Optocoupler
An optocoupler carries a digital or low-bandwidth analog signal from one electrical domain to another using light, with no metallic path between the two sides. That gap is the whole point — it isolates ground loops, blocks high-voltage transients, and protects fragile logic from noisy power.
On the input side, current through an LED produces light. On the output side, a phototransistor (or photo-darlington, or photo-triac) sees that light and conducts. A clear-plastic dielectric or air gap of typically 0.4 mm separates the two halves, rated for several kilovolts of isolation. Because only photons cross the gap, the input and output grounds can sit at vastly different potentials without leaking current to each other.
In plain terms
Two strangers in soundproof rooms passing notes by flashlight through a window. The light gets through; nothing else does.
Also called: opto-isolator, photocoupler, opto.
Why designers use it
- Break ground loops between subsystems that share a chassis but not a clean ground.
- Protect a microcontroller's input pins from a noisy or high-voltage source.
- Fire a TRIAC or thyristor from a logic-level signal in mains-side circuits.
- Carry feedback across the isolation barrier in offline switching power supplies.
- Sense a button on a 230 V panel without bringing mains anywhere near the MCU.
Best for
- Galvanic isolation
- Mains-side feedback
- Ground-loop fix
- Triac firing
Key specifications
- Isolation voltage: 2.5 kV – 7.5 kV (Vrms, 1 minute)
- Current transfer ratio: 20 % – 600 % (Output Ic / input If)
- Switching speed: 1 µs – 100 µs (10 kbit/s class)
- Input forward voltage: 1.1 V – 1.4 V (GaAs LED, ~10 mA)
- Operating temperature: −55 °C – +110 °C
- Common-mode rejection: 10 kV/µs typical
When not to use it
- Where you need MHz-class data rates — basic optocouplers max out around 10 kbit/s; reach for a digital isolator instead.
- Where you need analog linearity — the LED-to-phototransistor transfer is non-linear and drifts with temperature.
- Where the LED would be left on continuously for years — its light output ages, and the receiver eventually undertriggers.
Common mistakes
- Forgetting the input-side current-limit resistor — burns the LED out within minutes.
- Treating CTR (current transfer ratio) as constant — it's a wide spec range and degrades with age.
- Using a slow optocoupler in a feedback loop and chasing a phantom oscillation that's just propagation delay.
- Putting the optocoupler on the same ground plane on both sides — defeats the entire isolation barrier.
Where you will find it
- Every offline phone charger has a tiny optocoupler (often a PC817 or SFH615) carrying the regulation feedback signal from the secondary side back to the primary-side controller — without it, the controller can't see how loaded the output is, and the brick won't regulate.
- Industrial PLCs use one optocoupler per input channel to galvanically isolate every limit switch and proximity sensor on the factory floor — a single shorted sensor cable can dump kilovolts onto the line, and the optocoupler's 5 kV barrier is what stops that surge from frying the CPU.
- Hobbyist motor-control boards use a TLP250 (gate-drive optocoupler) to fire each MOSFET in an H-bridge from an Arduino-level signal: the optocoupler bridges the floating gate-drive supply on the high side to the logic-level 5 V on the controller side without ever connecting them electrically.
A short history
The optocoupler combines two earlier inventions: the LED (Holonyak, 1962) and the silicon phototransistor (Shive, 1950). The first commercial optocoupler — the General Electric H11A1, packaging a GaAs IR LED with a silicon phototransistor in a 6-pin DIP — appeared in 1968, and the basic four-pin variant followed in 1971 as the standard general-purpose isolator. Sharp introduced the PC817 in the late 1980s, which became the de facto cost-driven workhorse for SMPS feedback loops and remains in production at billion-unit annual volume. Modern silicon-photonic and capacitive digital isolators have largely replaced optocouplers in fast data paths, but the LED + phototransistor optocoupler remains dominant for AC-mains power-supply feedback because it tolerates large common-mode transients and its isolation barrier is well-understood by safety regulators (UL 1577, IEC 60747-5-5).
Good to know
- An optocoupler's only conducting path is a photon — the very same particle that lets your eye read this sentence. Nothing in electronics is more elegantly disconnected.
- The classic 4-pin optocoupler form factor (DIP-4) has been pin-compatible across vendors since 1971 — you can drop in a Sharp PC817 where an Everlight EL817 used to be without changing your PCB.
- The CTR spec drifts down by about 10 % per 10,000 hours of LED on-time — designers add 2× headroom so the part still meets spec at end of life.