What the MOC211M isolation and CTR mean for your BOM
The MOC211M is a single-channel optoisolator from onsemi with a 2500Vrms isolation rating, enough for basic galvanic isolation in industrial controls, PLC I/O modules, and power-supply feedback loops where the primary-to-secondary voltage is under 250 V peak. Its current transfer ratio (CTR) is guaranteed at a minimum of 20% when the LED is driven with 10 mA forward current — that means for every 10 mA into the LED, the output transistor can sink at least 2 mA at the collector, which sets the pull-up resistor value for a clean logic-level swing. The output is a phototransistor with the base brought out to a pin, giving you the option to add a base-emitter resistor for faster turn-off or to bias the transistor for improved linearity in analog isolation applications.
Switching speed and output drive capability
Typical turn-on time is 7.5 µs and turn-off is 5.7 µs, with rise/fall times of 3.2 µs and 4.7 µs respectively — this part is suited for 10–100 kHz switching signals, not for high-speed data links. The output transistor can handle up to 30 V at the collector and sink 150 mA continuous, which is enough to drive a small relay coil, a logic input, or a low-power LED indicator directly without a buffer transistor.