What is an Optocoupler and How it Works

By Megan Tung

An optocoupler (also called optoisolator) is a semiconductor device that allows an electrical signal to be transmitted between two isolated circuits. Two parts are used in an optocoupler: an LED that emits infrared light and a photosensitive device that detects light from the LED. Both parts are contained within a black box with pins for connectivity. The input circuit takes the incoming signal, whether the signal is AC or DC, and uses the signal to turn on the LED.

The photosensor is the output circuit that detects the light and depending on the type of output circuit, the output will be AC or DC. Current is first applied to the optocoupler, making the LED emit an infrared light proportional to the current going through the device. When the light hits the photosensor a current is conducted, and it is switched on. When the current flowing through the LED is interrupted, the IR beam is cut-off, causing the photosensor to stop conducting.

Optocoupler Current How it Works

There are four configurations of optocouplers, the difference being the photosensitive device used. Photo-transistor and Photo-Darlington are typically used in DC circuits, and Photo-SCR and Photo-TRIAC are used to control AC circuits. In the photo-transistor optocoupler, the transistor could either be PNP or NPN. The Darlington transistor is a two transistor pair, where one transistor controls the other transistor’s base. The Darlington transistor provides high gain ability.

Optocoupler Configuration How it Works

The term optocoupler and optoisolator are often used interchangeably, but there is a slight difference between the two. The distinguishing factor is the voltage difference expected between the input and the output. The optocoupler is used to transmit analog or digital information between circuits while maintaining electrical isolation at potentials up to 5,000 volts. An optoisolator is used to transmit analog or digital information between circuits where the potential difference is above 5,000 volts.

An Optocoupler Can Effectively:

  • Remove electrical noise from signals
  • Isolate low-voltage devices from high-voltage circuits. The device is able to avoid disruptions from voltage surges (ex: from radio frequency transmissions, lightning strikes, and spikes in a power supply)
  • Allow the usage of small digital signals to control larger AC voltages

Megan Tung is a summer intern at Jameco Electronics, attending the University of California, Santa Barbara (UCSB). Her interests include photography, music, business, and engineering.

Photo Credits: Electronics-Tutorials and Autodesk.