Optocoupler

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A photocoupler, or optocoupler is a device that allows two circuits to be connected and electrically isolated, we will have a control circuit with low currents which will drive a power circuit who will have current more important.

how does an optocoupler work ?

Provide galvanic isolation between two different circuits, while allowing the transmission of logical or analog information.

The transfer of information between the two circuits takes place in a closed environment and is achieved by a luminous flux of wavelength l.

The light is transmitted inside the case through a transparent plastic film.

  1. The LED lights up, the phototransistor receives light: the phototransistor is saturated (switch closed).
  2. The phototransistor is not receiving light (the LED is off or an object interrupts the light beam), the phototransistor is blocked (switch open)





Why use an optocoupler?

The role of an opto-coupler is either to provide galvanic isolation (no electrical connection) between two electrical systems.

Galvanic isolation protects the brain of the system (microprocessor, microcontroller) from possible disturbances caused by an external high voltage: sensors placed on the medium or high voltage electrical network, actuators supplying medium and high voltage)

The insulation produced can be tens of thousands of volts (see technical documentation)

The two components of this device are at the input, a photo-emitter, in the visible or infrared, and at the output a photo-receiver, photodiode or most often phototransistor. 



Electrical characteristics of an optocoupler

The specific characteristics of an optocoupler are mainly:

  • The input-output isolation voltage (Isolation Surge Voltage: VISO).
  • The maximum input current (Forward Current: IF).
  • The transfer rate (Current Transfer Ratio: CTR).
  • The maximum output current (Collector current: IC (in the case of a photo-transistor)).
  • The residual dark current in operation (Collector –Base Dark Current: ICBO).
  • The maximum power that the case can dissipate (Total Device Power Dissipation: PD)

The electrical characteristics for the transmitter (infrared LED):

- maximum current in the LED

- voltage across the LED (Vf)

The electrical characteristics for the receiver (phototransistor):

maximum current in the phototransistor

- leakage current in the phototransistor (dark current)

- power that the phototransistor can dissipate


And between the two, the CTR coefficient:

- transfer rate (CTR: current transfer ratio)

- isolation voltage of the optocoupler

The CTR is the ratio between the current in the phototransistor and the current in the LED. It is expressed in% (percent).

CTR = output current in transistor / current in LED

If the output current was proportional to the current in the LED, the CTR would be a constant.

a CTR of 160% corresponds to a collector current (phototransistor) which is worth 1.6 times the current injected into the LED.


Optocoupler applications

Optocouplers are found in various fields:

- switching power supplies (regulation of the output voltage)

- isolated digital data

- control of mains loads from an accessible low voltage circuit.




sources:

https://www.electromecanique.net/2018/10/photocoupleur-le-fonctionnement.html

http://pelletier1.free.fr/electronique/cours/fichiers_cours_electronique/Cours_Optocoupleurs.pdf

http://mistershoe.free.fr/Techno%20Electronique/Optocoupleur/optocoupleur.htm

http://stssnsb.free.fr/telecharger/blusson/cours/cours%20optocoupleur.pdf


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