The structure principle and application of bidirectional thyristor

Ordinary thyristors (VS) are essentially DC control devices. To control AC loads, two thyristors must be connected in parallel with opposite polarity so that each SCR controls one half-wave. To do this, two independent trigger circuits are required, which is not convenient to use.

Bidirectional thyristor is developed on the basis of ordinary thyristor. It can not only replace two thyristors connected in parallel with opposite polarity, but also only needs one trigger circuit. It is currently an ideal AC switch device. Its English name TRIAC means three-terminal bidirectional AC switch.

Construction principle
Although the bidirectional thyristor can be regarded as a combination of two ordinary thyristors in form, it is actually a power integrated device composed of 7 transistors and multiple resistors. Low-power bidirectional thyristors are generally packaged in plastic, and some are also equipped with heat sinks. The appearance is shown in Figure 1. Typical products include BCM1AM (1A/600V), BCM3AM (3A/600V), 2N6075 (4A/600V), MAC218-10 (8A/800V), etc. High-power bidirectional thyristors mostly use RD91 type packaging. The main parameters of bidirectional thyristors are shown in the attached table.

The structure and symbol of the bidirectional thyristor are shown in Figure 2. It is a five-layer NPNPN device with three electrodes, namely T1, T2, and G. Because the device can conduct in both directions, the two electrodes other than the gate G are collectively referred to as the main terminals, represented by T1 and T2. It is no longer divided into anodes or cathodes. Its characteristic is that when the voltages of the G pole and the T2 pole relative to T1 are both positive, T2 is the anode and T1 is the cathode. Conversely, when the voltages of the G pole and the T2 pole relative to T1 are both negative, T1 becomes the anode and T2 becomes the cathode. The volt-ampere characteristics of the bidirectional thyristor are shown in Figure 3. Since the forward and reverse characteristic curves are symmetrical, it can conduct in either direction.

Detection Methods

The following describes a method of using the RX1 position of a multimeter to determine the electrodes of a bidirectional thyristor, while also checking the triggering capability.

1. Determine T2 pole

As can be seen from Figure 2, the G pole is close to the T1 pole and far away from the T2 pole. Therefore, the forward and reverse resistances between G and T1 are very small. When using the RX1 gear to measure the resistance between any two pins, only between G and T1 is low resistance, and the forward and reverse resistances are only tens of ohms, while the forward and reverse resistances between T2-G and T2-T1 are infinite. This shows that if a certain pin is not connected to the other two pins, it must be the T2 pole. In addition, for bidirectional thyristors using TO-220 packages, the T2 pole is usually connected to a small heat sink, and the T2 pole can also be determined based on this.

2. Distinguish between G pole and T1 pole

(1) After finding the T2 pole, first assume that one of the remaining two pins is the T1 pole and the other is the G pole.

(2) Connect the black test lead to T1 and the red test lead to T2. The resistance is infinite. Then use the red test lead to short-circuit T2 and G. Add a negative trigger signal to G. The resistance should be about ten ohms (see Figure 4 (a)), proving that the tube is turned on and the conduction direction is T1 to T2. Disconnect the red test lead from G (but still connect it to T2). If the resistance remains unchanged, it proves that the tube can maintain the conduction state after being triggered (see Figure 4 (b)).

(3) Connect the red test lead to the T1 pole and the black test lead to the T2 pole, then short-circuit T2 and G, add a positive trigger signal to the G pole, and the resistance value is still about ten ohms. If the resistance value remains unchanged after disconnecting from the G pole, it means that after the tube is triggered, it can also maintain the on state in the direction of T2 to T1, so it has a bidirectional trigger property. This proves that the above assumption is correct. Otherwise, the assumption is inconsistent with the actual situation, and it is necessary to make another assumption and repeat the above measurement. Obviously, in the process of identifying G and T1, the triggering ability of the bidirectional thyristor is also checked. If the measurement is carried out according to any assumption, the bidirectional thyristor cannot be triggered to conduct, which proves that the tube is damaged. For 1A tubes, RX10 gear can also be used for detection. For tubes of 3A and above, RX1 gear should be selected, otherwise it is difficult to maintain the on state.

Typical Applications
Bidirectional thyristors can be widely used in industries such as industry, transportation, and household appliances to achieve multiple functions such as AC voltage regulation, motor speed regulation, AC switch, automatic opening and closing of street lamps, temperature control, table lamp dimming, and stage dimming. They are also used in solid-state relays (SSRs) and solid-state contactor circuits. Figure 5 is a proximity switch circuit composed of bidirectional thyristors. R is the gate current limiting resistor, and JAG is a dry reed switch. Normally, JAG is disconnected, and the bidirectional thyristor TRIAC is also turned off. Only when a small magnet moves closer, JAG is attracted, turning on the bidirectional thyristor and connecting the load power supply. Since
the current passing through the dry reed switch is very small and the time is only a few microseconds, the life of the switch is very long.

Figure 6 is the internal circuit of the zero-crossing triggered AC solid-state relay (AC-SSR). It mainly includes input circuit, photocoupler, zero-crossing trigger circuit, switch circuit (including bidirectional thyristor), and protection circuit (RC absorption network). When the input signal VI (generally high level) is added and the AC load power supply voltage passes through the zero point, the bidirectional thyristor is triggered and the load power supply is turned on. The solid-state relay has the characteristics of small driving power, no contact, low noise, strong anti-interference ability, short pull-in and release time, long life, and is compatible with TTL\CMOS circuits, and can replace traditional electromagnetic relays.