What Is an Optocoupler (Opto-Isolator)?
An optocoupler (opto-isolator) is a component that electrically (galvanically) separates the input and output circuits by transferring a signal via light. There's no metal connection between the two circuits.
How Does It Work?
Inside it there's an IR LED and, facing it, a phototransistor (or photodiode). When a signal arrives at the input the LED lights, and the light turns on the receiver at the output. Since the signal passes via light, the two sides don't touch.
Why Is Isolation Needed?
It's used to protect a low-voltage control circuit (e.g. a microcontroller) from the high-voltage power side, break ground loops and separate noise.
| Parameter | Meaning |
|---|---|
| Isolation voltage | The difference it withstands (kV) |
| CTR | Current transfer ratio |
| Speed | Switching frequency |
CTR (Current Transfer Ratio)
It's the ratio of the output current to the input LED current. CTR can drop somewhat over time and with temperature; a margin is left for this in the design.
Where Is It Used?
SMPS feedback lines, relay/triac triggering, industrial I/O interfaces, modem lines and between an MCU and high voltage.
Common Mistakes
- Not setting the LED current: the input LED must be limited with a series resistor.
- Ignoring CTR drop: CTR decreases with aging; a margin must be left in the design.
- Speed expectation: a standard optocoupler may not suit high-speed data.
Frequently Asked Questions
What's the difference between an optocoupler and a relay?+
Is there a digital optocoupler?+
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