Protecting Low-Voltage Microcontrollers
An optocoupler (photocoupler) integrates an infrared LED and a phototransistor in a single package. Because the connection is optical, there is no electrical path between input and output, protecting delicate 3.3V processors from 400V ground spikes.
Key Selection Parameter: Current Transfer Ratio (CTR)
CTR is the ratio of output collector current to input forward LED current, expressed as a percentage:
CTR = (Ic / If) × 100%
CTR degrades over the component’s operational lifespan and drops significantly at temperature extremes. Engineers must design circuits assuming a 30-50% CTR margin over 10 years of continuous operation.