AVR-Chat

--- In AVR-Chat@yahoogroups.com, Dave VanHorn <dvanhorn@d...> wrote:
> 
> Avoid tantalums.
> 
> You have bigger problems.
> Have you checked the max current rating and max heat dissipation 
> ratings for the regulator?
> 
> Also, the LM78XX series is not designed to run from vehicle power.
> National makes a few that are though, and it won't be a painful 
change.
> 
> 
> 0.1uF on input and output, located as close to the reg as 
possible, 
> to stop oscillation.
> You will need to add electrolytics on the input and output as 
> required by the circuit, but they do not take the place of the 0.1 
caps.
> You also need a reverse-biased diode across the regulator's input 
and 
> output, to prevent destruction of the reg, if there is a short on 
the 
> input side.

Thanks for the quick reply!

I am a definite beginner as far as design goes.  Why not tantalums?  
I suppose that the alternative would be ceramic?

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?

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 ask these questions not to indicate that I do not believe the 
answers, just the opposite.  I need to learn the reasons so I will 
not make the same sorts of mistakes in the future!!!!

As for the large electrolytics that the design may require.  Would I 
need one on BOTH the input and output?  I would have thought that 
one on the input would be good enough, but I can see that big 
transients may not get fed well through the reg, so a big cap on the 
output would be beneficial too....

-Tony