Optocoupler Symbol
Definition: The Optocoupler symbol represents an optoelectronic isolation device — standardised in IEC 60747-5 and depicted per IEC 60617 and ANSI Y32.2/IEEE 315 — that transmits signals between two galvanically isolated circuits using an internal infrared LED on the input side (Anode/Cathode) and a phototransistor on the output side (Collector/Emitter), with common types including the PC817 and 4N35.
Also known as: optoisolator, optical isolator, photocoupler, opto-isolator, 4N35, PC817.
What the Optocoupler symbol means
The optocoupler symbol depicts two electrically separate sub-circuits enclosed in a common package boundary: a light-emitting diode (LED) on the left (input) side and a phototransistor on the right (output) side, separated by a dashed line indicating the optical barrier. The LED converts an electrical signal into infrared light; the phototransistor converts that light back into a current, passing the signal without any direct metal-to-metal electrical connection.
In schematics the optocoupler symbol conveys that two circuit domains — for example a mains-voltage control circuit and a low-voltage microcontroller — are linked only by photons, providing high-voltage isolation (typically 1,500 V to 5,000 V RMS). The standard designator for an optocoupler is U (IC package) or OC/OK in older European conventions.
How to identify the Optocoupler symbol
The optocoupler glyph shows a rectangular package outline containing, on the left half, a standard LED triangle-and-bar symbol with two small arrows pointing diagonally to the right (representing emitted infrared light). On the right half sits a phototransistor symbol — a transistor body with arrows pointing toward the base region to indicate incoming photons. A vertical dashed centre line bisects the rectangle, symbolising the optical (non-electrical) coupling boundary. Four external pins emerge from the package: Anode (A) and Cathode (K) on the input (LED) side at the left, and Collector (C) and Emitter (E) on the output (phototransistor) side at the right.
Function in a circuit
An optocoupler transfers a logic or analog signal from an electrically isolated source circuit to a destination circuit. The input LED is forward-biased through a current-limiting resistor; its emitted infrared light drives the phototransistor into conduction, allowing current to flow from Collector to Emitter on the output side. Because the only link between input and output is light, the device blocks noise, surge transients, and dangerous voltages from reaching the receiving circuit, making it essential for mains isolation, ground-loop elimination, and level shifting between circuits at different potentials.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-05 (semiconductors) defines the optocoupler using the combined LED + phototransistor symbol within a dashed envelope; the LED input is drawn as a diode triangle with two radiation arrows, and the phototransistor output has inward-pointing arrows at the base. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 (ANSI Y32.2) represents the optocoupler with the same LED-plus-phototransistor construction inside a package box, using inward light-arrow convention identical to IEC; the overall appearance is functionally the same. |
| Key difference | IEC and ANSI/IEEE symbols for optocouplers are nearly identical; the main practical difference is that some IEC schematics omit the outer package rectangle and rely on proximity alone, whereas ANSI/IEEE always encloses the pair in a rectangle with a dashed isolation barrier. |
Terminals / pins
| Pin | Name |
|---|---|
| anode | Anode |
| cathode | Cathode |
| collector | Collector |
| emitter | Emitter |
Typical values
Forward current (IF) 5–60 mA; forward voltage (VF) ~1.2 V; isolation voltage 1,500–5,000 V RMS; collector-emitter voltage (VCEO) 30–80 V; current transfer ratio (CTR) 20–300%; switching time typically 2–10 µs for small-signal types.
Where the Optocoupler symbol is used
- Isolating microcontroller I/O from mains-referenced triac/SCR gate drive circuits
- CAN bus and RS-485 network transceivers requiring ground isolation between nodes
- Switch-mode power supply feedback loops — transferring the output voltage error signal to the primary side
- Medical equipment where patient-circuit isolation from mains is mandated by IEC 60601
- Motor drive and inverter gate drivers separating high-voltage IGBT gates from control logic
- Industrial 4–20 mA current-loop receivers providing signal isolation
- USB-to-RS-232 converters with galvanic isolation for measurement instruments
Example
In a mains-powered LED dimmer, the optocoupler (e.g. PC817) sits between the phase-detection zero-crossing circuit operating at mains potential and a 3.3 V microcontroller: the LED side is connected via a resistor to the mains-referenced detection point, and the phototransistor output drives a GPIO input, allowing the microcontroller to time triac firing without any direct mains exposure.
