The 4N28FM: Its primary job is to pass a DC signal across a galvanic isolation barrier while blocking ground loops and high-voltage transients. The 7500Vpk isolation voltage is the headline rating — this part creates a safety barrier rated for that peak voltage between input and output. In a motor drive or industrial PSU, that margin covers repetitive switching transients on the 400V bus without creeping into breakdown territory. Minimum current transfer ratio of 10% at 10mA forward current means the output transistor conducts at least 1mA collector current when the LED is driven at 10mA. That sets the minimum LED drive current needed to guarantee a logic-level output into a 10kΩ pull-up — budget 15-20mA forward current for margin across temperature and CTR degradation over life. Turn-on and turn-off times are both 2µs typical — this is a general-purpose isolator for low-frequency signals (DC to about 100kHz). It is not rated for high-speed data isolation; use a digital isolator or high-speed optocoupler for CAN, SPI, or Ethernet isolation.
Temperature range and package — board-fit for harsh environments
Operating temperature spans -55°C to 100°C, covering the full industrial and military temperature band. The 100°C upper limit is the junction temperature — in a sealed enclosure with 85°C ambient, the self-heating from the LED and output transistor adds about 10-15°C, keeping the junction below the 100°C ceiling with margin. The 6-SMD Gull Wing package has a seated height of approximately 3.5mm and a 0.050-inch (1.27mm) lead pitch. The Gull Wing footprint is standard for this class — the same pad layout serves the 4N25, 4N35, and 4N37 families, so a single PCB footprint covers multiple second-source options. Supplied in Tube packaging — 50 units per tube typical for this package. For volume production, confirm the tube quantity matches the pick-and-place feeder setup; the tube orientation (pin 1 at the tube stop) is standard for 6-SMD Gull Wing parts.
Lifecycle reality — obsolete, no direct successor
The 4N28FM is marked Obsolete by onsemi. For existing BOM lines, the functional replacement is a different part number — typically a 4N25, 4N35, or 4N37 variant in the same 6-SMD Gull Wing package, but with different CTR and Vce ratings. A board spin is not required, but the CTR bin and Vce(sat) must be re-verified against the circuit's minimum collector current requirement. Sourced through independent surplus and excess-inventory channels. Availability is lot-specific — date-code spread and quantity confirmed at RFQ.