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The 4N35 passes a signal from one circuit to another using light: current through its input infrared LED activates an output-side silicon NPN phototransistor. The circuits have no direct conductive connection, but “nothing crosses” is not literal—the light carries the signal, and Vishay specifies input-to-output coupling capacitance below 0.5 pF.
How does a 4N35 transfer a signal?
A 4N35 is a six-pin optocoupler. Vishay describes its two functional elements as a gallium arsenide infrared LED and a silicon NPN phototransistor. Current through the LED produces light; the phototransistor responds and provides an output current path. The signal therefore crosses optically rather than through a shared conductive path. Vishay’s datasheet describes the device construction.
This separation is called galvanic isolation. It does not mean there is no coupling of any kind: Vishay lists input-output coupling capacitance below 0.5 pF on its 4N35 product page. Nor does the device guarantee a fixed output current for every input. Current transfer depends on operating conditions; consult the datasheet’s specified current-transfer ratio and test conditions when designing a circuit.
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What is the Vishay 4N35 pinout?
For Vishay’s six-pin DIP package, viewed according to the package orientation in its datasheet drawing, the pins are:
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| Pin | Function |
|---|---|
| 1 | Anode (LED) |
| 2 | Cathode (LED) |
| 3 | No connection (NC) |
| 4 | Emitter (phototransistor) |
| 5 | Collector (phototransistor) |
| 6 | Base (phototransistor) |
The base is brought out as a separate terminal, so the output transistor is not limited to a two-terminal collector-emitter connection. Check the drawing for the exact manufacturer and package before wiring; do not assume every part carrying the 4N35 number has an identical document or specification.
What do the 4N35 isolation and electrical ratings mean?
Isolation figures are manufacturer-specific and must be read in context. Vishay lists an isolation test voltage of 5000 VRMS for its 4N35 family information. Broadcom’s 4N35 datasheet, dated April 14, 2023, specifies 3550 Vrms. These are not interchangeable universal ratings for every 4N35. See Vishay’s product information, its Rev. 1.2 datasheet dated January 7, 2010, and Broadcom’s datasheet.
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An isolation test-voltage value is not, by itself, a safe continuous working-voltage rating for a finished circuit. It also does not establish that a mains-connected design complies with applicable safety requirements. A design must account for the exact device, required approvals, board layout, clearance and creepage, and the relevant circuit conditions.
Vishay’s Rev. 1.2 datasheet gives these absolute maximum ratings:
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- LED forward current: 50 mA maximum.
- Collector-emitter breakdown voltage: 70 V.
- Phototransistor collector current: 50 mA maximum.
Absolute maximum ratings are limits, not recommended operating targets. Verify ratings and operating conditions against the datasheet for the exact manufacturer and part being used.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where is a 4N35 used?
Vishay lists AC mains detection, reed-relay driving, switching-mode power-supply feedback, telephone ring detection, logic-ground isolation, and logic coupling with high-frequency noise rejection as applications. These are manufacturer-stated use cases, not automatic endorsements for every circuit. Suitability depends on the circuit’s electrical, timing, and safety requirements.
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What should you check when choosing or wiring one?
- Manufacturer and package: Match the pinout and specifications to the exact datasheet and package drawing.
- Input drive: Choose LED current within the device’s specified operating conditions; do not treat an absolute maximum as a normal target.
- Output transfer: Check current-transfer ratio under conditions relevant to your input current and output circuit.
- Timing: Check switching response if the application depends on signal speed. Broadcom lists a typical rise time of 3 μs under stated conditions; that figure should not be compared with another device unless test conditions match.
- Isolation requirements: Confirm the required approvals and working-voltage, clearance, and creepage requirements for the complete design. A test-voltage number alone is not enough.
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