Construction principle of bidirectional thyristor and testing method of multimeter

Common thyristor (VS) is essentially a DC control device. To control the AC load, it is necessary to parallelize the reverse polarity of two thyristors so that each SCR can control one and a half waves. For this reason, two sets of 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 reverse polarity parallel thyristors, but also only need a trigger circuit. It is an ideal AC switching device at present. Its English name TRIAC means three-terminal two-way 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 seven transistors and multiple resistors. Small power bidirectional thyristor is generally packaged with plastic, and some are also equipped with heat dissipation plate, as shown in Figure 1. Typical products include BCMlAM (1A/600V), BCM3AM (3A/600V), 2N6075 (4A/600V), MAC218-10 (8A/800V), etc. RD91 type package is mostly used for high-power bidirectional thyristor. See the attached table for the main parameters of bidirectional thyristor.

The structure and symbol of the bidirectional thyristor are shown in Figure 2. It belongs to NPNPN five-layer device, and the three electrodes are T1, T2 and G. Because the device can conduct in both directions, the two electrodes other than gate G are collectively referred to as the main terminal, T1 and T2. Indicates that it is no longer divided into anode or cathode. Its characteristic is that when the voltage of G and T2 is positive relative to T1, T2 is the anode and T1 is the cathode. On the contrary, when the voltage of G pole and T2 pole relative to T1 is negative, T1 becomes anode and T2 is cathode. The volt-ampere characteristics of the bidirectional thyristor are shown in Figure 3. Because of the symmetry of the forward and reverse characteristic curves, it can conduct in any direction.

test method

The following describes the method of determining the bipolar thyristor electrode by using the Rxl gear of the multimeter, and also checks the triggering ability.

1. Determine T2 pole

As can be seen from Figure 2, G pole is close to T1 pole and far from T2 pole. Therefore, the forward and reverse resistances between G-T1 are very small. When measuring the resistance between any two pins with Rxl gear, only the resistance between G-T1 is low, 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 one foot and the other two feet are not connected, it must be T2 pole, In addition, the T2 pole is usually connected with the small heat dissipation plate by using the bidirectional thyristor packaged with TO-220, so the T2 pole can also be determined.

2. Distinguish between G pole and T1 pole

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

(2) Connect the black lead to T1 pole and the red lead to T2 pole. The resistance is infinite. Then use the pen tip of the red meter to short circuit T2 and G, add a negative trigger signal to the G pole, and the resistance value should be about 10 Ω (see Figure 4 (a)), to prove that the pipe has been connected, and the direction of connection is T1 - T2. Then disconnect the pen tip of the red meter from the G pole (but still connected to T2). If the resistance value remains unchanged, it proves that the tube can maintain the conduction state after triggering (see Figure 4 (b)).

(3) Connect the red lead to the T1 pole and the black lead to the T2 pole, and then make the T2 and G short circuit, add a positive trigger signal to the G pole, and the resistance value is still about 10 Ω. If the resistance value remains unchanged after being disconnected from the G pole, it means that the pipe can also maintain the conduction state in the T2-T1 direction after being triggered, so it has the nature of two-way trigger. This proves that the above assumptions are correct. Otherwise, the assumption is not consistent with the actual situation. It is necessary to make another assumption and repeat the above measurement. It is obvious that in the process of identifying G, T1, the triggering ability of the bidirectional thyristor is also checked. If the measurement is carried out according to any assumption, the bidirectional thyristor can not be triggered, which proves that the transistor has been damaged. For lA pipes, RXl0 can also be used for detection. For 3A and above pipes, RXl should be selected, otherwise it is difficult to maintain the conduction state.