Optocoupler Construction and Operating Principle
Optocoupler Construction and Operating Principle
What is optocoupler?
An opto-isolator or opto-coupler is a system that transmits electrical signals by light energy between two electrically separated circuits. Opto-isolators prohibit high voltages in one portion of the circuit from impacting or destroying other parts of the loop.
Basic Construction of Optoisolator
Opto-isolator is a device with two diodes: one is a light source or emitter, typically a light emitting diode (LED) and the other is a photodiode serving as a photosensor. The LED transforms the electrical input signal into light, and the photodiode senses the incoming light and either produces the resulting electrical energy depending on the incoming light.
An opto-isolator (also known as an opto-coupler, photo-coupler or optical insulator) is an electronic device that transmits electrical signals by the use of light between two separated circuits. Opto-isolators block high voltages from impacting the signal-receiving device. Commercially available opto-isolators with an input-to-output voltage of up to 10 kV and a voltage transient of up to 25 kV.
Operating Principle of Optoisolator
The operating theory of the opto-isolator is simple and fascinating. The output signal of one circuit can be regulated by a variable input signal in another circuit, where the two circuits are electrically separated. A strong light emitting diode (LED) is attached to a variable voltage source. The intensity of the light produced from the LED can be controlled by changing the input voltage around the LED. A photodiode is present in front of the LED such that the light from the LED is directly at the intersection of the photodiode. The photodiode is in reverse bias. The reverse bias circuit of the photodiode is the output circuit of the device. It is therefore assured that there is no other light falling on the photodiode junction and that the device is shielded from any external light except the light coming from the LED. Initially, no voltage is applied to the lead; thus, the lead does not shine. In this condition, when no light falls on the photodiode, only the dark current will pass through the output circuit. Dark current is the reverse saturation current of the reverse bias photodiode when it's dark. This is the inevitable reverse leakage current of the diode. Now, if we increase the voltage around the LED, the LED will start to glow and at the same time the strength of the light will increase with an increase in the input voltage.
Optocoupler Applications
- Lamp Ballasts
- Light Dimmers
- Valve or Motor Controllers
- Microcontrollers for interfacing with High Voltage Circuit
- The main application of optocoupler is to isolate two different electrical circuits.
- Optocouplers are used in solid state relays.
- Optocouplers are used for ground isolation purposes.
- Optocouplers are used for high voltage monitoring and sensing circuits.
- Optocouplers are used in lamp dimmer circuits.
Optocoupler Circuit