Posted on: July 1, 2014

What is an Optoisolator?

An optoisolator is an electronics device that transfers electrical energy from one circuit to another through a short optical transmission path while providing electrical isolation between two circuits. An optoisolator couples high voltages from one side of the circuit to the other without any direct electrical contact.

The devices convert the electrical energy into a beam of light using a light emitting diode, and then directing the light towards a light sensor such as a photodiode or phototransistor which converts the optical energy back into electrical energy. This isolate the two circuits, prevents voltage spikes, and decreases noise and interference associated with communication connections.

Optoisolators are widely used in power supplies, control and monitoring systems, communications, and other systems to safely couple one circuit section to another electrically, while preventing direct contact and high voltages from affecting the lower voltage side.

Optoisolator basic construction

A typical optoisolator consist a near-infrared light emitting diode LED, a photo sensor such as a photodiode, phototransistor or photodarlington transistor, a closed channel, and a power source. The two components are usually enclosed in an opaque casing that prevents external light from interfering with the emitted beam, and housed in a package similar to an IC or a transistor with extra leads.

There are different kinds of optoisolators based on the light responsive device and the configuration. The two common types are:

  • Photodiode: uses LEDs as light source and Silicon photodiodes as the light sensors
  • Phototransistor: uses as the phototransistor as the light sensor
what is an Optoisolator

Figure 1: Types of optoisolators

Basic Operation of an Optoisolator

The voltage from the primary circuit is applied to the power source to produce a near-infrared light beam which travels across the closed channel until it hits the photo sensor which converts the optical energy to electrical energy. Since the LED and the phototransistor or photodiode are separated and have no direct electrical connection, the device provides isolation of the two sections of a circuit while enabling transfer of the electrical energy from one section to the other.

Once the light from the LED strikes the phototransistor, it starts conducts electricity depending on the state and duration of the light. The optoisolators are packaged in a wide variety of styles which includes cylinders, rectangles or other special configurations. These are designed to allow for the isolation of higher voltages than what the optocoupler SMD and DIP packages can handle.

Optopcouplers and optoisolators are sometimes used interchangeably; however, the optocouplers handle voltages of up to about 5000V while the optoisolators handle voltages of over 5000V.

Parameters and specifications for the opto-isolators:

  • Current transfer ratio, CTR: ratio between the LED and sensor currents
  • Isolation voltage: the highest guaranteed voltage between the LED and the light sensor
  • Linearity
  • Output device maximum voltage
  • Input current
  • Bandwidth

Factors that affect Optoisolator operations

The breakdown potential exterior to the optoisolator depends on factors such as the temperature, humidity, distance, barometric pressure, type and concentration of the contaminants in the air. The devices are therefore affected by humidity and in particular at higher voltages of about 50,000 DC. High humidity in the air may lead to arcing around the isolator or along the surface of the circuit board, resulting to a conductive path and possible short circuit around the optoisolator.

Advantages of optoisolators include:

  • Providing electrical and physical isolation of two sections of a circuit and hence the safety
  • Minimizing noise susceptibility and EMI and reducing interference such as from electrical interference
  • Relatively small and inexpensive
  • Ability to limit voltage across multiple circuits
  • Provide isolation

Disadvantages of optoisolators

  • Have limitations and cannot be used in some electrical systems
  • Affected by factors such as humidity, air pollution and barometric pressure, each if which can cause arcing and interference with the isolation. There requires that the devices be used in climate controlled areas.

Applications of Optoisolators

The Optoisolators are used in a variety of optical applications including the power supplies to provide isolation, in the recording industry to reduce interference, and in computer systems to transfer data. Applications include:

  • Power supply feedback systems
  • Medical, industrial, applications
  • Isolating ground loop currents
  • High voltage level shifting
  • Signal isolation
  • Electrical power and noise isolation