How to identify the three poles of thyristor module
The thyristor module, also known as the silicon controlled rectifier (SCR), has developed into a large family since its introduction in the 1950s. Its main members include unidirectional thyristors, bidirectional thyristors, photo controlled thyristors, reverse conducting thyristors, switchable thyristors, fast thyristors, and so on. Today, we are using a unidirectional thyristor, also known as a regular thyristor. It is composed of four layers of semiconductor material, with three PN junctions and three electrodes facing the outside: the electrode led out by the first layer of P-type semiconductor is called anode A, the electrode led out by the third layer of P-type semiconductor is called control electrode G, and the electrode led out by the fourth layer of N-type semiconductor is called cathode K. From the circuit symbol of a thyristor, it can be seen that it is a unidirectional conductive device, just like a diode. The key is the addition of a control electrode G, which gives it completely different working characteristics from a diode.
A multimeter can distinguish the three electrodes of a thyristor
The three electrodes of a regular thyristor can be measured using a multimeter with an ohm range of R × 100. As we all know, there is a pN junction between thyristors G and K [Figure 2 (a)], which is equivalent to a diode. G is the positive pole and K is the negative pole. Therefore, according to the method of testing diodes, find two of the three poles and measure their forward and reverse resistance. When the resistance is small, the black probe of the multimeter is connected to the control pole G, the red probe is connected to the cathode K, and the remaining one is the anode A. To test the quality of the thyristor, you can use the teaching board circuit (Figure 3) that was just demonstrated. Connect the power supply SB, if the light bulb emits light, it is good; if it does not emit light, it is bad.
How to identify the three poles of a thyristor
The method of identifying the three poles of a thyristor is very simple. According to the principle of p-N junction, simply measure the resistance value between the three poles with a multimeter.
The forward and reverse resistance between the anode and cathode is several hundred kiloohms or more, and the forward and reverse resistance between the anode and the control electrode is several hundred kiloohms or more (there are two p-N junctions between them, and the directions are opposite, so the anode and control electrode are not connected in both directions).
There is a p-N junction between the control electrode and the cathode, so its forward resistance is approximately in the range of a few ohms to several hundred ohms, and the reverse resistance is larger than the forward resistance. However, the characteristics of the control diode are not ideal, as it is not completely blocked in reverse and can have a relatively large current passing through. Therefore, sometimes measuring a small reverse resistance of the control diode does not necessarily indicate poor control characteristics. In addition, when measuring the forward and reverse resistance of the control electrode, the multimeter should be placed in the R * 10 or R * 1 position to prevent the reverse breakdown of the control electrode due to high voltage.
If the positive and negative poles of the component are short circuited, or the anode and control pole are short circuited, or the control pole and cathode are short circuited in the opposite direction, or the control pole and cathode are open circuited, it indicates that the component is damaged.
Silicon controlled rectifier (SCR) is a high-power semiconductor device with a four layer structure consisting of three pN junctions. In fact, the function of a thyristor is not only rectification, but it can also be used as a switchable device to quickly turn on or off circuits, realizing the inversion of direct current into alternating current, converting one frequency of alternating current into another frequency of alternating current, and so on. Like other semiconductor devices, thyristors have advantages such as small size, high efficiency, good stability, and reliable operation. Its emergence has brought semiconductor technology from the weak current field to the strong current field, becoming a component that is eagerly adopted in industries such as industry, agriculture, transportation, military research, as well as commercial and civilian electrical appliances.
