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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.
Zero-crossing control (or burst-fire control) is an approach for electrical control circuits that starts operation with the AC load voltage at close to 0 volts in the AC cycle. [1] This is in relation to solid-state relays, such as TRIACs and silicon controlled rectifiers . [ 1 ]
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.
If no current/voltage is applied to the gate lead, the TRIAC switches off. On the other hand, if the trigger voltage is applied to the gate lead, the TRIAC switches on. TRIACs are suitable for light-dimming circuits, phase-control circuits, AC power-switching circuits, AC motor control circuits, etc.
A QUADRAC is a special type of thyristor which combines a DIAC and a TRIAC in a single package. The DIAC is the triggering device for the TRIAC. Thyristors are four-layer (PNPN) semiconductor devices that act as switches, rectifiers or voltage regulators in a variety of applications.
The control signal must be coupled to the controlled circuit in a way which provides galvanic isolation between the two circuits. Many SSRs use optical coupling. The control voltage energizes an internal LED which illuminates and switches on a photo-sensitive diode (photo-voltaic); the diode current turns on a back-to-back thyristor ( TRIAC ...
The most common of these operations include electric lighting, electric motor speed control, electric welding, and electric heating. Although it is always possible to control the amount of electrical power delivered to a load by using a variable transformer to create a variable secondary output voltage , these transformers are physically large ...
This makes the TRIAC a very convenient switch for AC circuits, allowing the control of very large power flows with milliampere-scale control currents. In addition, applying a trigger pulse at a controllable point in an AC cycle allows one to control the percentage of current that flows through the TRIAC to the load (phase control).