5-4. What Is a Photocoupler?

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 (Isolators/Solid State Relays)

5-1. Types of Optical Semiconductors
5-2. Light-Emitting Principal of LEDs
5-3. The wavelength range of LEDs
5-5. Why Are Photocouplers Necessary?
5-6. Types of Photocouplers
5-7. Types of Photocouplers (Packages)
5-8. Types of Photocouplers (Internal Structure)
5-9. Safety Standards of Photocouplers
5-10. Characteristics of Photocouplers (Current Transfer Ratio: CTR)
5-11. Principal Characteristics of Photocouplers (Trigger LED Current)
5-12. Aging Variation Data of Photocouplers
5-13. How to Use a Photocoupler
5-14. How to Use a Photocoupler “Input Current”
5-15. How to Use a Photocoupler “Output Current”
5-16. How to Use a Photocoupler “Output-Side Resistor”
5-17. How to Use a Photocoupler Check

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