The onsemi 4N35W is officially obsolete per the manufacturer's lifecycle status. No direct pin-compatible replacement is listed by onsemi.
What the 5300Vrms isolation means on a real line
5300Vrms isolation between input and output — this is a reinforced-grade rating for a DIP-6 optocoupler. It means the part can withstand a 5.3 kV transient between the control side and the load side without breakdown, which covers most industrial motor-drive and power-supply applications where the primary-secondary barrier must hold off surge voltages per IEC 60950-1 reinforced insulation requirements. The 100% minimum current transfer ratio at 10 mA forward current means the output transistor is guaranteed to saturate when the LED is driven with 10 mA, even at the low end of the CTR distribution. This is the figure that decides whether the downstream logic sees a clean logic-low — if the CTR drops below 100%, the output may not pull low enough to register as a '0' on a 5 V logic input.
Switching speed and temperature range for field service
2 µs turn-on and 2 µs turn-off typical — this is a general-purpose optocoupler, not a high-speed digital isolator. It handles 115/230 VAC zero-crossing detection, relay driver feedback, and DC bus voltage sensing at line frequencies, but it will not pass a 1 MHz PWM signal cleanly. The 2 µs propagation delay limits the data rate to roughly 250 kbps in most circuits. Rated for -55°C to 100°C operating temperature — the full industrial range with margin on both ends. In a field-service context, this means the part can sit in an unheated panel in a Minnesota winter and still switch correctly when the system powers up. The 100°C upper limit covers most enclosed power-supply compartments, though derating the LED drive current above 85°C is good practice.
Package and board-fit — DIP-6 through-hole
6-DIP package, 0.400-inch (10.16 mm) body width — the standard wide-body DIP footprint. Through-hole mounting means it can be socketed or soldered; in a field-service kit, a DIP socket is the preferred option so the optocoupler can be swapped without desoldering. The tube packaging is standard for DIP devices — 50 parts per tube typically, though the exact count varies by factory fill.