# Reverse Polarity & Load Dump Protection (TVS)

Based on this and other material I’ve read, I should have a diode for reverse polarity protection and a TVS for load-dump protection in an automotive circuit. I have prototyped the circuit and its associated load (an ATtiny85 at NODE1) without the diodes and everything works as desired, but I want to add appropriate protection. I would like the circuit to be compatible with both 12V (~14V) and 24V (~28V) automobiles with replacement of the TVS. I believe D2 could be one of any number of diodes like the 1N4148 shown or a 1N400x series diode. Because I don’t want any voltage leaking past D1 under normal conditions, I think a P6KE18A (VR = 15.3V) would work for a 12V system and a P6KE36A (VR = 30.8V) for a 24V system.

Do those diodes seem reasonable? Is there a better approach or different values I should use?
With 28V approaching the 30V max input of the LP2950, should I also consider something else to lower the input voltage of the 2950?

simulate this circuit – Schematic created using CircuitLab

How to select power line polarity protection diodes http://www.eetimes.com/document.asp?doc_id=1279734

Transient Voltage Suppressors (TVS) for Automotive Electronic Protection http://www.vishay.com/docs/88490/tvs.pdf

Fairchild TVS Data Sheet http://www.farnell.com/datasheets/1725637.pdf?_ga=1.259177292.759527797.1468595075

How do I protect against an automotive load dump? How do I protect against an automotive load dump?

• Please see this: electronics.stackexchange.com/questions/236469/… What is your current consumption? Can you afford a resistor in series? Jul 15, 2016 at 16:37
• Your circuit might not protect you from a load dump - your D1 is likely to be severely over-stressed. See Fig.8 on Pg4 of that Vishay TVS doc. Jul 15, 2016 at 16:43
• @winny The load is the ATtiny and a couple NPNs to switch relays; less than 20 mA.
– unix
Jul 15, 2016 at 17:06
• @unix Then you are in luck! A few ohms, say 47 first and then a good zener diode and you are done. Another diode in series if you need to save it from reverse polarity as well. Done! Jul 15, 2016 at 17:21
• @winny I'm afraid I don't follow. Are you saying a 47Ω resistor and zener in series in place of the TVS? If so, what is a "good" zener value? D2 remains as is?
– unix
Jul 15, 2016 at 21:21