The VOS618A-3T is a single-channel phototransistor optocoupler from Vishay in a compact 4-SSOP surface-mount package. It provides 3750 Vrms of galvanic isolation between a DC input LED and a phototransistor output, making it a standard choice for breaking ground loops, level-shifting logic signals, and providing safety isolation in industrial controls, power supplies, and PLC I/O modules.
CTR range — the spec that decides the drive margin
The current transfer ratio is specified at 100% minimum and 200% maximum, both measured at a forward current of 1 mA. That bin (VOS618A-3T is the '3' rank) means the output transistor can sink at least as much collector current as the LED forward current, and at most twice it. For a design driving a logic input or a low-side relay, this CTR range gives you a predictable on-state voltage drop — the Vce(sat) is 400 mV max — so you can size the input resistor with confidence.
Switching speed and output ratings
Rise and fall times are 5 µs and 7 µs typical; turn-on and turn-off times run 5 µs and 8 µs. That puts this part in the general-purpose optocoupler class — fine for 10 kHz PWM or 115.2 kbps serial isolation, but not for high-speed gate drives. The output transistor is rated for 80 V collector-emitter and 50 mA continuous current per channel, which covers most 24 V industrial logic and 5 V / 3.3 V level-shifting needs.
The 4-SSOP body (4.40 mm width) saves board area compared to the common DIP-4 optocoupler, but the pin spacing is tighter — verify creepage distance against your safety standard if you need reinforced isolation.