Page 1019 - Basic Electrical Engineering

14.13.9 Optocouplers

An optocoupler is a combination of two light-activated devices, viz a
phototransistor and an LED. These two are placed together in one package.

We know light emits from an LED and a phototransistor is activated by a
light source. The current flowing through an LED will cause light to be

emitted from the device. When these lights are directed to illuminate the
phototransistor junction, current will flow in the transistor as shown in Fig.

14.57.
The coupling between the LED and the phototransistor is optical, i.e.,
through the emitted light. The two circuits are electrically isolated. The

voltage level of the two circuits may be made different. Thus, from a low
voltage side, control can be made of the high-voltage side. Fig. 14.57 (b)

shows an optocoupler contained in a transparent insulating base material.
This base material will allow light emitted by the LED to be transmitted to

the base of the phototransistor. However, the two, i.e., the LED and the
phototransistor are electrically isolated.

An optocoupler can be operated as an electronic switch. A pulse of current
through the LED may cause the transistor to be switched on which is
otherwise off. An optocoupler can be made as a combination of (i) an LED

and a photo-darlington; (ii) a LED and any other device that can be activated
by light.