Automotive IO input protection design for ESD, transients, and reverse polarity compatible with ISO 7637

Question

I want to accept an signal that could have any of the transients defined in ISO 7637-3 on it. It is a reset signal that could be tied to 24V in the case of a wiring harness meltdown but typically should be either floating or ground.

I was thinking of using the circuit below but I fear it may not be robust enough. (The arrow points to where the transients can couple )

^{simulate this circuit – Schematic created using CircuitLab}

So yeah, if this is exposed to ESD, +-24V (for 60s) or any automotive IO transients it needs to survive. What should I add/change?

Answer 1

I would lose the TVS and add a diode from the base of Q1 to ground to prevent damaging Q1 (the TVS does not protect Q1). I don't see the point in D1.. The voltage is clamped at +0.6 so R2 cannot inject current into the supply rail. R2 needs to be rated for 1W, and to withstand the transient pulses, so preferably a surge resistant type.

Evaluate the transistor to ensure it can withstand 200mA base transients without damage.