Complete original post attached due to BCC to Gavin Automotive environment is severe. Search on "load dump" and "2 battery jumps". NatSemi and others make regulators specified for an auto environment. Many of NatSelis LM29xx regulators are specifically automotive rated. I've copied a comment from the LM2940 data sheet below. Datasheet here http://www.national.com/ds/LM/LM2940.pdf "Designed also for vehicular applications, the LM2940/ LM2940C and all regulated circuitry are protected from reverse battery installations or 2-battery jumps. During line transients, such as load dump when the input voltage can momentarily exceed the specified maximum operating voltage, the regulator will automatically shut down to protect both the internal circuits and the load. The LM2940/LM2940C cannot be harmed by temporary mirror-image insertion. Familiar regulator features such as short circuit ..." An alternator is highly inductive - sudden removal of heavy loads - whther due to a fuse blowing or just eg switching off headlights or similar, can lead to a VERY large voltage surge. Despite warnings people occasionally weld cars with battery leads on and earth return currents and side effects have been known to do "interesting things" to electronics. Note that the LM2940 is designed to shut down when transients exceed certai= n values. Temperatures in some locations can be extremely extreme. The Firewall is th= e classically referred to location, but you can probably find other unfriendl= y places. If you use 85 C rated parts you MUST specify locations where it is acceptable to install it. The Firewall is not one of those places :-). Unde= r the front passenger's carpet against the floor works quite well often, or against the side walls in the front passengers foot well. . Protection against an occasional a soaking in this location is mandatory - but it's a really really really good idea in any other location as well. If current drain is not too high a quite effective filter is two stages of zener diodes. Rseries, zener to ground, rseries, zener to ground. THEN add filtering :-). I'll BCC this to a friend who makes his living designing and selling commercial systems which work in cars - he may have things to say. Russell On 23 October 2010 10:39, Adam Field wrote: > I have a hobby project I've been developing that I want to turn into a > self assembly kit, eventually. So far google hasn't answered all my > questions about the hostility of an automobile environment. How stable > is the voltage coming off the alternator? How bad are the transients? > Are older cars worse? I assume you would see anywhere from 12-15VDC > normally depending on engine speed / alternator load, say nothing of > transients. If I was using a voltage regulator with a max 35VDC input, > is it safe to assume I would rarely see a condition to damage the > regulator? If not, what sort of input filtering should I use? Would a > "pi" http://en.wikipedia.org/wiki/Capacitor-input_filter filter be a > good idea? Or simply a 16-30V zener diode may be enough? > > My other question is temperature extremes. If I stuck with the > Microchip Industrial range, -40C to 85C, I would also assume I would > not worry about the lower limit, as cars probably have trouble > starting below -40C. The upper limit (85C, 185F) though may be > approached during summer, in the interior, in full sun (on dashboard) > and probably with a black plastic case to make the situation worse. > Does anyone know what to expect? > -- --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .