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The TRIAC's gate can be connected through an opto-isolated transistor, and sometimes a resistor to the microcontroller, so that bringing the voltage down to the microcontroller's logic zero pulls enough current through the TRIAC's gate to trigger it. This ensures that the TRIAC is triggered in quadrants II and III and avoids quadrant IV where ...
A zero-cross circuit works to correct this problem, so that the TRIAC functions as well as possible. This is typically done with thyristors in two of the three phases. Many opto-TRIACs come with zero-cross circuits built in. They are often used to control larger, power TRIACs.
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.
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.
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. When triggered, thyristors turn on and become low-resistance ...
A Triggering device is an electronic circuit, such as a Schmitt trigger, ... Thus, a transistor is not a thyristor even though it can operate like a switch (ON-OFF ...
In addition to its use as the active device in relaxation oscillators, one of the most important applications of UJTs or PUTs is to trigger thyristors (silicon controlled rectifiers (SCR), TRIACs, etc.). A DC voltage can be used to control a UJT or PUT circuit such that the "on-period" increases with an increase in the DC control voltage.
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).