What Is a Photocoupler?

A photocoupler is a device incorporating a light-emitting diode (LED) and a photodetector in one package. Unlike other optical devices, light is not emitted outside the package. The external appearance is similar to that of non-isolators/solid state relays. Although a photocoupler is an optical device, it does not handle light, but handles electrical signals.

Examples of a photocoupler’s operation:

(1)The LED turns on (0 ⇒ 1).
(2)The LED light enters the phototransistor.
(3)The phototransistor turns on.
(4)Output voltage changes 0 ⇒ 1.

(1)The LED turns off (1 ⇒ 0).
(2)The LED stops light emission to the phototransistor.
(3)The phototransistor turns off.
(4)Output voltage changes 1 ⇒ 0.

*The cutaway image on the right shows a transistor-output photocoupler of the transmissive type in a double-mold structure.

What Is a Photocoupler?

Chapter V : Optical Semiconductors

Types of Optical Semiconductors
Light-Emitting Principal of LEDs
The wavelength range of LEDs
Why Are Photocouplers Necessary?
Types of Photocouplers
Types of Photocouplers (Packages)
Types of Photocouplers (Internal Structure)
Safety Standards of Photocouplers
Characteristics of Photocouplers (Current Transfer Ratio: CTR)
Principal Characteristics of Photocouplers (Trigger LED Current)
Aging Variation Data of Photocouplers
How to Use a Photocoupler
How to Use a Photocoupler “Input Current”
How to Use a Photocoupler “Output Current”
How to Use a Photocoupler “Output-Side Resistor”
How to Use a Photocoupler Check

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