How to Determine Triac Electrodes with a Multimeter
Ordinary thyristors (VS) are essentially DC control devices. To control the AC load, two thyristors must be connected in reverse polarity in parallel, so that each SCR can control a half-wave. For this purpose, 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 thyristors connected in reverse polarity in parallel, but also only needs one trigger circuit. It is an ideal AC switching device at present. Its English name TRIAC means three-terminal two-way AC switch.
Structure principle
Although the triac 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 triacs are generally packaged in plastic, and some also have a heat sink, as shown in Figure 1. Typical products are BCMlAM (1A/600V), BCM3AM (3A/600V), 2N6075 (4A/600V), MAC218-10 (8A/800V) and so on. Most of the high-power triacs are packaged in the RD91 type. The main parameters of the bidirectional thyristor are shown in the attached table.
The structure and symbol of the bidirectional thyristor are shown in Figure 2. It belongs to the NPNPN five-layer device, and the three electrodes are T1, T2, and G respectively. Because the device can conduct bidirectional conduction, the two electrodes except the gate G are collectively referred to as the main terminals, which are T1 and T2. Indicates that it is no longer divided into anodes or cathodes. Its characteristic is that when the voltages of G pole and T2 pole are positive relative to T1, T2 is the anode and T1 is the cathode. Conversely, when the voltages of the G and T2 poles are negative relative to T1, T1 becomes the anode and T2 is the cathode. The volt-ampere characteristics of the bidirectional thyristor are shown in Figure 3. Due to the symmetry of the forward and reverse characteristic curves, it can be turned on in any direction.
Detection method
The following introduces the method of using the multimeter RX1 file to determine the electrode of the triac, and also checks the triggering ability.
1. Determine the T2 pole
It can be seen from Figure 2 that the G pole is close to the T1 pole, and is far from the T2 pole. Therefore, the forward and reverse resistances between G-T1 are very small. When using the RX1 gear to measure the resistance between any two feet, only the low resistance is shown between G-T1, the forward and reverse resistances are only tens of ohms, and the forward and reverse between T2-G and T2-T1 The resistances are all infinite. This shows that if a foot and the other two feet are not connected, it must be the T2 pole. , In addition, using TO-220 package triac, the T2 pole is usually connected with the small heat sink, and the T2 pole can also be determined accordingly.
2. Distinguish G pole and T1 pole
(1) After finding the T2 pole, first assume that one of the remaining two feet is the T1 pole and the other is the G pole.
(2) Connect the black test lead to the T1 pole and the red test lead to the T2 pole, the resistance is infinite. Then short-circuit T2 and G with the tip of the red meter, and apply a negative trigger signal to the G pole. The resistance value should be about ten ohms (see Figure 4(a)), which proves that the tube has been turned on, and the conduction direction is T1-T2. Then disconnect the red meter tip from the G pole (but still connect 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 test lead to the T1 pole and the black test lead to the T2 pole, then short-circuit T2 and G, and apply a positive trigger signal to the G pole, 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, the conduction state can also be maintained in the T2-T1 direction, so it has a bidirectional triggering 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 triac is also checked. If the measurement is made according to which assumption, the triac can not be triggered and turned on, which proves that the tube has been damaged. For 1A tubes, RX10 can also be used for detection. For 3A and above 3A tubes, RX1 should be selected, otherwise it is difficult to maintain the conduction state.
