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Hello planning to use an optocoupler to interface a button meters away from the circuit. The optocoupler ILx series already comes with Common Mode Transient Immunity which would work great for my application. Driving the input of the optocoupler is a Constant current IC BCR401 that is set to 10mA from a separate power supply.

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We already know that opto has built in immunity to transient, is my decision of omitting the TVS diode on the twisted pair (place near the optos input cathode) as it would be redundant with the built in one correct?

The BCR401 datasheet has no mention of any form of immunity against transients. Would it be protected by the opto's protection circuitry too? If it need its own protection it would most probably some sort of a TVS diode but i could not figure out where its anode and cathode would be connected because the induction would come from the twisted pair so it has to go to VDD making it not needed ??

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    \$\begingroup\$ Use any signal diode for reverse LED protection from ESD and V= LdI/dt \$\endgroup\$ – Tony Stewart EE75 Jul 8 '20 at 6:29
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 I get that this is to protect the diode from being reversed voltage due to long wire inductance, but im a bit confused about the direction. the moment the switch opens because the wire has inductance current cant suddenly be stop so a large voltage happens at the button(?) but the current direction is still to ground(?). As for ESD, can an ESD also create current on the opposite direction? wont i have more problem then because the constant current driver and the power supply is being reversed voltaged forcefully? \$\endgroup\$ – Jake quin Jul 8 '20 at 6:57
  • \$\begingroup\$ ESD can be either positive or negative. ESD does not have enough energy to damage a power supply (usually, it's eaten by the decoupling capacitors). \$\endgroup\$ – CL. Jul 8 '20 at 6:57
  • \$\begingroup\$ @CL. The optos data sheet doesnt show the complete schematics of the internals, In that case ommitting the TVS at the start of the TP is a bad idea since now the ESD can go through the whole system. \$\endgroup\$ – Jake quin Jul 8 '20 at 7:01
  • \$\begingroup\$ Analog optocoupler indeed have nothing else inside. And where excatly would you want to place the TVS? \$\endgroup\$ – CL. Jul 8 '20 at 7:08
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From the optocoupler's point of view, common-mode transients happen when the voltages at the inputs change relative to the voltages at the outputs, but when the voltage between the A and K inputs stays the same. In this circuit, this implies that the voltage between VDDiso and GNDiso does not change.

In other words: the LED driving circuit is not affected by common-mode noise.

If you want to protect your circuit against noise, you have to care about differental-mode noise (most likely due to ESD). Optocoupler LEDs have a very bad reverse voltage rating, so you should add an antiparallel diode. And a low-pass filter (as simple as a capacitor) will prevent the optocoupler falsely switching on or off due to short spikes.

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  • \$\begingroup\$ Is protection against common mode noise a feature that natural to an optocoupler (because that opto seems to place it as an highlight). This would now be the schematics, please have a look. The tvs would have to be rated to clamp at 40v since that is the max voltage the driver can handle. \$\endgroup\$ – Jake quin Jul 8 '20 at 7:18
  • \$\begingroup\$ There is already a switch debouncer that handles debouncing and validating a button press on the optos output so i might not need the low-pass filter (unless it has other purpose i might not be aware of) \$\endgroup\$ – Jake quin Jul 8 '20 at 7:25
  • \$\begingroup\$ All optocouplers (and other isolators) protect against common-mode voltages; that is their entire purpose. \$\endgroup\$ – CL. Jul 8 '20 at 7:43
  • \$\begingroup\$ The BCR401 has a lower rating for reverse voltages. Anyway, ESD and transients are different things; does your environment generate the latter? \$\endgroup\$ – CL. Jul 8 '20 at 7:48
  • \$\begingroup\$ it will be in a residential environment (my house to be specific) and the inductive power wires(only thing i could think of that will generate transients) are going to be far away(feets away). and lightning strikes(maybe twice a year for a relatively close one) which nowhere in my home is safe \$\endgroup\$ – Jake quin Jul 8 '20 at 8:00

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