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So be I recently asked a question about using a PCB design for a fusebox (link here: PCB design for automotive fusebox?) but since recently come across two other fuse alternatives: PPTCs and automotive circuit breakers.

As the current fusebox design goes, our max ratings range from 3A to 30A. Would a design using either PPTC or automotive circuit breakers be feasible?

As I understand for PPTCs the operating temperature would be required (not sure of top of my head but I assume it's below 85°C. Can update when I get the results back). Automotive circuit breakers at the moment seems the better option thus far. Any suggestions?

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  • \$\begingroup\$ Would a design using X or Y be feasible AND "any suggestions" is not really definitive as questions go. Clearly an automotive fuse box is probably definitive enough to imply a decent specification but unless you're in the automotive inductry there will be nuances and subtleties (known to those guys) that make any analysis by an ordinary EE pointless. Just my gut feeling. \$\endgroup\$ – Andy aka May 16 '15 at 18:46
  • \$\begingroup\$ @Andyaka I better point out this is in part for my fusebox design for my uni's formula sae team. We'd if all be it, stick with the standard fusebox design but checking to see whether PPTC or automotive circuit breakers won't be limiting us. This is a competition is about using creative ideas so the team would like it if I'd be able to come up with an alternative design. \$\endgroup\$ – Kristopher Rahim Afful-Brown May 16 '15 at 18:51
  • \$\begingroup\$ Yes I understand that but to design something you need to consider the environment it goes in. This means vibration levels, temperatures, humidities, shock levels, reliability expectations to name what has just come off my head. Do you know these things? \$\endgroup\$ – Andy aka May 16 '15 at 18:54
  • \$\begingroup\$ @Andyaka the only tests that need to be taken as of now is the temperature and humidity (although we don't have the equipment to measure the latter). Also as it's a comp, it only needs to survive that one day because it will be an on-going project for next year. As it's screwed into place on the chassis, shock and vibrations levels will be that of the car (I'd have to get to the chassis team about those values if you require them). \$\endgroup\$ – Kristopher Rahim Afful-Brown May 16 '15 at 18:59
  • \$\begingroup\$ I don't require them because I know I'm not qualified to answer whether PPTC or automotive circuit breakers are feasible. I'm pointing out that many people on this site will likely be wary of answering because of their lack of tech knowledge on automotive stuff. This probably kind of makes this question one which solicits opinion rather than good engineering answers and, of course opinions don't make good answers AND questions that solicit opinion don't make good questions. \$\endgroup\$ – Andy aka May 16 '15 at 19:03
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Would a design using either PPTC or automotive circuit breakers be feasible?

OK I'm going to answer and say "No", PTC resettable fuses just won't cut the mustard. I'm basing this on a search of Farnell. I narrowed down the search by telling Farnell's search engine to look for PTC fuses that were rated at 60V or more. From memory, 60V is a decent enough margin in automotive applications to allow electronics to survive i.e. a 60V rating gave something reliability in the automotive environment.

Once narrowed down, the largest fuse available was 3.75 amps and this barely eats into the rangle of 3A to 30A specified in the question. Given also that these types of fuse seem to hit the end stops at 85C I would also consider them to be "fragile" in these applications.

Passive aging, thermal shock and humidity aging is something else that makes them look unsuitable but I'm no expert here. This specification gave no indication of how they cope with vibration either and, I'm sure this is important. Also, on page 1 of the data sheet (normally where all the intentional lies occur), "Automotive" was not listed or implied as an application.

My search is not exhaustive so I would encourage you to use my methodology to see if something looks like it might be suitable.

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  • \$\begingroup\$ 60V is a pretty high voltage limit (as we only have a 12V supply, splitting between 5V and 12V on varying components) And I imagine I'd still need to do some testing to see exact current draw on the bigger components (current draw on the fan for example was very large). In regards to the vibrations that shouldn't be a factor as it's a one time design. But yh, I see a bit more research will be needed. \$\endgroup\$ – Kristopher Rahim Afful-Brown May 16 '15 at 19:46
  • \$\begingroup\$ Here is a document that sets 42V as a protective limit for automotive ti.com/lit/an/snva382a/snva382a.pdf Another document cites 100V: maximintegrated.com/en/app-notes/index.mvp/id/4240 \$\endgroup\$ – Andy aka May 16 '15 at 21:29
  • \$\begingroup\$ electronics.stackexchange.com/questions/32550/… is a SE.EE question that implies 42V is a minimum (so 60V seems OK to my way of thinking unless you believe the maxim document above (this document cites 100V for battery disconnect load dumps)) \$\endgroup\$ – Andy aka May 16 '15 at 21:32
  • \$\begingroup\$ cheers for those links, much appreciated. I can relay this onto the team and check whether it seems a likely alternative. in back-up we'd still have our old design we can fall on (though getting a ptc design to work would be do well for judging) \$\endgroup\$ – Kristopher Rahim Afful-Brown May 16 '15 at 21:39

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