What is an opto-isolator and how does it work? what are its applications

What is an optoisolator

Opto-isolator is also known as an optical isolator, or optocoupler, it is a device which transfers an electric signal or voltage from one circuit to another by using the light, while it isolates the two circuits from each other. It can prevent high voltages or rapidly changing voltages damaging the components by isolating the overvoltage signals. Opto-isolators can withstand input to output voltages up-to 10KV and voltages transients with speed up to 10KV.

Why do we need optoisolators

Most of the machine controlling equipment generates induced noise that may cause malfunctions with the help of optocouplers we can isolate this noise in the input signals.

Mostly optical isolators are used to prevent the high voltage or rapidly changing voltages on one side of the circuit from damaging a component or distorting transmission on the other side.

How does optocoupler or optoisolator work

The two major important elements needed for an optoisolator is light emitter and a light detector. Optoisolator has a source of light, which is almost always a near-infrared light-emitting diode that can convert the electrical input signals into light. A photosensor and an optical channel can detect the light and can generate the electrical energy directly or it can modulate the electric current which flows from an external power supply.

The sensor can be a photo-resistor, photo-diode, photo-transistor or an SCR. LED can sense light in addition to emitting it. An opto-coupled solid state relay has a photo-diode opto-isolator which drives a power switch, usually a complementary pair of MOSFET. Source of light and a sensor is present in a slotted optical switch and its optical channel is open which allows the modulation of light by external objects which obstructs the path of light or reflects the light into the sensor.

What are the types of optoisolators

Photodiode opto-isolators

This type of optoisolator uses LED as a source of light and silicon photodiode as sensors. When the photodiode is reverse biased with an external voltage source, incoming light increases the reverse current flowing through the diode. Diode won’t generate energy itself, it modulates the flow of energy from an external source. This mode of operation is called a photoconductive mode. In the absence of external bias, the diode converts the energy of light into electrical energy by charging its terminals to a voltage of up to 0.7V. Energy is created with the help of an external high impedance path. These types of optoisolators can be used to interface analog signals.

Phototransistor optoisolators

Phototransistors are slower than photodiodes, it is the earliest type and they are not quick. In order to achieve the maximum speed of phototransistor, it must be properly biased and loaded. Opto isolators using field-effect transistors as sensors are rare, Opto FET’s turn on without injecting switching charge in the output circuit and it would be useful in a sample and hold circuits.

Bidirectional opto-isolators

The optoisolators which is described above are uni-directional. They are all uni-directional because the optical channel always works one way, from the source to the sensor. But the LEDs are capable to detect the incoming light. Which makes possible construction of a two way optoisolators from a pair of LEDs. The simplest bidirectional optoisolator is a pair of LED’s placed face to face and held together with heat shrink tubing and the gaps between the two LED’s can be extended with a glass fiber insert.

What are the advantages of optoisolator

  • Control circuits can be protected by electrical isolation
  • Wideband signal transmission is possible
  • Interfacing with logic circuits is easily possible
  • It is a small and lightweight device
  • It provides very good response at lower frequencies
  • Because of the unidirectional signal transfer, noise from the output side does not get coupled to the input side.
  • They are compact and cheaper

What are the disadvantages of optical isolator

  • There is a chance for signal coupling for high power signals
  • Higher frequency response is poor

What are the applications of optoisolators

It can be used to isolate the lower power circuits from higher power circuits and at the same time the control signals are coupled from the control circuits to the high power circuits. Examples for such applications are high power choppers and inverters, and AC to DC converters used for DC motor speed control. One of the important applications of optoisolators is to couple the base driving signals to a power transistor connected in a DC-DC chopper.