Opto-coupler

A opto-coupler (or optocouplor) is a simple device of Opto-électronique being used to ensure a galvanic insulation between two electrical circuits.

Composition

The basic principle of the opto-coupler is very simple: It is composed of
  • a box tight with the light with inside and face to face:
  • a Electroluminescent diode (LED)
  • a component sensitive to the light emitted by the LED (phototransistor or photodiode)

Operation

A opto-coupler is characterized by its tension of insulation, its type (analogical/digital, at active exit or passivates, its commutation rate) and its profit while running, called CTR (Current Transfer Ratio).

  • the LED is a Diode emitting infra-red light (spontaneous emission) when it is subjected to a direct polarization. When a power is on in the LED, and starting from a certain tension (in general 1,5 Volt), the LED ignites.

  • the phototransistor: This last is composed of 3 pins: the transmitter, the base and the collector. The phototransistor is an alternative of the Transistor NPN: the power is on from the collector towards the transmitter, but provided that the base receives, either current, but of the visible light or Infrarouge. In the majority of the applications, the phototransistor functions in Commutation. When the LED lights (crossed by a current), then the current can cross the transistor. One can then regard the transistor as a closed switch.
On the diagram of right-hand side, the diode is represented between pins 1 and 2 while the phototransistor is located between pins 3 (collector) and 4 (transmitter). The phototransistor has a high CTR (from 10% to 150%), but a commutation rate average (0,1ms typical).
  • the photodiode: This component is following the example phototransistor a receiver of light. Like this last, it lets pass the current only if it is enlightened. Its advantage is to be much faster (0,1 microsecond with 1 microsecond) that the phototransistor. On the other hand the current which it commutates is smaller, i.e. its CTR is smaller (from 0,1% to 10%). Many phototransistors are equipped with photodiode followed by a transistor (or a more complex electronics) which amplifies the current provided by the photodiode, which combines a commutation rate fast with a high CTR.
  • the photo-relay: It is about a optocouplor whose receiver is composed of several photodiodes in series, which generate a tension when they are illuminated. This tension is applied to (them) the grid (S) of one or more transistor (S) to field effect integrated in the optocouplor. This component behaves thus like a relay with semiconductors. The commutation rate slow (compared with the other opto-couplers), but a little faster is compared with the electromechanical relays. (1ms with 10ms).
  • the photoone: The receiver is composed of a phototransistor, follow-up of a normal transistor which amplifies the current. This optocouplor has a very high CTR, but a commutation rate average.
  • the analogical optocouplor: It is used to reproduce a precise current through a galvanic insulation. It is composed of a transmitter (a LED) and of two receivers of characteristics as identical as possible. One of the receivers provides a current which is compared with the current of reference to transmit. The other receiver is with dimensions one to insulate.
  • insulation: It is about the tension of common mode between the entry and the exit of the optocouplor. The manufacturer in general specifies a continuous tension maximum and/or alternative for which no breakdown occurs. This tension must for the test being applied during a minimum time (1 second or 1 minute). To obtain a good insulation, the optical components are separated by a transparent screen, even in certain optocoupleurs run in the same resin.

Alternatives

  • multiple Opto-coupler: Regrouping under the same case of 2,4,8 or 16 opto-couplers intended to treat transfers of information in parallel.
  • Opto-coupler with slit: The case is opened by a slit which makes it possible to stop the luminous ray (in this case there, use of the infra-red to avoid the disturbances). The introduction of an opaque object into the slit then transforms the coupler into Interrupteur.

Uses

The opto-couplers are used, where a galvanic separation of two electrical circuits is necessary: by protection or to connect two circuits which do not have the same masses. The opto-couplers can order only continuous currents, but are characterized by very high speeds of order and a relatively broad field of tension.

Some references of components

  • TIL111

See too

  • Opto-electronic Electroluminescent diode

  • Photodiode

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