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ON VIDEO What is a Optocoupler ? How Optocoupler Works ? The Optocoupler Explained

 




An optocoupler is formed by an infrared LED and a phototransistor or a photodiode.
 The optocoupler provides a connection between the LED and the phototransistor while ensuring electrical isolation between the two.

The optocoupler is characterized from an electrical point of view by the infrared LED part and the phototransistor part.

Electrical characteristics of an optocoupler
The electrical characteristics are for the emitter (infrared LED):

- maximum current in the LED

- voltage across the LED (Vf)

The electrical characteristics are for the emitter (infrared LED):

- 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)

- optocoupler isolation voltage

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

For example, a CTR of 50% indicates that for 1mA in the LED, it will be able to flow 0.5mA in the phototransistor (from the collector to the emitter). In practice, the CTR varies with the current in the LED, and varies from one optocoupler to another (see below).

The optocoupler does not switch alternating current, it is not bidirectional.



Principle of operation of the optocoupler

An optocoupler is based on an LED and a phototransistor or photodiode. When a current is passed through the LED, it shines (it emits infrared) in a box that is well sealed against light. The light emitted by the LED is captured by the phototransistor which then becomes conductive. It is therefore possible to transmit an electric current while electrically insulating. In principle, the optocoupler performs the successive conversions: electric current - infrared light - electric current.

CTR of an optocoupler: definition
The current that the phototransistor of an optocoupler can deliver increases if the current in the LED increases. The greater the current in the infrared LED, the more infrared the infrared LED emits and the more the phototransistor is "illuminated", which creates a higher current, as far as the circuit where it is allows.

The CTR (current transfer ratio) of an optocoupler is the following ratio:

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. The CTR is often expressed in %: a CTR of 160% corresponds to a collector current (phototransistor) which is 1.6 times the current injected into the LED.



 




An optocoupler is formed by an infrared LED and a phototransistor or a photodiode.
 The optocoupler provides a connection between the LED and the phototransistor while ensuring electrical isolation between the two.

The optocoupler is characterized from an electrical point of view by the infrared LED part and the phototransistor part.

Electrical characteristics of an optocoupler
The electrical characteristics are for the emitter (infrared LED):

- maximum current in the LED

- voltage across the LED (Vf)

The electrical characteristics are for the emitter (infrared LED):

- 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)

- optocoupler isolation voltage

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

For example, a CTR of 50% indicates that for 1mA in the LED, it will be able to flow 0.5mA in the phototransistor (from the collector to the emitter). In practice, the CTR varies with the current in the LED, and varies from one optocoupler to another (see below).

The optocoupler does not switch alternating current, it is not bidirectional.



Principle of operation of the optocoupler

An optocoupler is based on an LED and a phototransistor or photodiode. When a current is passed through the LED, it shines (it emits infrared) in a box that is well sealed against light. The light emitted by the LED is captured by the phototransistor which then becomes conductive. It is therefore possible to transmit an electric current while electrically insulating. In principle, the optocoupler performs the successive conversions: electric current - infrared light - electric current.

CTR of an optocoupler: definition
The current that the phototransistor of an optocoupler can deliver increases if the current in the LED increases. The greater the current in the infrared LED, the more infrared the infrared LED emits and the more the phototransistor is "illuminated", which creates a higher current, as far as the circuit where it is allows.

The CTR (current transfer ratio) of an optocoupler is the following ratio:

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. The CTR is often expressed in %: a CTR of 160% corresponds to a collector current (phototransistor) which is 1.6 times the current injected into the LED.



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