Two-way thyristor schematic

Two-way thyristor schematic - nicknamed "two-way thyristor"

Bidirectional thyristor, also known as a TRIAC, is a four-layer, three-terminal device made from silicon. It functions as an AC switch and was invented in 1957. Unlike traditional thyristors, which are unidirectional, the TRIAC can conduct in both directions, making it ideal for AC control applications. This allows it to replace two thyristors connected in parallel with opposite polarity, simplifying the circuit design and reducing the number of components needed.

The TRIAC has three terminals: T1, T2, and G (gate). It operates in two states: on and off. The main types include bolt-shaped, flat plate, and flat-bottomed designs. Due to its bidirectional nature, it doesn’t have a fixed anode or cathode like a standard thyristor. Instead, T1 and T2 are considered the main terminals, and their roles depend on the voltage polarity applied.

Because of its ability to conduct in both directions, the TRIAC is often referred to as a "silicon-controlled rectifier" (SCR) in some contexts. However, unlike SCRs, which only conduct in one direction, the TRIAC can be triggered regardless of the voltage polarity. This makes it particularly useful in AC power control systems such as dimmers, motor speed controllers, and heating systems.

Figure 1: Two-way thyristor outline drawing

Two-way thyristor schematic - structural schematic

The two-way thyristor belongs to the NPNPN five-layer structure and has three electrodes: T1, T2, and G. Although it can be thought of as two thyristors connected in anti-parallel, it's actually a more complex integrated device composed of multiple transistors and resistors. Its symmetrical design allows it to operate in both forward and reverse directions without requiring separate circuits for each direction.

The gate (G) controls the conduction between T1 and T2. Depending on the polarity of the voltages at G and T2 relative to T1, either T1 or T2 acts as the anode or cathode. This bidirectional behavior is what gives the TRIAC its unique advantage over conventional thyristors.

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

Two-way thyristor schematic - characteristics

The TRIAC is a three-terminal component that can be turned on regardless of whether the voltage across T1 and T2 is positive or negative. Its symbol and structure are shown in Figure 3. When a gate signal is applied, the TRIAC conducts between T1 and T2, and when no signal is present, it remains in a high-impedance state.

(a) Symbol (b) Construction

Figure 3: TRIAC

Two-way thyristor schematic - triggering characteristics

The TRIAC can be triggered in four different combinations of gate voltage (VG) and the voltage between T1 and T2 (VT1T2). These combinations are:

• 1. VT1T2 is positive and VG is positive
• 2. VT1T2 is positive and VG is negative
• 3. VT1T2 is negative and VG is positive
• 4. VT1T2 is negative and VG is negative

It is generally best to use symmetrical conditions (such as 1 and 4 or 2 and 3) to achieve balanced performance in both positive and negative half-cycles. The most convenient method is using condition 1 or 4, where the gate signal aligns with the polarity of the voltage across T1 and T2.

Figure 4: TRIAC VI characteristic curve

Two-way thyristor schematic - phase control

Phase control of the TRIAC is similar to that of an SCR, but it can be triggered even when the voltage between T1 and T2 is negative. This allows it to conduct in both the positive and negative half-cycles of an AC waveform, making it suitable for full-wave power control. As a result, the TRIAC offers greater flexibility and convenience in AC power management compared to traditional thyristors.

Figure 5(a) shows a TRIAC phase control circuit that adjusts the excitation angle by varying the RC time constant. Figures 5(b) and 5(c) illustrate the voltage waveforms across the AC supply and the load when the excitation angle is set to 30 degrees. In general, the TRIAC is used for lower-voltage and lower-current applications, typically up to around 600V and 40A.

(a)

(b) Voltage waveform across the AC

(c) Voltage waveform across the load

Figure 5: TRIAC phase control circuit

Two-way thyristor schematic - detection method

To identify the electrode positions of a bidirectional thyristor using a multimeter in RX1 mode, follow these steps:

1. Determine the T2 pole: The resistance between G and T1 is low, while the resistance between T2 and G or T2 and T1 is infinite. This helps identify the T2 terminal.
2. Distinguish between G and T1 poles: After identifying T2, assume one of the remaining pins is T1 and the other is G. Connect the black lead to T1 and the red lead to T2. If the resistance is infinite, apply a trigger signal to G. If the resistance drops to tens of ohms, the thyristor is conducting.

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

Two-way thyristor schematic - working principle

A bidirectional thyristor consists of a P1N1P2N2 structure with three PN junctions. It can be modeled as a combination of a PNP transistor and an NPN transistor. When a forward voltage is applied to the anode, both BG1 and BG2 transistors enter an amplification state. A trigger signal applied to the gate initiates conduction, leading to a sharp increase in current due to positive feedback between the two transistors.

Figure 7: Bidirectional thyristor equivalent diagram

Two-way thyristor schematic - naming rules

Triac (TRIAC) stands for "Three Terminal AC Switch." BCR stands for "Bidirectional Controlled Rectifier," and BT represents "Bidirectional Thyristor." Different manufacturers use various naming conventions, such as BCR for Mitsubishi, BT for ST and Philips, and others. These naming schemes help identify the type and function of the device in various applications.

Figure 8: Two-way thyristors have different naming schemes

Recommended reading:

• Control LED lighting with three-terminal and four-terminal thyristor switching elements
• Design scheme of two-way thyristor trigger circuit
• Design of bidirectional thyristor zero-crossing trigger circuit
• multimeter