4170Vrms isolation in a hand-solderable SMD package
The onsemi 4N28SVM is a single-channel optoisolator in a 6-SMD gull wing package — a footprint that hand-solders cleanly with a fine iron tip, no hot air required. The 4170Vrms isolation rating means it cleanly separates a 24 VDC PLC input from a 5 V logic side, with enough margin to survive repetitive 1 kV transients on the field wiring without breakdown.
CTR and switching speed — what they mean for the BOM
Minimum current transfer ratio is 10% at 10 mA forward current — so with 10 mA into the LED, the output transistor can sink at least 1 mA. That is enough to pull a 5 V logic input low through a 4.7 kΩ pull-up, but leaves little margin if the LED current droops or the phototransistor ages. The 2 µs typical turn-on and turn-off times suit relay-coil sensing, limit-switch readback, and moderate-speed digital isolation up to about 100 kHz.
onsemi has marked the 4N28SVM as obsolete. Sourcing is through independent distribution — quantities are lot-specific and confirmed at RFQ. The 6-SMD gull wing package and 4170Vrms isolation are the primary fit criteria; any replacement candidate must match these two parameters to drop into the same PCB land pattern and safety spacing.