Light-controlled thyristor

Photo-controlled thyristor, also known as GK type optical switch tube, is a photosensitive device.

1. Structure of light-controlled thyristor

Usually, a thyristor has three electrodes: control electrode G, anode A and cathode C. However, since the control signal of a light-controlled thyristor comes from light irradiation, there is no need to lead out the control electrode, so it only has two electrodes (anode A and cathode C). However, its structure is the same as that of a common thyristor, which is composed of four layers of PNPN devices.

Related pictures for this topic are as follows:

The appearance of the light-controlled thyristor is shown in Figure 1 (a). Its circuit symbol is shown in Figure 1 (b).

In terms of appearance, the photo-controlled thyristor also has a light-receiving window, two pins and a shell, which is very similar to a photodiode.

2. Working principle of light-controlled thyristor

Related pictures for this topic are as follows:

When a positive voltage is applied to the anode of the photo-controlled thyristor and a negative voltage is applied to the cathode, the photo-controlled thyristor in FIG2(a) can be equivalent to the circuit in FIG2(b).

The following formula can be deduced from Figure 1 (b):

Ia = Il / [1-(a1+a2)]

Where, Il is the photocurrent of the photodiode; Ia is the anode current of the photo-controlled thyristor, that is, the output current of the photo-controlled thyristor; a1 and a2 are the current amplification factors of BG1 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 photo-controlled thyristor also increases accordingly. At the same time, the increase of Il increases the current amplification coefficients a1 and a2 of BGl and BG2. When the sum of al and a2 is close to l, the Ia of the photo-controlled thyristor reaches the maximum, that is, it is fully turned on. The minimum light intensity that can turn on the photo-controlled thyristor is called the turn-on light intensity. The photo-controlled thyristor is the same as the ordinary thyristor. Once triggered, it will be in the conducting state. As long as there is a light source of sufficient intensity to illuminate the light receiving window of the tube, it will immediately become in the conducting state, and then it can remain conducting even if the light source is evacuated, unless the voltage applied between the anode and the cathode is zero or reversed, it can be turned off.

3. Characteristics of light-controlled thyristors

In order to make the light-controlled thyristor trigger and conduct under weak light, it is necessary to make the light-controlled thyristor conduct reliably under extremely small control current. Therefore, the light-controlled thyristor is limited by high temperature and withstand voltage. Under 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 those of ordinary thyristors. Therefore, when using them, you can choose according to the ordinary thyristors, just pay attention to the fact that it is light-controlled. Light-controlled thyristors have certain requirements for the wavelength of the light source, that is, they are selective. Infrared with a wavelength of 0.8-0.9um and laser with a wavelength of about 1um are both ideal light sources for light-controlled thyristors.