Light controlled thyristor

Photo-controlled thyristor, also called GK-type optical switch, is a photosensitive device. 1. The structure of a photocontrolled thyristor usually has three electrodes: control electrode G, anode A and cathode C. Since the control signal of the light-controlled thyristor comes from the illumination of light, there is no need to lead out the control electrode, so there are only two electrodes (anode A and cathode C). But its structure is the same as that of ordinary silicon controlled silicon, consisting of four layers of PNPN devices. Pictures related to this topic are as follows: The appearance of a photocontrolled thyristor is shown in Figure 1(a). Its circuit graphic symbol is shown in Figure 1(b). From the appearance point of view, the light-controlled thyristor also has a light-receiving window, two pins and a shell, which resembles a photodiode. 2. The relevant pictures on this topic of the working principle of light-controlled thyristor are as follows: When a positive voltage is applied to the anode of the light-controlled thyristor and a negative voltage is applied to the cathode, the light-controlled thyristor in Figure 2(a) can be equivalent to Figure 2(b) ) circuit. The following formula can be deduced from Figure 1(b): Ia = Il / [1-(a1+a2)] In the formula, Il is the photocurrent of the photodiode; Ia is the anode current of the photocontrolled thyristor, that is, the output of the photocontrolled thyristor Current; a1 and a2 are the current amplification coefficients of BGl and BG2 respectively. It can be seen from the above formula that Ia is proportional to Il, that is, when the photocurrent of the photodiode increases, the output current of the photocontrolled thyristor also increases accordingly. At the same time, the increase of Il causes the current amplification coefficients a1 and a2 of BG1 and BG2. also increased. When the sum of a1 and a2 is close to l, the Ia of the photocontrolled thyristor reaches the maximum, that is, it is fully conductive. The minimum illumination that can turn on the light-controlled thyristor is called the conduction illumination. Photocontrolled thyristors are the same as ordinary thyristors. Once triggered, they become conductive. As long as there is a light source of sufficient intensity to illuminate the light-receiving window of the tube, it will immediately become conductive, and then it can maintain conduction even if the light source is removed. Unless the voltage applied between the anode and cathode is zero or reversed, it can be turned off. 3. Characteristics of light-controlled thyristors In order for the light-controlled thyristors to trigger conduction under weak light, the light-controlled thyristors must be able to reliably conduct under extremely small control currents. In this way, the photocontrolled thyristor is limited by high temperature and withstand voltage. Under the current conditions, it is impossible to make it as high-power as ordinary thyristors. Except for the different trigger signals, the other characteristics of light-controlled thyristors are basically the same as ordinary thyristors. Therefore, you can choose ordinary thyristors when using them. Just pay attention to the fact that they are light-controlled. Photo-controlled thyristors have certain requirements on the wavelength of the light source, that is, they are selective. Infrared rays with a wavelength of 0.8-0.9um and lasers with a wavelength of about 1um are ideal light sources for photo-controlled thyristors.