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Old 03-21-2011, 02:13 PM   #9 (permalink)
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Originally Posted by bobski View Post
Yes, it would see the increased voltage on the power supply connections, but that would be taken care of by the MPGuino's on board voltage regulator. It's a simple linear regulator so the excess voltage would be dissipated as heat. The additional heat could still cause problems, but it wouldn't immediately damage anything.
The inputs on the other hand are unregulated and designed for TTL-level input - 0 to 5V. A divide-by-3 voltage divider would turn that range into 0-15 volts, which a malfunctioning alternator can exceed. It's entirely possible the inputs can handle voltages in that range without damage, but if we can avoid such an occurrence by simply choosing different resistor values, why take the chance?
Just clamp the input pins to the power rail with diodes. Then you set your divider to what you need for nominal operation.
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