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:
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)