AVR-Chat

On the reverse diode between the input and output of the
regulator: this is normally used to prevent reverse current
flow through the regulator. This could occur if the input
is suddenly forced to a low voltage while the output cap is
still charged. The LM78xx family is not as sensitive to
this as some and I rarely see this diode in 7805 circuits. 

Also, don't forget power dissipation! If your load current
is 1A, and the input voltage is 13.5V, the voltage drop is
8.5V. The 1A and 8.5V makes 8.5W. This is a lot of power
and requires some careful thinking and design to make sure
the head is removed and the junction temperature is not too
high. If It does get too warm, it will shut down, often
leading to mysterious behaviour of the circuit.

Tantalum caps are nice because of small size and low ESR.
ESR isn't much of a requirement in linear regulators. But,
they cost more, have limited voltages and values, and have
a tendancy to destroy themselves (ie, explode or burst into
flames) if not in a current-limited circuit. Thus, putting
them in a place where they directly "see" battery power is
asking for trouble. It is less of an issue on the output
since the regulator, itself, provides current limiting.
They also tend to be more attractive on the output side
since the larger value is needed there. A general rule
commonly used by designers is to specify a tantalum voltage
rating that is about 2X the actual working voltage. 

Jim

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