I have a connector which has the following lines:

  • 12V
  • 3.3V
  • GND
  • SDA (I2C)
  • SCL (I2C)

The connector might get wet with water and I would like to protect the circuit from the resulting over-voltage in case the 12V line gets connected to the other lines.

Based on this schematic from this question Why is a resistor needed in zener protection circuit?: zener protection circuit

I've designed this connector using a 3.6V zener diode and 330Ohm resistors. I don't understand why R1 and R2 are required and I've removed them in mine:

Proposed solution

The connection would look like this:

Connection example

Would this solution protect the connections from the 12V line? Is there a better approach?

  • \$\begingroup\$ Connector corrosion insulation will be the problem caused by water, not excessive conductance between conductors. I2C is open drain and cannot pull up on its own. \$\endgroup\$ Commented Aug 25, 2018 at 15:26
  • \$\begingroup\$ Maybe I asked the question wrong. I know for sure that if I connect the 12v line to the other lines the circuit blows up. I'm using an esp8266 at the other end of the circuit and I think it's not real i2c but simulated with the GPIO. My problem is that I need to protect my circuit somehow \$\endgroup\$
    – Blasco
    Commented Aug 25, 2018 at 16:51
  • 1
    \$\begingroup\$ Over voltage protection is resolved by either TVS or Sch. Diode to Vdd some current limit from source. A water-protected connector pair may be needed. \$\endgroup\$ Commented Aug 25, 2018 at 17:40
  • \$\begingroup\$ So is this zener diode not a good approach? could you give me more details about how solution with the Sch. diode would look like? I'm new to electronics. Thank you very much for all the information \$\endgroup\$
    – Blasco
    Commented Aug 25, 2018 at 18:37

1 Answer 1


Pull-ups are required on SDA and SCL to ensure that they are high when no signal is present. See the I2C details (try Wikipedia).

  • \$\begingroup\$ I see, but I don't need them for the 3.3v line right? And what do you think about the protection. Would it work? \$\endgroup\$
    – Blasco
    Commented Aug 25, 2018 at 14:01

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