Opto-Isolator Circuit Working and Applications

In general, electrical signals are transmitted from one circuit to another circuit using various components or mediums. If we consider the television system operated using IR, then the infrared remote which consists of IR emitter is used to transmit electrical signal from remote circuit to the TV internal circuit. Similarly, every remote operated system uses either RF or IR or laser or etc., for transmitting electrical signals from one circuit to another circuit. In this article, let us discuss about Opto isolator circuit working and applications.

What is an Opto Isolator?

In electronics, an Opto isolator is also termed as an optical isolator or photocoupler or Optocoupler. It is one of electrical and electronics components used for transferring electrical signals from one circuit to another circuit (which are isolated) using light. In general, combination of LED (source) and phototransistor (sensor) is used in typical Opto-isolator IC packed in a single opaque package.

In fact, there are few other sources such as LED, lamp, etc., which are detected by sensors such as photo diode, LASCR, photoresistor, and so on. These Opto-isolator circuits are used generally for transferring digital signals, but can be used to transfer analog signals with a few techniques. The rapidly changing voltages or other electrical parameters on one side of the circuit may cause damage to components on the other side of the circuit due to direct electrical contact. So, opto isolator circuit can be used to isolate the high or rapidly changing voltages of one side of circuit causing damage to the components on the other side of the circuit.

Types of Opto Isolators

Types of Opto Isolators

There are various types of opto isolators which are classified based on various criteria such as number of channels, isolation voltage, type of packing, output voltage capacity, current transfer ration (CTR), and so on.

Opto Isolator Circuit

As we discussed earlier in this article, source and sensor of opto isolator are used for transferring the signals. Let us consider an opto isolator circuit with a source of light-LED which emits light and a photosensor (SCR or TRIAC or photoresistor or phototransistor or photodiode). This photosensor is used to detect the incoming light and thereby directly generate electrical energy or modulate electrical energy of external power supply. The LED can not only emit light, but also sense light, hence, it is possible to construct symmetrical and bidirectional Opto Isolators.

Opto Isolator Circuit

If the light emitted from the LED (source) falls on the (sensor) phototransistor base, then the (sensor gets turned ON) transistor starts conduction. If the current passing through the LED is made to zero or turned off, then the transistor stops conduction.

Opto Isolator Applications

These opto isolator circuits can be used on their own and also for switching other electronic devices (transistors, etc.,) or circuits to provide electrical isolation between low voltage control signal and high output signal (voltage or current). In general, for switching microprocessor input/output, AC or DC power control, computer communications, and for the regulation of power supply opto isolator circuits are used.

Practical Application of Opto Isolator Circuit – Thyristor Controlled Power for Induction Motor Project by Efxkits.com

Thyristor based power control for induction motor using an opto isolator circuit. In this electrical project (circuit is shown in the above figure), AC power supplied to the load is controlled using thyristor firing angle control technique. The comparator is used to detect the zero crossing point of the waveform which is used in this project. The output of this comparator is fed to the 8051 microcontroller used in the project.

Practical Application of Opto Isolator Circuit – Thyristor Controlled Power for Induction Motor Project Block Diagram by Efxkits.com

The block diagram of the project circuit is shown in the above figure which consists of various blocks such as a power supply block, microcontroller block, opto isolator circuit block connected to load and the microcontroller through a thyristor circuit, and zero crossing detector. Thus, the triggering signal required by the pair of SCRs is provided by the microcontroller through the opto isolator circuit interface.

Then, the power required by the load is supplied through the SCRs and this supplied power can be increased or decreased using push button switch. Here, for the demonstration purpose lamp is used instead of induction motor.

Opto Isolator Circuit based DC Switch

DC Switch using Opto Isolator Circuit

The opto isolator circuit can be used to detect DC signals and data, to control the AC powered devices and mains lamps. In case of high powered loads, the gate pulse to larger triac is provided by (another opto isolator circuit) opto triac through current limiting resistor as represented in the above figure.

Triac Optocoupler Application

Triac Optocoupler Application

The simple solid state relay basis can be formed using optocoupler configuration shown in the above figure. These types of opto isolator circuit can be used for controlling the AC loads such as motors, lamps and so on. The triacs or SCR conducts for both half cycles of AC mains cycle and thus, using zero crossing detection technique full power to the load can be supplied by avoiding heavy inrush currents which occurs due to inductive load switching.

These opto isolator circuits are typically used for AC power control applications. The noise and voltage spikes (if any) of the AC power supply can be completely isolated using photo-SCRs or photo-triacs. The thermal stress caused due to heavy inrush current caused by switching can be avoided by using zero crossing detection technique. The opto isolator circuits provide high electrical isolation between input terminals and output terminals. Thus, small digitals are allowed for controlling high AC currents, voltages, and power.

Do you know any practical opto isolator applications? Are you interested in designing electronics projects? Then, share your ideas, views, comments, and suggestions by posting in the comments section below.