Optocoupler
An optocoupler (also called an optoisolator) transfers a signal between two electrically isolated circuits using light. An LED on the input side shines onto a phototransistor on the output side. Used to isolate high-voltage circuits from low-voltage control logic and to eliminate ground loops.
Properties
| Property | Description | Default | Range |
|---|---|---|---|
| CTR | Current Transfer Ratio — output collector current / input LED current (%) | 100 % | 10 % – 500 % |
| Forward voltage | Input LED forward voltage (V) | 1.2 V | 0.9 V – 2.0 V |
| Max input current | Maximum current through the input LED before failure (mA) | 60 mA | 5 mA – 100 mA |
| Isolation voltage | Maximum voltage between input and output sides (V) | 3 750 V | 500 V – 10 000 V |
Simulation behavior
When forward current flows through the input LED, the output phototransistor turns on. Output collector current = CTR / 100 × input LED current. The two sides of the circuit are completely isolated — no direct electrical connection exists between them.
If input current exceeds max input current, the input LED fails open and the output goes dark. If the voltage across the isolation barrier exceeds isolation voltage, the component fails and the isolation is broken.
Tips
- Add a current-limiting resistor in series with the input LED: R = (V_supply – V_forward) / I_desired.
- Higher CTR means more output current per unit of input current — useful when the phototransistor must drive a downstream load directly without a buffer.
- Use optocouplers whenever your circuit mixes mains voltage (230 V / 120 V AC) with microcontroller logic — the isolation voltage keeps the low-voltage side safe.