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An optocoupler (opto-isolator) transfers a signal across an isolation barrier using light: an input LED shines onto an output phototransistor. The key spec is the Current Transfer Ratio (CTR) — the output collector current divided by the input LED current. A CTR of 50% means 10 mA into the LED produces up to 5 mA at the output.
| Quantity | Formula |
|---|---|
| LED resistor | RLED = (Vin − VF)/IF |
| Available output current | IC = CTR × IF |
| Current to saturate output | IC(needed) = (VCC − VCE(sat))/Rpull-up |
| Required LED current | IF = margin × IC(needed)/CTR |
CTR falls with age and temperature and varies widely between parts, so always design with a margin (2× or more) so the output still saturates over the product\'s life.
A device that transfers an electrical signal between two isolated circuits using light — an internal LED drives a phototransistor, with no electrical connection between input and output.
The ratio of output collector current to input LED current, usually a percentage. CTR=50% means 10 mA into the LED gives up to 5 mA out.
It determines how much LED current you need for a given output current. Designing without margin can fail as CTR drops with age and temperature.
The LED's light output falls with age and heat, so the same LED current produces less output current. Plan for 20–50% CTR loss over the product life.
RLED = (Vin − VF)/IF, using the LED forward voltage (~1.2 V) and your chosen forward current. Round down to the nearest standard value.
The available output current (CTR×IF) must exceed the current needed to pull the output down through its load/pull-up. Use the saturation tab and a 2× margin.
About 1.2 V for the infrared LED in common couplers like the PC817 and 4N35, at a few milliamps. Check the datasheet at your current.
A larger pull-up needs less output (and LED) current but gives slower edges; a smaller one is faster but needs more drive. 4.7 k–10 k is common for logic-speed signals.
Standard phototransistor couplers handle a few kHz to tens of kHz. For fast data use high-speed logic-output or IC optocouplers rated in Mbps.
Commonly 2.5–5 kV between input and output, letting them safely separate mains-side and low-voltage circuits. Check the datasheet for your safety requirement.
Only small loads — the output transistor handles a few tens of mA. For larger loads, use the opto output to drive a transistor, MOSFET, or relay.
Both isolate, but an optocoupler is solid-state, fast, and low-current; a relay handles high current and switches AC or DC but is slower and mechanical.
Usually a pull-up (or load) resistor sets the output logic level. The phototransistor pulls the node low when the LED is on and the pull-up returns it high when off.
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