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This circuit is called a leading-edge dimmer or forward phase dimming. 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.
Forward-voltage triggering occurs when the anode–cathode forward voltage is increased with the gate circuit opened. This is known as avalanche breakdown, during which junction J2 will break down. At sufficient voltages, the thyristor changes to its on state with low voltage drop and large forward current.
DIACs and SIDACs are often used to deliver a pulse once a capacitor has charged to the breakdown voltage, giving both controlled delay set by the charging resistor and a fixed pulse energy set by the capacitor and breakdown voltage. This is common in simple phase angle controls for AC lamp dimmers and motor speed controls.
In addition, applying a trigger at a controlled phase angle of the AC in the main circuit allows control of the average current flowing into a load (phase control). This is commonly used for controlling the speed of a universal motor, dimming lamps, and controlling electric heaters. TRIACs are Bipolar devices.
Phase-fired control (PFC), also called phase cutting or phase-angle control, is a method for power limiting, applied to AC voltages. [1] It works by modulating a thyristor , SCR , triac , thyratron , or other such gated diode -like devices into and out of conduction at a predetermined phase angle of the applied waveform.
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Adjusting the brightness of light emitted by a light source is then merely a matter of setting at what voltage (or phase) in the AC half-cycle the dimmer begins to provide electric current to the light source (e.g. by using an electronic switch such as a triac). In this case the PWM duty cycle is the ratio of the conduction time to the duration ...
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