What is an Optocoupler and How does it Works?


Optocouplers also known by photocoupler are made of LEDs, photodiodes or phototransistors linking circuits by a short optical path but isolating them electrically hence the name optoisolators .

The photocouplers or optocouplers are typically contained in a single package, often about the size of an integrated circuit, although there is a large degree of variation according to the intended application.

Photocoupler / Opto-coupler basics

components of a optocoupler

Light emitter : a light emitting diode that takes the incoming signal and converting it into a light signal. Used in the input side of the circuit.

Light detector : The light detector which can be a photodiode to a phototransistor, photodarlington, etc. detects the light from the emitter and converting it into an electrical signal.

Maximum coupling is achieved by matching wavelengths of the light emitter and detector

Other components of The opto-coupler are series resistor for the LED ,output amplifier etc.

Both detector and emitter comes in same packages due to mechanical arrangements constraints but when needed separate components are utilised for specific applications.

Photocoupler / optocoupler terminology

Stictly, there are differences between the terms optoisolator and optocoupler. The distinguishing factor between the opto-coupler and opto-isolator is in the voltage difference expected between the input and output:

Opto-coupler: The opto-coupler is generally thought of as being used to transmit analogue of digital information between circuits while maintaining electrical isolation at potentials up to 5 000 volts.

Opto-isolator: The opto-isolator is generally used in power systems and used to transmit analogue or digital information between circuits where the potential difference is above 5 000 volts.

Opto-coupler symbol

The symbol is a LED, which is the light emitter and the relevant device that acts as the receiver often a phototransistor or photodarlington, or light sensitive diacs, etc may also be used.


Optocoupler and opto-isolator specifications

Current transfer ratio, CTR : The current transfer ratio of an optocoupler is the ratio of the current that flows in the output device divided by the current on the input device. The CTR will differ depending on the opto-coupler used in the output. Outputs with photodarlingtons will have CTR valus higher than ordinary phototransistors. Values may be anywhere between 10% and 2000% - 5000%. The CTR have a tendency to to vary with the input current level. Although it will vary according to the device usually it peaks for input current levels around 10mA falling either side of this value.

Bandwidth : The maximum data rates for which an opto-coupler can be used its bandwidth is necessary. For opto-couplers using phototransistors it is around 250 kHz, and for those using photodarlingtons it may be a tenth of this. Typically the lower the CTR, the faster the rise and fall times

Input current : This is the current required for the input transmitter device - LED. The value is used to calculate the series resistor used to limit the current.

Output device maximum voltage : For opto-couplers with transistors, the maximum value is equal to the VCE(max) of the transistor. For opto-couplers using other devices in the output, the equivalent rating should be used.

Note: The devices should not operate close to their maximum ratings.

Types of opto-isolators

Device type Source of light Sensor type Speed Current transfer ratio
Resistive opto-isolator(Vactrol) Incandescent light bulb or Neon lamp or GaAs infrared LED CdS or CdSe photoresistor(LDR) Very low <100%
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%
Transistor opto-isolator GaAs infrared LED 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

Aplications of optocouplers

  • used to link two circuits

  • used within optical encoders, where the optocoupler provides a means of detecting visible edge transitions on an encoder wheel to detect position, etc.,

  • Detect link breakage

  • Transfer data optically

What aspects should you know to select an optocoupler?

There are various types of optocouplers, with the decision depending on the intended use. In any event, all optocouplers have the maximum parameters listed below:

  1. The forward current (IF) of the emitting diode & the reverse voltage (VR) should not be exceeded.
  2. The optocoupler has a phototransistor output, collector current (IC), & collector-emitter voltage (VCE).

Additionally, the functioning of these properties at various operating temperatures needs to be considered. Typically, the manufacturer’s datasheets provide de-rating curves that illustrate the impacts.

Finally, probably the most essential characteristic in an optocoupler is CTR (Current Transfer Ratio), which is the percentage ratio of an optocoupler’s output current (IC) to its input current (IF).

CTR% = [(IC(in mA)/IF(in mA))] X 100