Key facts
- An optocoupler uses infrared light to couple a signal across a galvanic isolation barrier, with no direct electrical connection between input and output.
- The standard designator is U (or OC/OK in older European practice); the symbol shows an LED and a phototransistor inside a shared package rectangle with a dashed isolation line.
- Pins are: Anode (A) and Cathode (K) on the LED input side; Collector (C) and Emitter (E) on the phototransistor output side — 4 terminals total for a standard single-channel device.
- Isolation voltage ratings for common small-signal optocouplers range from 1,500 V RMS (PC817) to 5,000 V RMS (6N136, HCPL-0201).
- Current Transfer Ratio (CTR) — the ratio of output collector current to input LED current — typically spans 20–300% and degrades with device age and temperature.
- IEC 60617-05 and IEEE 315-1975 (ANSI Y32.2) both represent the optocoupler with the combined LED + phototransistor symbol inside a dashed-barrier envelope; the two standards are visually nearly identical.
- Optocouplers are used to break ground loops in audio equipment, protect microcontrollers from high-voltage spikes, and implement feedback paths in isolated SMPS designs.
- High-speed optocouplers (e.g. HCPL-7710) achieve propagation delays below 50 ns and are used in gate-drive circuits for IGBTs operating at hundreds of volts.
Frequently asked questions
What does the optocoupler symbol look like?
The optocoupler symbol shows a rectangular package outline split by a vertical dashed line. On the left (input) side is an LED symbol — a diode triangle-and-bar with two small arrows pointing right to indicate light emission. On the right (output) side is a phototransistor symbol with arrows pointing inward toward the base, indicating photon reception. Four pins exit the package: Anode and Cathode on the left, Collector and Emitter on the right.
What does the optocoupler symbol mean in a circuit diagram?
The optocoupler symbol means a signal passes between two electrically isolated circuit domains via infrared light rather than a direct wire connection. The dashed line in the symbol represents the optical — not electrical — coupling barrier, indicating complete galvanic isolation between the input LED circuit and the output phototransistor circuit.
What is the difference between the IEC and ANSI symbols for an optocoupler?
IEC 60617-05 and IEEE 315 (ANSI Y32.2) both use a combined LED-plus-phototransistor symbol inside a rectangular package with a dashed isolation barrier, and the two representations are nearly identical. The minor practical difference is that some IEC schematics may omit the outer package rectangle; ANSI/IEEE schematics consistently include it.
How many pins does an optocoupler have?
A standard single-channel optocoupler has 4 pins: Anode (A) and Cathode (K) for the LED input, and Collector (C) and Emitter (E) for the phototransistor output. Dual-channel devices such as the 4N26 dual have 6 or 8 pins. The 4-pin arrangement is defined by IEC 60617-05 and matches common parts like the PC817 and 4N35.
What is CTR in an optocoupler?
CTR stands for Current Transfer Ratio — the percentage ratio of the output collector current (IC) to the input LED forward current (IF). A CTR of 100% means 10 mA into the LED produces 10 mA at the collector. Typical small-signal optocouplers have CTR values of 20–300%; CTR decreases over the device lifetime and at higher temperatures.
What is the designator letter for an optocoupler?
The standard reference designator for an optocoupler is U (used in modern IEC and IEEE 315 practice for all integrated-circuit packages). Older European and DIN 40719 schematics may use OC or OK. The letter U is consistent with other single-package IC components such as amplifiers and logic gates.
What standard defines the optocoupler symbol?
The optocoupler symbol is defined in IEC 60617-05 (Semiconductors) for IEC schematics and in IEEE 315-1975 (ANSI Y32.2) for North American schematics. Both standards use the same combined LED-plus-phototransistor enclosed-package representation with a dashed optical isolation barrier between input and output.
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