Obsolete — sourcing through surplus channels
onsemi lists the 4N25FM as Obsolete.
7500Vpk isolation — what it means for the BOM
The 4N25FM is rated for 7500Vpk isolation between input and output. That is a high-voltage barrier suited for separating control-side logic from a high-voltage DC bus or mains-referenced circuit — think motor drive feedback, power-supply error amplifiers, or medical patient-interface isolation where reinforced insulation is required. The minimum current transfer ratio is 20% at 10 mA forward current. Switching times are 2 µs turn-on and 2 µs turn-off typical. That puts the usable data rate around 250 kbit/s — fine for status feedback, relay replacement, or isolated I/O, but not for high-speed serial buses.
Package and temperature grade
Housed in a 6-SMD gull wing package, the 4N25FM is a surface-mount variant of the industry-standard 4N25 DIP. The supplier device package is 6-SMD.