All You Need to Know About TRIACs

A “Triode for AC” is a component that is commonly abbreviated into TRIAC, where ‘TRI’ denotes a device with three terminals, and AC implies that it controls alternating current or conducts it in both directions. TRIACs are semiconductor devices used for switching control and power applications, and they are used extensively in fan speed controls, lamp light controls, phase controls, and even modern computerized control units of several household devices. Since TRIACs find widespread application in several industries, learning more about their construction and function becomes imperative.

How Are TRIACs Constructed

TRIACs are four-layered, three-terminal, bilateral semiconductor devices which incorporate two silicon controlled rectifiers in inverse parallel connection with a common gate terminal. A typical TRIAC has six doped regions with a gate terminal embedded in a single chip device, having contact with both the P and N terminals, a design feature that allows polarity reversal to instigate conduction. Unlike conventional circuit layouts which use a cathode or anode, TRIACs are bilateral devices and use different terminology for their terminals, such as ‘MT₁’ for main terminal 1, ‘MT₂’ for main terminal 2, and ‘G’ for the gate. Usually, all currents and voltages in the circuit are specified with MT₁ as the reference nodal point for convenience, even though TRIACs are bidirectional devices.

MT₁ can be connected to P₂ and N₂, whereas MT₂ can be connected to P₁ and N₃. Since MT₁ and MT₂ are connected to both P and N terminals, their current polarity can be determined by the device’s layers. Meanwhile, the gate terminal ‘G’ is close to the MT₁ node while connected to the P₂ and N₄ regions. A minimum threshold current should be maintained at the gate to keep the TRIAC conductive because if the gate is removed, the TRIAC gets switched off. The gate can also be triggered by either negative or positive voltage.

Functional Operation of TRIACs

When the input voltage applied to the TRIAC equals its breakdown voltage, then the TRIAC becomes conductive. However, the standard method to turn on a TRIAC is by using a proper gate current where the higher the gate current is, the lower the voltage supply that is needed to turn on the TRIAC. Therefore, it can conduct current regardless of the voltage polarity between MT₂, the gate, and the polarity between MT₁ and MT₂. As a result, four different permutations and combinations of TRIAC layouts can exist with MT₁ as a reference, where both MT₂ and the gate are either positive (trigger mode 1) or negative (trigger mode 3), or where only MT₂ is positive (trigger mode 2) or negative (trigger mode 4) while the gate develops the opposite polarity to that of MT₂ in each case. In some cases, TRIACs may be called two-quadrant switching gate controlled devices. This is because they can be triggered into conduction by both negative and positive voltage applications and the positive and through negative trigger pulses applied to the gate.

Generally, trigger modes 2 and 3 have a high sensitivity, though trigger mode 1 exhibits the highest of the three. Trigger modes 2 and 3 are the most conventionally used types because of their uniform gate and bidirectional control. With their high sensitivity, negative gate pulses are used for their marginal triggering capability. Meanwhile, triggering mode 1 requires a positive gate trigger, even though its sensitivity is higher than 2 and 3 combined.

Types of TRIACs

TRIACs are perfect for AC switching applications since they can conduct current in both bisects of an alternating current circuit. Standard 4 quadrant (4Q) TRIACs can conduct electricity in all four trigger modes described above. They contain resistor-capacitor snubbers at all the main terminals alongside an inductor placed in series with the component. Meanwhile, three-quadrant (3Q) TRIACs use the 1, 2, and 3 trigger mode without the need for any additional protective devices connected to the circuit. Some of the other common types of TRIACs used today have been described below:

1. TRIAC BT136: Digital circuits are used to drive TRIAC BT136 devices because they use a maximum current of 4A with a low minimum threshold voltage. These TRIACs are most suitable for applications where the AC load uses sub-6A current through microcontrollers or microprocessors. Furthermore, the maximum voltage at the gate during the on-state is 1.4 V, while the current is 4A. The maximum terminal voltage is 600 V and the gate holding current is 2.2 mA.

2. TRIAC BT139: TRIAC BT139 is a sensitive 4Q TRIAC suitable for phase control and switching applications. Such TRIACs are designed for high voltage blocking capacity and high thermal cycling, making them ideal for switching, motor control, and industrial & domestic lighting applications. TRIAC BT139 is connected in series with logic ICs and low power gate triggers, with a minimum gate current of 35 mA, and a maximum current and voltage of 16A and 800V, respectively.

Advantages of Using TRIACs

TRIACs boast flexibility since they can function with alternating current waveforms in both half cycles. Moreover, TRIACs have minimal part requirements, needing only a single sink and fuse. Furthermore, a parallel diode is not required for such applications as safe breakdown of current is possible in either direction.

Wrapping Up

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