Light Activated SCR or LASCR construction and working

(Last Updated On: June 19, 2022)

Light Activated SCR or LASCR:

An SCR which conducts or turns on, when light radiates on its open door, is known as a light-activated SCR or LASCR. If the light reflecting on the SCR gate becomes higher than its peculiar threshold level, the flow of current start in it. Remember that light-activated SCR is also called photo SCR. In other words, LASCR is such an SCR, which can be triggered by the use of light energy. It is a high-gain device, through which a high quantity of electrical energy can be controlled by using a very small number of light rays or radiant energy.

Construction light-activated SCR or LASCR

From a construction viewpoint, a light-activated SCR or LASCR is similar to a conventional SCR. The only exception is that a reverse bias PN junction exists in the center of its structure, which can be turned on by reflecting light on it. In other words, the construction of LASCR and an ordinary SCR is similar. The only difference between these two is that a window has been built on light-activated SCR, by means of which light rays reach silicone (because silicone is extremely sensitive to light) and activate it (that’s makes it radiant). In figure 7.19, the construction of a LASCR has been represented. It consists of an anode, cathode, and gate along with three PN junctions. Light rays penetrating through the window made on light-activated SCR, reach directly its J2 junction area. In figure 7.20 LASCR’s symbols and specific shapes have been illustrated. Often, a gate terminal is also built along a LASCR, so that conventional current can be obtained through it during times of need.

Figure 7.19- construction of light-activated SCR (LASCR)

Light Activated SCR or LASCR

Light Activated SCR or LASCR

Figure 7.20- light-activated SCRs

Light Activated SCR or LASCR

Working light-activated SCR or LASCR

We know that J1 and J2 junctions of an ordinary SCR are forward biased because of the potential provided on its anode and cathode, while its J2 junction due to being reverse biased stops the flow of current in SCR. When light radiation occurs on a light-activated SCR’s reverse-biased junction J2, electron holes are created on it (remember the number of electron holes in the junction depends on the intensity of light on it), thus, the junction’s resistance declines and it also becomes forward biased. Resultantly, flow of current from anode to cathode in light-activated SCR starts (in other words when light collides with the depletion layer of the junction passing via the window, junctions’ valence electrons detach from their orbits due to this light energy and become free electrons. As a consequence of the flow of these free electrons positive feedback starts and photo SCR either turns on or off). It must be kept in mind that just as an ordinary SCR is turned on by providing positive signals on its gate and after it turning on, even if these positive gate signals are removed, even then it will remain on. Similarly, a LASCR turns on or starts working when light radiates from its gate. After this, even if light radiation stops or ceases, it works incessantly similar to an ordinary SCR. Until its anode current value is lowered compared to its holding current, then it turns off (i.e. once an SCR triggers and comes into its own mode, it functions similar to an ordinary SCR)

light-activated SCR or LASCR is most sensitive to light at a time when its gate terminal is open. Therefore, when maximum sensitivity is required, its gate is kept open as can be seen in figure 7.21 (A). However, if the trigger point of light-activated SCR or LASCR is desired to be adjusted, a variable resistor is fitted along with its gate (as depicted in figure B). Thus, applying resistance between gate and ground, some of the electrons generated via the intensity of light, go astray and change their route, and the sensitivity of the circuit lessens compared to the arriving light (that’s in this way, the level of light at which SCR triggers on, can be changed) Remember that a lead has been fixed with light-activated SCR gate just like a common SCR so that it can be triggered on during times of need by giving positive signals on its gate.

Figure 7.21 A. simple LASCR circuit at maximum sensitivity B. simple LASCR with variable sensitivity

Light Activated SCR or LASCR

Uses of light-activated SCR or LASCR

LASCR is used at certain places in specific circuits due to its long life, fast response (fast switching), and its small size. Therefore, where ever a circuit is to be turned on or off very quickly via light, an input source has also to be kept isolated from the rest of the circuit, at such a place’s application of LASCR is common. Light-activated SCRs are mostly used for energizing latching relays or electrical mechanical relays. Thus, output also remains completely isolated from the input, and characteristics of a good current are also achieved.

In figure 7.22, a light-activated SCR or LASCR circuit has been shown which energizes a latching relay. These relays operate an alarm or door opener after having been energized. A lamp has been fixed with the input source. When the switch of the lamp is closed, the lamp illuminates. Lamplight radiates directly on LASCR by means of which, it triggers. Thus, the anode current energizes the relay coil, resultantly a latching relay form. Whenever the relay is required to be de-energized, off, or reset, the switch fitted on the anode circuit is pressed again and thus LASCR turns off.

In figure 7.23, another such type of a simple circuit has been displayed. When switch S1 is closed, the lamp illuminates and turns LASCR on. Thus, the flow of current through the load starts. When there is a need to reset the LASCR, push button S2 is pressed momentarily, by means of which LASCR’s current lessens compared to its holding current and it turns off. The biggest advantage of this sequence/series is that complete isolation between the control circuit and load becomes possible.

Figure 7.22- a light-activated SCR

Figure 7.23- the standard symbol and an application of light-activated SCR or LASCR


Light Activated SCR or LASCR



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My name is Shahzada Fahad and I am an Electrical Engineer. I have been doing Job in UAE as a site engineer in an Electrical Construction Company. Currently, I am running my own YouTube channel "Electronic Clinic", and managing this Website. My Hobbies are * Watching Movies * Music * Martial Arts * Photography * Travelling * Make Sketches and so on...

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