> The issue now is the input capacitor to the 7812.  The input capacitor I
> am not sure I need to add into my circuit because the input voltage is
> from an Unregulated Adaptor of 12 V. The distance from the Adaptor to
> the 7812 circuit board is 3 meter wire (10 feets wire length).   The
> worry of adding of the input cap is if the cap short, it will burn the
> circuit.

Any component failing could cause, well, failures.

> What is the best value to add the input cap,

As big as you can afford, given price, weight, size, etc.  At the very
least the total output capacitance on the unregulated supply needs to be
high enough to droop no less than to the minimum voltage at the maximum
current.  If the load current has spikes that exceed the unregulated
supply's average current capacity, then that needs to be taken into
account too.  Also think about the 6 meters of wire between your circuit
board and the power supply.  That will have some resistance and
inductance, therefore increasing the apparent impedence of the supply.
The cap will decrease the impedence at higher frequencies.


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