Opto-isolator

An optocoupler is a component of optoelectronics and serves to transmit a signal between two isolated circuits. It consists of an optical transmitter, it is typically a light emitting diode (LED) and an optical receiver as a phototransistor, which are both housed in an opaque casing.

Optocouplers both digital and analogue signals can be transmitted.

Construction

The internal design, a distinction between the face-to -face design and the Coplanar design. In the first case, the transmitter and receiver are as shown in the diagram opposite. In the second variant the sender and receiver are on one level. Here, the light beam is similar to the principle of an optical waveguide transmitted by reflection.

For some types of an amplifier is directly downstream of the optocoupler to switch small loads, such as relays, directly.

Characteristics

DC Current Transfer Ratio

The Current Transfer Ratio ( engl. current transfer ratio, CTR ) is the ratio between the input and output current with DC signals or low signal frequencies. The values ​​are between 0.2%, depending on the receiver (photodiode) for about 100% ( phototransistor ) up to more than 1000 % ( Darlington phototransistor ). In digital optocouplers not CTR indicated, but a minimum LED current, which is required to level change at the output.

Insulation voltage

This voltage is dependent on distance and arrangement of the transmitter and receiver, the insulating material and the spacing of the terminals. Usual isolation voltages are 1500, kVolt to 4000 V in special cases up to 25.

For reliable mains isolation optocouplers are manufactured with more distant connections than is the case with the pictured below DIL package, to be on the board to comply with the 8 mm creepage distance required by a rule.

Insulation resistance

The insulation resistance between the input and the output is very high and is up to 1013 Ω.

Cut-off frequency and switching times

The cutoff frequency is the highest working frequency at which an optocoupler can still work. It is optocouplers with phototransistor at about 50 ... 200 kHz, those with photodiode considerably higher, usually above 10 MHz. At photodiodes or digital opto-couplers, the cutoff frequency is limited by the switching times of the transmission LED.

The slowest are optocouplers with PhotoMOS transistor, these have response times in the millisecond range.

Blocking voltages

The emitting diode only tolerate reverse voltages of about 5 volts; however, it is already operated in the direction of flow. The reverse voltage of the receiving phototransistor is usually 30 to 50 volts. In optocouplers built thyristors and triacs have blocking voltages up to about 400 volts.

Optocoupler digital work on the receiver side the most to a voltage of 5 volts.

Designs

Optocouplers are offered in packages similar to those of integrated circuits. For high reverse voltages from about 4 kV also elongated housing types are manufactured.

Optocouplers are manufactured with an open ( accessible ) optical beam path, they are then called hybrid coupler or reflection coupler (see photocell).

Advantages of optocouplers

  • Small size
  • Digital and analog signal transmission possible
  • Low coupling capacitance between input and output
  • No inductors
  • Compared to also galvanically isolating relay shorter delay times of the output signal
  • No interference from magnetic fields
  • Galvanic separation of input and output circuits
  • In contrast eg to relay no mechanical wear, so much more switching cycles possible

Disadvantages of optocouplers

  • Higher voltage drops in the output circuit than the relay
  • Only one direction of current in the output circuit (except for Triac and PhotoMOS receiver)
  • Transmitting diode requires external series resistor ( Exception: Solid -state relay )
  • In PhotoMOS partly, low cut-off frequencies in the range of a few kHz
  • Input and output circuits are compared to relay sensitive to overload and glitches

Areas of application

Optocouplers are found usually where circuits electrically (electric) must be separated from each other. Applications are, for example:

  • Protection of Modules: If thermally destroyed in a span of LED input of the optocoupler by increased current, the output part ( phototransistor ) and the underlying circuit remains protected - only the LEDs need to be replaced.
  • Interface cards or network cards of computers. Here, the circuits must be electrically isolated from each other because the connected devices may have different ground potentials.
  • Modules that must be protected against transient voltages and common mode noise pulses often have opto coupling of their inputs and outputs ( industrial controls, PLC)
  • In medical devices, the patient must be protected, particularly against fault voltages.
  • Control of circuit parts that are on different voltage potentials (eg signal transmission from and to network voltage circuits )
  • Electronic overload relays are also driven by integrated optical coupler
  • SMPS: transfer of control information from the secondary to the primary part
  • Musical Instrument Digital Interface (MIDI ) to avoid ground loops

Alternatives

In addition to mechanical relay, there are other devices that transmit signals to non- optical way floating. These include electronic coupler and isolation amplifiers that operate with inductive or capacitive transmission, as well as semiconductor circuits, the changing potential differences by means of high-blocking transistors overcome (level shifter, high side switches, half bridge driver)

Photodiode#Other modes of operation Analogsignal Solid state (electronics) Ground loop (electricity)
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