Quote:
Originally Posted by pcrider
The LM2940.. was designed by National Semiconductor for auto applications with attendent power interruptions and spiking and is available with a 5v output.
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I just took a look at the datasheet for the LM2940, and as I read it, there's no spike protection (but, since I'm an embedded software guy, with only modest hardware knowledge, I
might be mistaken). I believe the "automotive thang" was the low dropout so the 5V wouldn't go squirrelly during crank.
Back in the 1980s and 1990s, I used to do a lot of work with a friend who
is a real EE, developing systems for (mostly automotive) engine control. The power input circuit he always used (with great results: he had pretty near zero units get smoked in the field) was:
1. A series diode (to protect against reversed wires during installation, and negative spikes that can occur in some vehicle electrical systems). Any old 1N4000 series will do. Since I'm not personally building a million of these, I'd spring the extra 2 or 3 cents, and go for a 1N4003 or better: it's cheap insurance.
2. A series resistor to help absorb spike energy, and dissipate some heat instead of the regulator doing it all. About 10 Ohms, since the MPGuino peaks at about 200mA.
3. A 1/2W Zener in the 25-30V range.
4. An input filter cap. The 330uF already specified might be okay, but I'd probably go for a smaller one, and use a tantalum (there are some formulas around for picking the value). Make sure its voltage rating is similar to the Zener's, since it will have to absorb spikes of up to the Zener voltage.
The output from this network went into an ordinary 3-terminal regulator (like a 7805 or an LM317).
Maybe the newer vehicles that most folks are building for have cleaner electical systems, but I wouldn't think of putting even a $40 MPGuino in my old-enough-to-vote vehicles without this sort of circuit.
Ran