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I have a microcontroller monitoring a few digital inputs. The max voltage on these inputs is VCC + 0.3v. VCC is 3.6v so these inputs can't be higher than 3.9v. These digital input are just switch closures on a button, toggle, or any other various human operated switch. Input response time is definitely not critical. The switch will be connected to the uC via a length of cable. Cable length will vary based on customer's needs; anywhere from a few feet to 1,000 feet. This will be industrial application so lightning is the main concern.

I wanted to use capacitor --> series resistor --> TVS --> series resistor --> capacitor --> uC input. However, I understand TVS diodes will give fastest response, zeners will be too slow as I understand. The problem is, all TVS diodes that have a working voltage of 3.65-3.8v have a breakdown voltage and clamping voltage way too high. I see some breakdown ratings of 3.8, but clamping is upwards of 6-15v! I'm assuming my input will be fried before the VTS reaches full clamp.

With such low input Vmax, am I limited to zener/schottky diode protection?

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  • \$\begingroup\$ What is your microprocessor? \$\endgroup\$
    – Voltage Spike
    Oct 8, 2019 at 3:52
  • \$\begingroup\$ Usually one can use diodes to VCC and GND resepectively. Though they might already be implemented in your MCU. They can absorb some energy though might need a current limiting resistor.I would recommend to look for application notes on that topic. ESD, EMC, Protection of HMI interface or similar. \$\endgroup\$
    – stowoda
    Oct 8, 2019 at 4:31
  • \$\begingroup\$ ESD events are in ns and HMI events with 10ms lag is ok, so a LPF will sufficiently attenuate any ESD voltage in 10 ms. electronics.stackexchange.com/questions/220795/… \$\endgroup\$ Oct 8, 2019 at 6:02
  • \$\begingroup\$ Its a AVR 128A. There are diodes in the chip. But these are not sufficient for industrial application from my reading \$\endgroup\$
    – James Pie
    Oct 8, 2019 at 14:03
  • \$\begingroup\$ I have a new question. The reason this VCC + 0.3 is stated is because any higher would cause too much current to flow through the input's protection diodes which are rated at 1ma. If use a series resistor on the input, this will allow a higher input voltage, correct? For example, a 3.3v MAX input with 1ma max current - if I have a 1K ohm resistor inline with this input, I could go up to 4.3v before reaching 1 ma max? So if I used a 10K, this would give me 43V before hitting my limit, which would allow me to use a TVS with higher clamp voltages, correct? \$\endgroup\$
    – James Pie
    Oct 8, 2019 at 21:57

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