Isolation barrier and output drive
The 4N26SM is a single-channel optocoupler from onsemi with a DC input and a phototransistor output that brings the base terminal out. The typical forward voltage of 1.18V and maximum forward current of 60mA define the input-side resistor and LED current budget.
Switching speed and saturation
Turn-on and turn-off times are both 2µs typical, which limits the usable signalling rate to roughly 250 kbps before pulse-width distortion becomes significant. The 500mV maximum Vce saturation at the output ensures low voltage drop when the transistor is on, keeping power dissipation in the output stage manageable. Compared to the 4N35SM, which specifies a 300mV Vce saturation, the 4N26SM has a higher saturation ceiling — a consideration when driving loads near the 30V output maximum where headroom is tighter.
Package, temperature range, and compliance
The 4N26SM is supplied in a tube of surface-mount 6-SMD gull-wing packages.