1. Structure of Light Controlled Thyristor

Typically, thyristors have three electrodes: control electrode G, anode A, and cathode C. However, since the control signal of the light controlled thyristor comes from the irradiation 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 ordinary thyristors, consisting of four layers of PNPN devices.

From the appearance, the light controlled thyristor also has a light receiving window, two pins, and a housing, resembling a photodiode.

2. Working principle of light controlled thyristor

When a positive voltage is applied to the anode and a negative voltage is applied to the cathode of the photo controlled thyristor, the photo controlled thyristor in Figure 2 (a) can be equivalent to the circuit in Figure 2 (b).

Calculate the following equation:

In the formula Ia=Il/[1- (a1+a2)], Il is the photocurrent of the photodiode; Ia is the anode current of the photo controlled thyristor, which is the output current of the photo controlled thyristor; A1 and a2 are the current amplification factors of BGl and BG2, respectively. As can be seen from the above equation, Ia is proportional to Il, that is, when the photocurrent of the photodiode increases, the output current of the thyristor also increases accordingly. At the same time, the increase of Il also increases the current amplification factors a1 and a2 of BGl and BG2. When the sum of al and a2 approaches l, the Ia of the thyristor reaches its maximum, indicating complete conduction. The minimum illuminance that can make the light controlled thyristor conduct is called conducting illuminance. Light controlled thyristors, like ordinary thyristors, become conductive once triggered. As long as there is a light source of sufficient intensity shining on the light receiving window of the tube, it immediately becomes conductive, and then can maintain conductivity even if the light source is removed, unless the voltage applied between the anode and cathode is zero or in reverse phase, it can be turned off.

3. Characteristics of Light Controlled Thyristors

In order for the light controlled thyristor to trigger conduction under weak light illumination, it is necessary to ensure that the light controlled thyristor can reliably conduct under extremely small control currents. In this way, the light controlled thyristor is limited by high temperature and voltage resistance, and under current conditions, it is impossible to make high-power thyristors like ordinary thyristors.