Two-way thyristor schematic

Two-way Thyristor Schematic - Nicknamed "Two-way Thyristor"

The bidirectional thyristor, also known as a TRIAC, is a four-layer, three-terminal semiconductor device made from silicon. It functions as an AC switch and was first developed in 1957. Unlike a standard thyristor, which only conducts in one direction, the TRIAC can conduct in both directions, making it ideal for AC power control applications. It operates using a single gate trigger circuit, replacing the need for two reverse-connected thyristors. The main types of thyristors include bolt-shaped, flat-plate, and flat-bottom configurations.

Due to its symmetrical characteristics, the bidirectional thyristor is often referred to as a silicon-controlled rectifier (SCR) or simply a thyristor. Its name comes from its ability to switch current in both directions, making it a versatile component in modern electronics.

Figure 1: Two-way thyristor outline drawing

Two-way Thyristor Schematic - Structural Diagram

The two-way thyristor belongs to the NPNPN five-layer structure with three terminals: T1, T2, and G. Although it can be viewed as a combination of two standard thyristors, it is actually a complex integrated device made up of multiple transistors and resistors. This design allows it to conduct in both directions without requiring separate anode and cathode definitions.

When the voltage at the gate (G) is positive relative to T1, T2 acts as the anode, and T1 as the cathode. Conversely, if the gate voltage is negative relative to T1, T1 becomes the anode and T2 the cathode. Due to this symmetry, the device can be triggered regardless of the polarity of the applied voltage.

As shown in Figure 2(a), the structure of the two-way thyristor differs from that of a unidirectional thyristor. When simplified, it appears as two thyristors connected in anti-parallel, as seen in Figure 2(b) and 2(c).

Figure 2: Schematic diagram of the two-way thyristor structure

Two-way Thyristor Schematic - Characteristics

The TRIAC is a three-terminal device with T1 (second terminal or second anode), T2 (first terminal or first anode), and G (gate). Unlike a standard SCR, it can be triggered regardless of whether the voltage between T1 and T2 is positive or negative. This makes it highly suitable for AC power control applications.

Its symbol and construction are illustrated in Figure 3. If a gate signal is present, the device will conduct between T1 and T2, otherwise it remains in a high-impedance state. This feature makes it ideal for use in dimming circuits and motor speed controls.

(a) Symbol (b) Construction

Figure 3: TRIAC

Two-way Thyristor Schematic - Triggering Characteristics

The TRIAC can be triggered in four different voltage combinations: when VT1T2 is positive and VG is positive, when VT1T2 is positive and VG is negative, when VT1T2 is negative and VG is positive, or when both are negative. For optimal performance, it is recommended to use symmetrical triggering conditions to ensure balanced operation in both half-cycles.

Figure 4: TRIAC VI characteristic curve

As shown in Figure 4, the VI curve of a TRIAC is similar to that of an SCR, but it can be turned on in both forward and reverse voltages. This means the third quadrant of the curve resembles the first quadrant, allowing the device to function as a bidirectional switch.

Two-way Thyristor Schematic - Phase Control

Phase control of a TRIAC is similar to that of an SCR, but it has the advantage of being able to trigger even when the voltage between T1 and T2 is negative. This allows for full-wave control of AC power, making it more efficient than traditional SCRs.

Figure 5(a) shows a phase control circuit that adjusts the firing angle by varying the RC time constant. Figures 5(b) and 5(c) illustrate the voltage waveforms across the AC supply and the load at a 30-degree firing angle. TRIACs typically have lower voltage and current ratings compared to SCRs, usually around 600V and 40A or less.

(a)

(b) Voltage waveform across the AC

(c) Voltage waveform across the load

Figure 5: TRIAC phase control circuit

Two-way Thyristor Schematic - Testing Method

To identify the terminals of a bidirectional thyristor using a multimeter on RX1 scale, follow these steps:

1. Determine the T2 terminal: The resistance between G and T1 is low, while between T2 and G or T2 and T1 is high. This helps identify T2 as the terminal with the highest resistance.

2. Distinguish between G and T1: Connect the black probe to T1 and the red probe to T2. If the resistance is infinite, then apply a short circuit between T2 and G to trigger the device. If the resistance drops, it confirms the correct identification of T1 and G.

Figure 6: Using a multimeter to determine the bidirectional thyristor electrode

Two-way Thyristor Schematic - Working Principle

The bidirectional thyristor consists of four layers (P1N1P2N2) and three PN junctions. It can be modeled as a combination of a PNP and an NPN transistor. When a forward voltage is applied to the anode, both transistors enter an amplification state.

Figure 7: Bidirectional thyristor equivalent diagram

If a gate signal is applied, the base current of BG2 increases, causing a chain reaction that leads to saturation and conduction. Once triggered, the thyristor remains on until the current falls below the holding level.

Two-way Thyristor Schematic - Naming Rules

TRIAC stands for Triode for Alternating Current. BCR stands for Bidirectional Controlled Rectifier. BT refers to Bidirectional Triode. These naming conventions help identify the type and function of the device.

For example, Mitsubishi’s BCR series and ST’s BT series are common models used in various industrial and consumer applications. Understanding these naming conventions helps in selecting the right component for specific applications.

Figure 8: Two-way thyristors have different naming schemes

Recommended reading: Control LED lighting with thyristor switching elements, Design of bidirectional thyristor zero-crossing trigger circuit, Multimeter usage in testing thyristors.