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Optocoupler Circuits: Isolation for Safety

Key Takeaways

  • An optocoupler circuit breaks direct conduction paths by pairing an LED and a photosensitive receiver for signal transmission.

  • In addition to isolation, an optocoupler simultaneously acts like a diode and filter, fixing the direction of current flow and removing high-frequency noise.

  • Depending on the needs of the circuit, different optocoupler parameters may be more beneficial.

A row of optocouplers in packaging

Optocoupler circuits provide safe isolation from high-voltage sources, protecting equipment and users.

Light and electromagnetic radiation are synonymous; in some cases, performance or functionality improves by transmitting information using light flashes instead of direct conduction. Abstractly similar to transformers, optocouplers can physically isolate sections to break conduction pathways while maintaining signal coherence. Systems can benefit from optocoupler circuits in applications where operating conditions – typically high voltage – on one end of the circuit exceed the ratings of components on the other side.

Comparing Optocouplers and Transformers

Comparing Optocouplers and Transformers



  • Passes signals non-conductively through
  • electromagnetic radiation
  • Cannot transfer power
  • Can pass DC or low-frequency signals
  • The signal usually only flows from LED to phototransistor, but a bidirectional signal transfer is possible
  • Passes signals non-conductively through an inductive coupling
  • Can transfer power
  • Only couples with high-frequency signals
  • By design, the transformer can operate in “forward” or “reverse” coupling depending on the direction of current flow into the transformer

What Does an Optocoupler Circuit Do?

An optocoupler combines an LED and phototransistor in the same package; when the LED turns on, photons strike the light-sensitive phototransistor, causing it to produce current. Optocoupler circuits primarily break conduction pathways and isolate circuit sections to prevent high-voltage with considerable noise from wreaking havoc on less electrically resilient components. The isolation bundles multiple circuit functions in one package:

  • Asymmetric conduction - An optocoupler often only passes signals from LED to photosensor like a diode. However, since LEDs are also capable of sensing light, this is optional; designers can select components that suit the conduction pathway of the circuit. 

  • Step-down converter - The isolation between the sensor and emitter needs to disrupt the direct conduction pathway and transform the voltage to make it suitable for the receiving end of the optocoupler. A typical circuit configuration uses a series resistor on the input side to develop current across the LED according to Ohm’s Law. This current causes the LED to turn on and flicker at a specific rate (determined by the current’s frequency), which results in a corresponding current in the photosensitive component (diode, transistor, resistor) on the output side of the optocoupler. The output current develops a voltage across the output resistor, and the total output voltage is equal to the voltage across the photosensitive receiver (typically non-linear) minus the voltage drop across the resistor. The amount of light the LED generates fluctuates with the input voltage, meaning the output voltage does as well.

  • Filtering - High-voltage signals are especially noisy. The pulse rate of an LED caps in the hundreds of kHz; effectively, the optocoupler acts as a low-pass filter, transmitting the coherent signal while attenuating higher-order harmonics that cause distortion and loss. Low-voltage digital circuits are especially susceptible to the noise inherent in higher-voltage sensor detection. 

Considerations and Ideas for Optocoupler Usage

While it generally takes the place of a relay-controlled contact or transformer when isolation is the foremost goal, the exact roles an optocoupler fulfills will differ depending on the needs of the circuit. Optocouplers come in many styles and parameters. You will want to weigh a few optocoupler characteristics to discover the most suitable choice:

  • Current transfer ratio (CTR) - The CTR measures the optocoupler circuit's efficiency. For discrete components, matching the absorption range of the photosensitive receiver to that of the light emitted by the diode (which typically resides in the infrared spectrum) ensures maximum energy transfer. For specification, the CTR usually refers explicitly to the input-to-output CTR, which is a ratio of the current measured at the output of the phototransistor (or a different photosensitive component) and the input current flowing into the LED.

  • Input-to-output isolation voltage - This value represents the maximum realizable voltage drop between the input and output of the optocoupler, ranging anywhere from several hundred to several thousand volts.

  • Collector-emitter voltage - The maximum voltage that develops across the transistor for phototransistor optocouplers, usually in tens of volts.

  • Bandwidth - The range of signal frequencies the optocoupler can pass through from input to output. The bandwidth’s variance is considerable compared to other optocoupler metrics and relies heavily on the construction of the component.

  • Response time - Signal transmission's rise and fall time typically lasts a few microseconds.

Of similar importance is how the signal transmits from the LED to the photosensitive receiver. Arguably, the default configuration is a closed pair configuration, which monolithically encapsulates the LED and photosensitive receiver to save layout space. Additionally, the encapsulation prevents stray light from triggering the photosensor, preventing false positive events.

Variants of the closed pair configuration allow for additional functionality by making signal transmission selective. A slotted optocoupler features a physical gap between the LED and photosensitive receiver; there is a complete encapsulation of the optocoupler with transparent material, and a recession between the two sub-components means the transmission path disrupts when an opaque object inserts itself in the gap. Another alternate optocoupler style orients the LED and photosensitive receiver to the same focal point and uses a reflective surface to transmit the light. The slotted optocoupler lends to various detection functions (presence, vibration, etc.), while the reflective optocoupler is more suitable for detecting motion or monitoring the presence of reflective substances (like smoke).

Cadence Solutions Optimize Optical Circuitry

Optocoupler circuits offer a wealth of circuit functionality that protects equipment and users from dangerously high voltages through a non-conductive transmission pathway. Isolation is a critical safety feature for many high-voltage electronics or those that interface with them. While transformers can offer galvanic isolation (with power transfer capabilities), the unique structures of optocouplers open up many new circuit topologies. Regardless of which component design teams decide, they can rely on Cadence’s PCB Design and Analysis Software suite to provide comprehensive simulation and models to characterize circuits before layout fully. This data seamlessly transfers to  OrCAD PCB Designer for an accelerated production cycle with powerful and easy-to-use ECAD tools.

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