR conducts, D1 will discharge C1 via the SCR. 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. This circuit is called a Leading-Edge Dimmer or Forward Phase Dimming.  Leading edge dimmers work well with incandescent lamps, but not with all types of LED lamps.
  
  Dimmers based on Insulated-gate bipolar transistors (IGBTs) do away with most of the noise present in TRIACs by chopping off the falling side of the sine wave. These circuits are called Trailing-Edge Dimmers or Reverse Phase Dimming, and work well with 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.
caption: Waveform of the output voltage of a thyristor dimmer set for 60 volts RMS output, with 120 V input.  The red trace shows the output device switching on about 5.5 ms after the input (blue) voltage crosses zero.  Switching the thyristor on earlier in each half cycle gives a higher output voltage and brighter lights.
exif: {}