Single-channel phototransistor optocoupler for isolated signal transfer
The Vishay VOS618A-2X001T is a single-channel DC-input phototransistor optocoupler in a 4-SSOP surface-mount package, rated for 3750Vrms isolation between input and output. It is designed for basic signal isolation in industrial controls, power-supply feedback loops, and digital-logic interfaces where galvanic separation is needed without high-speed switching. Key parametrics include a current transfer ratio (CTR) range of 63% to 125% at a forward current of 1 mA, a maximum output voltage of 80 V, and a continuous output current of 50 mA per channel. The forward voltage is typically 1.1 V, and the maximum DC forward current is 50 mA. Switching times are moderate — typical rise/fall times of 5 µs and 7 µs, and turn-on/turn-off times of 5 µs and 8 µs — so it fits low-speed isolation (DC level sensing, status feedback, relay drivers) rather than high-frequency data links.
3750Vrms isolation — what it buys you in the BOM
The 3750Vrms isolation rating is a standard reinforced single-channel level for mains-isolated circuits in appliance, industrial, and medical equipment that require basic operator protection or functional isolation. It is not a medical-grade (5 kV+) or reinforced-for-mains (5 kV–6 kV) rating, but it covers 240 VAC secondary-side isolation with margin. For designs that only need functional isolation between logic domains, this rating is sufficient and keeps the package small.
CTR range at 1 mA — design margin for LED aging
The specified CTR of 63% to 125% at 1 mA forward current is the key parameter for ensuring the output transistor saturates reliably over temperature and LED degradation. A minimum 63% means the designer can size the input resistor to guarantee a logic-level output even when the LED has aged and its light output has dropped. The 125% maximum sets the upper bound for the pull-up resistor value to avoid excessive output saturation voltage. This CTR band is tighter than many general-purpose optocouplers, which helps reduce variation across production lots.
Package and footprint — 4-SSOP on 4-SOIC land pattern
The VOS618A-2X001T is supplied in a 4-SSOP package that is footprint-compatible with the standard 4-SOIC (0.173" body width, 4.40 mm) land pattern. This is a common footprint shared by many single-channel optocouplers, so board layout is straightforward and second-sourcing is possible without a PCB change.