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I have been digesting input power protection circuits for automotive use for several days now. There are a lot of snippets here and there with either complete info (no parts) or incomplete circuits (little info). I'm trying to piece it all together into an actual circuit design. I'm a fair noob when it comes to circuit design, and it's been a long, long time since my college electrical engineering classes.

The RBO40 seems a great place to start things around. It provides over-voltage, reverse polarity, and pulse protection. The application notes for the device don't seem to indicate that any additional devices are required. Unfortunately, it's not entirely clear from the PDF how true or not that is.

My particular constraints:

  • I need an actual +12V
  • I also need +5V
  • I'd like to protect from stupidity and nasty conditions
  • I'm trying to deal with JLCPCB basic parts to avoid extended part charges wherever possible. Unfortunately, the dollar signs are adding up fast. Total cost isn't a super important factor, but $40 worth of extended part charges would be annoying.
  • No automotive-grade/range TVS, Zeners, or varistors are basic JLCPCB parts.
  • I can hand-solder things myself, but tiny parts will likely be a challenge.
  • This first design is a prototype, but I might want to produce the items small-batch later. Having to hand-solder a dozen RBO40 is not so bad. However, having to hand-solder five parts a dozen times gets worse.

This document from ST goes into even more gory detail about the various tests, including some indications of +/-100V pulses.

As best I can tell, the RBO40 is sufficient all by itself to protect a +12V supply. However, we know that the car battery at rest is potentially (pun intended?) more than +12V, and the alternator usually generates +13.8V for charging. In my case, I have some LEDs and circuitry that are designed around actual +12V. So I assume that the RBO40 will pass +13.8V through if that's what it gets in (up to ~25V).

Here is a picture of a design I'm noodling on:

rbo40-based automotive input protection circuit

This is a schematic for a popular engine control module (ECM) - the Megasquirt. There are many, many thousands of these out in the market, and they are not known for frying. They're used in cars, boats, and all kinds of wacky projects. Unfortunately, their input circuit has $9 in extended part charges alone (Zeners and the varistor).

Thoughts and questions:

  • The RBO40 provides general input protection, reverse polarity, and ESD/pulse protection, right?
  • The LM2940 can tolerate up to 26V, which is higher than the 25V max output of the RBO40, right? Is any additional protection (like a Zener or TVS) needed on the output of the RBO40? This thread talks about it, but there is not (in my mind) a definitive answer.
  • The entire current project on my bench is pulling less than 200mA, so both of these 1A-capable LDOs should be sufficient, right?
  • How do my capacitors on inputs and outputs of the regulators look? I don't think I need the 100 + 300 in parallel on the output/input of the +12/+5 regulator - that just gives me 400nF total (although the indicator LED is in between right now...)
  • Is any kind of output fusing off the RBO40 warranted? It's possible that some kind of short could develop due to damage or conditions inside the rest of the device/circuitry, so perhaps some kind of electronic/resettable fuse?
  • Similarly, is any kind of input fusing into the RBO40 warranted? Again, some kind of electronic/resettable fuse?

Sorry for all of the questions. I'd like to try to get this right the first time.

Other background threads/information:

Automobile Project (Electrical Parts Requirement) https://www.reddit.com/r/AskElectronics/comments/3u0o3r/help_with_automotive_circuit_protection_for_small/ Automotive Load Dump Protection Circuit TVS Diode Selection for Automotive In terms of protection, how does this automotive power supply circuit look? Reverse Polarity & Load Dump Protection (TVS) Need help sizing Zener diodes and varistor for 12V automotive power supply protection (also references Megasquirt)

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    \$\begingroup\$ There are quite a few questions here, which should be split into multiple question posts. You don't necessarily have to post a question for each individual question -- a few highly related questions can be posted together -- but there are too many here. I'd suggest posting the question(s) about the RBO40 in one post, the question(s) about the regulators in another post, etc. You can link from one question post to another in order to avoid giving the full background on each question. \$\endgroup\$
    – Null
    Feb 10 at 15:13

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  1. Yes the protection part is compliant to automotive std. 7637
  2. Not 25Vmax? RBO40 is rated for 40V clamp max and 35V for T1,T2 @ 1mA .
  3. You may want to find an alternate with low Vio dropout (0.5V) rated for 40V Define load for each V to improve chances as 5V could be 40V input at lower current.
  4. 1 cap is fine, use recommended. But LED current of 20 mA make R9 dump almost 200 mW burning hot. Reduce to 10 mA.
  5. I presume source is fused to this board, but you could add a PTC for faults in proto-builds.

TY @jean for the reference that describes both ISO 10605 ESD and the ISO 7637 used by official automotive electrical standards groups.

a. ISO 10605: standard for “Electrostatic discharges” due to human body discharging inside a vehicle applied to a complete system
b. ISO 7637 standard for “Electrical disturbances from conduction and coupling

All the hazards indicated above are described by several standards bodies such as the Society of Automobile Engineers (SAE), the Automotive Electronic Council (AEC) and the International Standard Organization (ISO).

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  • \$\begingroup\$ 2. I thought that was accurate (40V clamp) but I was not totally sure. Which brings me to 3. Good point. The regulator I chose is already an extended part at JLCPCB so choosing a different 12V regulator is an option. Although I might need to go with an adjustable regulator to get 12V. 4. Good point. They were rated at 26mA but 10mA would also be fine. They're just indicators. 5. The end-user should inline a fuse on the supply to the board, but people do dumb things. On-board resettable adds safety. Thanks! \$\endgroup\$ Feb 10 at 15:47
  • \$\begingroup\$ This application note (from ST as well) is more detailed concerning automotive surges : st.com/resource/en/application_note/… \$\endgroup\$
    – Jean
    Jul 18 at 13:51

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