Isolation and signal transfer for legacy designs
It provides 5300Vrms of galvanic isolation between the DC input LED and the output side, which is rated for up to 30V on the collector-emitter path. With a minimum current transfer ratio of 20% at 10mA forward current, the 4N26SD can switch logic-level signals or drive a downstream transistor stage across the isolation barrier. Typical turn-on and turn-off times are 2µs each, making it suitable for moderate-speed digital isolation up to about 250 kHz. The operating temperature range of -55°C to 100°C covers industrial and some military-grade environments, though the part is now classified as obsolete by onsemi.
Package and surface-mount integration
Housed in a 6-SMD gull wing package (supplier device package 6-SMD), the 4N26SD is intended for surface-mount assembly. The gull-wing leads provide a visual solder-joint inspection point and moderate standoff for cleaning under the body. The base product number is 4N26, and the 'SD' suffix indicates the SMD gull-wing variant.
Lifecycle reality for the BOM line
Onsemi lists the 4N26SD as obsolete. No official successor part number is provided in the product record. For existing designs that require this exact package and pinout, the only sourcing path is through independent distribution or surplus channels. The forward LED can handle up to 100 mA DC continuous, and the output transistor saturates at 500mV max at rated current — typical for driving a 5V logic input or a small relay coil with an external base resistor.