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What are the Functions and Applications of TRIAC Circuits?

Posted on February 11, 2022 Michael Rodriguez Electronics

The term “TRIAC” stands for “triode for alternating current,” and as the name suggests, it is utilized as a control element in AC circuits. These semiconductor devices feature three terminals, each of which perform varying functions. However, they operate by utilizing only one terminal called the gate to trigger the flow of electrical current in the other two terminals, which are called the main terminals or anodes.

While TRIACs are similar to electronic switches such as silicon controlled rectifiers (SCRs), their distinctive quality lies in their ability to optimally conduct current in both directions. Typically, they find use as switches in a variety of electrical equipment, such as lamps, fans, and motors. Nonetheless, all TRIACs operate using the working principle outlined above, and can be categorized into one of two types: TRIAC switch circuits or TRIAC variable power circuits.

When used as simple electronic switches, TRIACs are triggered by an AC voltage present at the gate, and a resistor works in tandem with the gate to limit the current flow to the other terminals. Furthermore, the TRIAC enables current to flow in either direction, changing with the polarity of the gate voltage. The gate voltage is sourced from the AC voltage that is applied to the TRIAC’s load terminals. In the case that the application at hand only necessitates current flow in one direction, a diode is connected alongside the gate to convert the AC voltage into direct current (DC). Such a configuration allows a fixed current to flow through the TRIAC.

As power level or dimmer controllers, TRIACs consist of a gate trigger that is tasked with producing varying phases in relation to load voltage. Moreover, trigger voltage is derived from the load voltage, but with a phase shift applied to it. In this configuration, the phase-shifting circuitry is composed of a variable resistor and a capacitor. The capacitor voltage is utilized as the trigger voltage, which has its phase varied by the changing variable resistor. Usually, a diode for AC (DIAC) switch is affixed between the capacitor and the TRIAC’s gate to turn the TRIAC on.

Although TRIACs are found in a variety of applications, they are not without disadvantages. As such, it is important to be aware of the common issues TRIACs pose and how to resolve them. The first problem TRIACs encounter is rate effect. This effect occurs when a sudden change in voltage across the main terminals switches the TRIAC on. This occurrence can be remedied by connecting a resistor-capacitor (RC) snubber circuit between the terminals. 

The next most common issue is backlash effect. Backlash typically occurs in phase control circuits when the resistance is set to the maximum in order to reduce the power levels of the connected device to a minimum. Due to the absence of a discharge path for the TRIAC’s capacitance across the load terminals, backlash ensues, preventing the connected devices from operating. By connecting a high-value resistor in series with the DIAC or a capacitor between the gate and main terminals, an alternative discharge path is achieved, allowing the devices to turn on as usual.

Another problem TRIACs present is non-symmetrical firing in phase control circuits. This issue happens because TRIACs have different turn-on voltages for each direction. This design renders a poor electromagnetic radiation profile for the TRIAC. To resolve this issue, one can use a DIAC in conjunction with the TRIAC’s gate to even out the firing ability of the TRIAC. Lastly, the final problem TRIACs face is harmonic filtering. As the TRIAC turns on when the voltage across the terminals is not zero, harmonics are generated that are unsuitable for sensitive electronic equipment like wireless communications circuitry. That being said, it is important to note that harmonic filters suppress electromagnetic interference.

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