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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?

schematic

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?

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unix
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    Please see this: http://electronics.stackexchange.com/questions/236469/obd-port-power-source/236477 What is your current consumption? Can you afford a resistor in series? – winny Jul 15 '16 at 16:37
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    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. – brhans Jul 15 '16 at 16:43
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    @winny The load is the ATtiny and a couple NPNs to switch relays; less than 20 mA. – unix Jul 15 '16 at 17:06
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    @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! – winny Jul 15 '16 at 17:21
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    @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 '16 at 21:21
  • Yes. 18-20 V. Keep it or move it after your resistor to save some peak current. – winny Jul 15 '16 at 21:44
  • The basic circuit for spike surpression begins with a choke. – Marko Buršič Jul 16 '16 at 06:53
  • I think you'll find this Maxim [AppNote 7084 - LOAD-DUMP PROTECTION FOR 24V AUTOMOTIVE APPLICATIONS](https://www.maximintegrated.com/en/design/technical-documents/app-notes/7/7084.html) handy. It gives a good overview of the topic with calculations. Watch out: some of the images in the article use `Vp` to denote peak voltage, but the equations refer to it as `Vs`, which can be confusing at first. – pfabri Jul 07 '21 at 10:57

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If you have a voltage peak the TVS will clamp it by draining current. Since you don`t have a resistor your diode most likely will blow. Find the current that you need and calculate the resistance and power for your circuit. Check the maximum clamp voltage of you TVS. It should be lower than your regulator. A couple of capacitors may help to filter some noise from alternator and spark plugs. They will work as a RC low pass filter.

Gerson Bystronski
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Simply use a bidirectional TVS diode instead of unidirectional and place the reverse polarity protection diode after the TVS, your issue will be solved.TVS diode selection will be depend upon your voltage input for the LDO.

manu
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