Use this electronics calculation for a first-pass component, converter, signal, motor, or sensor estimate when the stated operating conditions and units match the device data.

Collector current (mA)

Current flowing through the output transistor collector.

LED forward current (mA)

Forward current through the input LED.

Compute the current transfer ratio of an optocoupler from LED forward current and collector current. The tool evaluates the supplied inputs together and returns the named outputs below; it does not infer omitted operating conditions or change the units shown.^[1]

Current transfer ratio (%)

Ratio of collector current to LED current as a percentage.

• Optocoupler current-transfer ratio varies with input current, temperature, aging, device spread, collector voltage, and switching speed. Isolation voltage, creepage, clearance, transient immunity, and safety approvals cannot be inferred from CTR.
• Use finite inputs in the displayed units and preserve more precision than the final presentation requires. Independently verify safety-critical, financial, compliance, or production decisions.

Check the result against the current datasheet and worst-case operating corners, then verify the circuit or measurement with simulation and bench testing where failure matters.

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