4N35 Optocoupler

The set 4N35, 4N36, 4N37 are popular and versatile optocouplers for isolation of circuit blocks.
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nThe most important feature is the Isolation test voltage 5000 VRMS that means 5KV. But PCB Design is very important to get this isolation for your products too. The PCB area under the
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nThat way Air Insulation makes Isolation good. The Line of Isolation has to be observed in all SMPS under Magnetics and Optos. Creepage of HV can occur due to Grime and Humidity on PCB, use RTV Silicone Rubber or Epoxy impregnation to make your product Industrial or Military Grade. – delabs design Notes
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n4N35 Optocoupler Vishay
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nOther Features – Interfaces with common logic families, Input-output coupling capacitance < 0.5 pF, the low level capacitance means you can switch this opto at high speeds.
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nOpto-Isolaters are used in SMPS to feedback sample of output to control circuits.  In Industrial Automation Boards they are built into Output Modules and most I/O Modules. Isolation is related to Product Safety and Reliability. It protects circuits and systems from real world high-energy exposures.