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Many opto-TRIACs come with zero-cross circuits built in. They are often used to control larger, power TRIACs. In this setup TRIAC turn-on delays compound, so quick turn-on times are important. The corresponding phase-angle circuits are more sophisticated and more expensive than zero-cross circuits.
A TRIAC (triode for alternating current; also bidirectional triode thyristor or bilateral triode thyristor [1]) is a three-terminal electronic component that conducts current in either direction when triggered. The term TRIAC is a genericised trademark.
A zero-crossing is a point where the sign of a mathematical function changes (e.g. from positive to negative), represented by an intercept of the axis (zero value) in the graph of the function. It is a commonly used term in electronics, mathematics, acoustics, and image processing.
In AC circuits, SCR or triac relays inherently switch off at the points of AC zero cross when there is zero load current. The circuit will never be interrupted in the middle of a sine wave peak, preventing the large transient voltages that would otherwise occur due to the sudden collapse of the magnetic field around the inductance.
One example is the TRIAC, which is comparable to two thyristors connected back-to-back (in other words, reverse parallel), but on a single piece of silicon. Two LEDs can be paired this way, so that each protects the other from reverse voltage. A series string of such pairs can be connected to AC or DC power, with an appropriate resistor.
A Voltage controller thyristor based dimmer rack An electrical schematic for a typical SCR-based light dimmer. A voltage controller, also called an AC voltage controller or AC regulator is an electronic module based on either thyristors, triodes for alternating current, silicon-controlled rectifiers or insulated-gate bipolar transistors, which converts a fixed voltage, fixed frequency ...
In the following example crowbar circuit, the LM431 adjustable zener regulator controls the gate of the TRIAC. The resistor divider of R 1 and R 2 provide the reference voltage for the LM431. The divider is set so that during normal operating conditions, the voltage across R 2 is slightly lower than V REF of the LM431. Since this voltage is ...
GTO thyristors suffer from long switch-off times, whereby after the forward current falls, there is a long tail time where residual current continues to flow until all remaining charge from the device is taken away. This restricts the maximum switching frequency to about 1 kHz. It may be noted, however, that the turn-off time of a GTO is ...