An optocoupler (or optoisolator) is an electronic component used to provide electrical isolation between two separate electrical circuits. It transmits an electrical signal from one circuit to another optically while providing electrical isolation. Optocouplers are used to enable high voltage circuits to safely communicate with low voltage circuits, especially in sensitive electronic devices.
How Does an Optocoupler Work?
Optocouplers function as follows:
Input Section (LED): The electrical signal from the first circuit triggers a LED (light emitting diode) inside the optocoupler. The LED generates light after receiving this electrical signal.
Optical Insulation: The light emitted from the LED is transmitted to the photosensor in the second circuit, through an optical interface inside the optocoupler. This optical interface provides electrical insulation.
Output Section (Photosensor): A photosensor that detects light (usually a phototransistor, photodiode, or phototriac) converts the light energy it receives into an electrical signal. This signal is used in the second circuit.
This process makes it possible to transmit signals between two circuits without any direct electrical connection.
Internal Structure of the Optocoupler
The internal structure of the optocoupler mainly consists of two main components:
LED (Light Emitting Diode): It is located at the input section of the optocoupler. When it receives an electrical signal, it generates light.
Photosensor: It is located at the output section of the optocoupler. It detects the light produced by the LED and converts it into an electrical signal. The photosensor can be a phototransistor, photodiode, phototriac, photodarlington or another photosensor, depending on the type of the optocoupler.
Advantages of Optocouplers
High Insulation: They provide excellent electrical insulation between high voltage and low voltage circuits.
EMI Reduction: They prevent electromagnetic interference (EMI) and voltage fluctuations.
Safety: Prevents high-voltage systems from damaging low-voltage control circuits.
Optocouplers are divided into different types according to the type of photosensors used, as well as their functions. Below are the main types of optocouplers:
Phototransistor Optocoupler
Operating Principle: The light emitted by the LED is detected by a phototransistor and converted into an electrical signal.
Areas of use: Digital signal insulation, low-speed data transmission.
Advantages: Common, low cost, simple circuit design.
Photodiode Optocoupler
Operating Principle: The light from the LED is detected by a photodiode and generates an electric current depending on the light intensity.
Areas of use: High-speed data transmission, analog signal insulation.
Advantages: High speed and precision.
Phototriac Optocoupler
Operating Principle: The light emitted by the LED triggers a phototriac, which is used to control AC power circuits.
Areas of use: Insulation and control of AC loads (e.g. dimmer circuits, engine drivers).
Advantages: They can isolate and control high voltage AC circuits.
Photodarlington Optocoupler
Operating Principle: It has a pair of darlington transistors that detect LED light, to provide higher gain and output current.
Areas of use: Insulation and amplification of low current signals.
Advantages: High gain, higher current output.
Optocoupler with Logic Output
The working principle of the optocoupler with logic output is based on light-based insulation and digital signal transmission:
LED Triggering: When an electrical signal is applied to the input of the optocoupler, this signal triggers the LED on the input side. The LED emits light based on the electrical signal applied on it.
Perception of Light: This LED light is detected by a photodetector (usually a phototransistor or photodiode) inside the optocoupler. The photodetector switches to transmission process when illuminated by the light received from the LED.
Digital Output Production: The signal obtained from the photodetector is converted into a digital logic signal. The output signal is either ”0“ (low level) or ”1” (high level), which provides a suitable digital signal for micro-controllers or other digital circuits.
Insulation: Signals from high-voltage or noisy circuits are safely insulated due to lack of an electrical connection between the input and output sides.
Areas of use: Micro-controller Interfaces, PLCs (Programmable Logic Controllers), Digital Signal Isolation, Power Electronics.
Advantages: High speed, easy interfacing, high insulation, noise immunity.
FotoSCR (Silicon-Controlled Rectifier) Optocoupler
Operating Principle: The light emitted by the LED triggers a photo-SCR, and this allows the current to flow in only one direction.
Areas of use: Controlled rectifying in AC circuits, power control.
Advantages: High voltage and current carrying capacity.
Application Areas of Optocouplers:
Optocouplers are commonly used in many fields due to their properties of providing electrical insulation and signal transmission. Here are the main areas of use of optocouplers:
- Power Electronics
AC-DC Power Converters: They is used to insulate high-voltage power circuits from low-voltage control circuits.
SMPSs (Switched Power Supplies): Optocouplers are used to safely transmit the feedback signal to the control circuit.
- Industrial Automation
Engine Drivers: Optocouplers are used in the circuits used for speed and torque control of engines, to insulate the control signals.
PLCs (Programmable Logic Controllers): They ensure safe control of high-voltage inputs and outputs.
- Telecommunication
Modems and İnsulated Signal Processing: They are used to ensure signal insulation and data integrity in high-speed data transmission.
Phone Lines: They are used in interface circuits by insulating the signals on telephone lines.
- Medical Devices
Patient Isolation Systems: In places where the medical devices directly contact with the patients, the optocouplers are used to ensure the safety of the control circuits of the devices.
Medical Equipment In devices such as MRI, ECG etc., they are used to isolate the patient from high voltages.
- Security Systems
Alarm Systems: They are used for insulating the signals from the sensors and transmitting them to the central control unit.
Fire Detection Systems: They are used for signal transmission and insulation in the applications where high reliability is essential.
- Consumer Electronics
Household Appliances: They are used in devices such as microwave ovens, washing machines etc., to insulate the high-voltage power circuits from user control circuits.
TVs and Computers: They offer insulation and signal transmission in power supply units.
- High Voltage Applications
High Voltage Power Systems: In power transmission and distribution systems, optocouplers are used to insulate the high-voltage circuits from control circuits.
HVDC (High Voltage Direct Current) Systems: They provide safe insulation in power control and monitoring circuits.
