AVR-Chat

Tony,

I don't know all the answers to your questions but here are a few:

> --- In AVR-Chat@yahoogroups.com, Dave VanHorn <dvanhorn@d...> wrote:
>
> I had not checked the maximum input voltage on the 7805, Is it less
> than 13.6v?  Or is there some other automotive power issue at work
> here?

Maximum is well above 13.6V. However the larger the difference, the more 
heat needs to be disappiated by the regulator. You also mentioned that 
you will need 1.5A which exceeds the specifications of the LM7805 which 
is limited to 1A. You will need something like a MC78T05 that is rated 
at 3A.

> Also, another sign that I know nothing, the reverse bias diode
> across the input and output of the reg.  I suppose that if the input
> goes to zero while there is +5 on the output this will cause the
> damage?  How big a diode do I need.  A 1A rectifier rather than
> a "switching" diode, not that I know the difference except in
> current ratings and possibly switching times....  The design apps I
> have seen did not show such a diode, but it seems to make sense!

I use 1N4001 or 1N4002 to protect both against polarity reversal and to 
protect the regulator against reverse current flow (connect the cathode 
to the regulator input). For your case you are going to need beefier 
diodes such as 1N5402.

Regards,
Mike