Electronics engineering

Optocoupler or Optoisolator – Construction, Working Principle and How it works?


An optocoupler or Optoisolator, is an electronics component that are use to interconnection in between two separate electrical circuit i.e. light sensitive optical interface. It is also use for transfer electrical signal between two isolated circuits by using light. It is use for prevents high voltage from affecting the system receiving the signal. The optocoupler is also called an optocouplerphotocoupler, or optical isolator.

The optocoupler consist a light emitting diode (LED) and phototransistor in the same opaque package. The other types of optocouple are the combination of LED-photodiode, LED-LASCR, and lamp-photoresistor pairs. The optoisolator are use for transfer digital and analog signals.


As we know transformer has no physical connection between primary and secondary side. In step down Transformer they provide electrical isolation between the higher voltage on the primer side and low voltage on secondary side.

The function of transform is based on mutual induction between two cores. This means transformer isolate the primer input voltage from secondary output voltage using electromagnetic coupling and this is achieved using the magnetic flux circulating within their laminated iron core.


Beside the transform we can also provide the electrical isolation between input and output side by using light and phototransistor and it is called optocoupler.

What is an isolated circuit?

Isolation circuits are that circuit which does not have any physical common conductor in between one side to another side and proper isolation is maintained.

We know the massage signal highly contain distortion and noise in it which can be beyond the tolerance limit of the logic circuit at the output end during transmission. In this case the optocoupler is use to prevent the noise and distortion. The optocoupler can work on DC and AC high current.

Optocoupler or Photocoupler basics

The photocoupler or optocoupler contain two basic thing which is described below-

Light emitter: the light emitter is on the input side which takes electrical signal and this electrical convert into light signal. Typically the light emitter is a light emitting diode.

Light detector: the function of light detector within an optocoupler or photocoupler is converting the incoming light signal form light emitting diode into original electrical signal. The light detector may be photo diode, photo transistor or photodarlington etc.

The light emitter and light detector are tailored to match one another, having matching wavelengths so that the maximum coupling is achieved.

Optocoupler symbol

The symbol of optocoupler is use for indication of structure and function. The symbol contains the light emitter element LED and light detector such as phototransistor, photodiode and light sensitive device etc. the symbol are shown below.


The optocoupler use in AC power application is based around a diac.


Internal Structure of Optocoupler


The above figure shows the internal structure of optocoupler. The input pins are 1 and 2. It is a light emitting diode terminal. This led emit infrared light to photosensitive transistor on the right side. The output is taken form pin 3 and 4 which is the collector and emitter terminal of phototransistor. The output is depends on the light emitting diode. The LED is control by an external circuit. There is no any conducting material present in between photodiode and phototransistor. It is electrically isolated. The hole structure is packed in glass or transparent plastic, the electrical isolation is much higher, typically 10 kV or higher.

Working of optocoupler

The photosensor is the output circuit that can detect the light, the output will be DC and AC. Firstly the current is applied to the LED that emit the infrared light proportional to the current going through the device. When light fall on photosenstor a current flow, and switched ON. When input is interrupted, the IR beam is cut-off, causing the photosensor to stop conducting.


Optocoupler and Optoisolator specifications

There are several specifications that need to be taken into account when using opto-couplers and opto-isolators:

  • Current transfer ratio, CTR: The current transformer ratio is a ratio of output current in photosenstor to input current in LED. The current transfer ratio (CTR) are vary according to the type of optocoupler is use in output side. The photdarlington type has much higher than ordinary phototransistor.
  • Bandwidth: In order to understand the maximum data rate in it. It is necessary to know the bandwidth. The phototransistor type bandwidth is 250 kHz and photodarlingtons bandwidth is around tenth digits.
  • Input current: It is current required for LED. This current is control by using resistor.
  •  Output device maximum voltage: The optocoupler use photosensor using transistor. The maximum output is equal to VCE(max) for the transistor.

Differences between optocouplers and solid state relays

There is slightly difference between optocoupler and solid state relays which is described below-

  • The solid state relay are use as a electronics switch to control the AC and DC power.
  • The solid sate relay provides the high resistance or isolation between input and output. It is also based on opto-coupling technology
  • The main difference between optocoupler and solid state switch is that optocoupler is use for low power application and solid state relay are use for high power application up to 100 of volts
  • The optocoupler are pack in thin IC. However the solid state relay is high package and it use heat sink.

Types of opto-isolators

Device type Source of light Sensor type Speed Current transfer ratio
Resistive opto-isolator
Incandescent light bulb CdS or CdSe photoresistor (LDR) Very low <100%
Neon lamp Low
GaAs infrared LED Low
Diode opto-isolator GaAs infrared LED Silicon photodiode Highest 0.1–0.2%
Transistor opto-isolator GaAs infrared LED Bipolar silicon phototransistor Medium 2–120%
Darlington phototransistor Medium 100–600%
Opto-isolated SCR GaAs infrared LED Silicon-controlled rectifier Low to medium >100%
Opto-isolated triac GaAs infrared LED TRIAC Low to medium Very high
Solid-state relay Stack of GaAs infrared LEDs Stack of photodiodes driving
a pair of MOSFETs or an IGBT
Low to high Practically unlimited



Phototransistor Optocoupler


The above figure shows the phototransistor. This contains the LED and phototransistor. At input terminal we provide voltage the LED produce infra ray. This infra ray active the phototransistor and gave some output. If the input current interrupts, there is no output current.

Optocoupler Types

The optocoupler is available in four type, each one having an infra-red LED source but with different photo-sensitive devices. The four optocouplers are the:

  1. Photo-transistor
  2. Photo-darlington
  3. Photo-SCR
  4. Photo-triac

The four types are shown below.



  1. It allow easy interfacing with logic circuits.
  2. It provides Electrical isolation to for circuit protection.
  3. It is suitable for wideband signal transmission.
  4. It is small in size and lightweight device.


  1. Its operation is slow.
  2. It is not suitable for high power applications.


Optocouplers or of optocoupler are used in,

  1. It is use for Lamp Ballasts
  2. In Light Dimmers
  3. It is use in Valve or Motor Controllers
  4. Microcontrollers for interfacing with High Voltage Circuits.

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