4N35

The 4N35 is the classic six-pin phototransistor optocoupler: a gallium arsenide infrared emitting diode optically coupled to a silicon NPN phototransistor inside an industry-standard dual-in-line package, with the transistor base brought out on pin 6. The pinout is 1 anode, 2 cathode, 3 not connected, 4 emitter, 5 collector, 6 base. That base lead is the reason the 4N35 survives alongside four-pin parts such as the PC817: it lets the designer trade gain for speed, suppress leakage in hot environments, and even bias the output for quasi-linear operation.

Vishay's datasheet (document 81181) specifies a 5000 Vrms isolation test voltage, at least 7 mm of creepage and clearance with 0.4 mm minimum isolation thickness, a comparative tracking index of 175, and input-to-output capacitance of 0.6 pF typical at 1 MHz. On the input side the LED takes 50 mA maximum continuous forward current, 1 A surge for pulses under 10 µs, and 70 mW dissipation, with a 1.3 V typical and 1.5 V maximum forward voltage at 10 mA. On the output side the guaranteed collector-emitter breakdown (BVCEO) is 30 V minimum at IC = 1 mA, collector current is 50 mA continuous and 100 mA for pulses under 1 ms, dark current is 50 nA maximum at VCE = 10 V, and power dissipation is 70 mW. DC current transfer ratio is 100 % minimum at VCE = 10 V and IF = 10 mA — a JEDEC registered value — but falls to 40 % minimum and 50 % typical across -55 °C to +100 °C, which is the number to design with. Switching time is 10 µs maximum for both turn-on and turn-off at VCC = 10 V, IC = 2 mA and RL = 100 Ω.

In practice the 4N35 is a DC-to-few-kilohertz device: the propagation-delay curves in the datasheet are given at 10 kHz, and turn-off stretches into the tens of microseconds with a large collector resistor because of the Miller effect through the collector-emitter capacitance and stored base charge. Leaving the base open gives the highest CTR but the slowest turn-off and the worst noise immunity; a 100 kΩ to 1 MΩ base-to-emitter resistor restores speed and stability at some loss of gain. For anything faster, move to a 6N137 or 6N138 logic-output optocoupler, or to a capacitive or magnetic digital isolator. Typical uses are AC mains presence detection, telephone ring detection, reed relay and solid-state switch driving, logic ground isolation, PLC digital inputs, and low-bandwidth supply feedback.

Sourcing is straightforward but isolation ratings are not uniform across brands. Vishay rates 5000 Vrms under UL file E52744 with BSI and FIMKO approvals; other manufacturers publish lower dielectric ratings for their own 4N35 equivalents, so if a safety standard governs your design, qualify the exact brand and its certificate rather than substituting on part number alone. Second sources include the onsemi 4N35M, the Lite-On LTV-4N35 and the Isocom ISO4N35. Counterfeit screening for optocouplers centers on electrical measurement: check CTR at IF = 10 mA (must exceed 100 %), dark current at VCE = 10 V (must stay under 50 nA), and run a hipot sample test. MOQ is one standard DIP-6 tube; Shenzhen stock ships in 1 to 2 days.