On 5/1/07, Russell McMahon wrote: > >> The 78xxx family are not "automotive rated" in the normal sense of > >> the > >> term. > >> While they can be used with an eg automotive 12V supply as input > >> they > >> would require protection against the normal nasties that occur in > >> that > >> environment. Some (only) of which include load dumps, polarity > >> reversals, high energy spikes to "most interesting" voltage levels, > >> ... . > > >> I'd > >> start with a series diode for reverse polarity protection and TWO > >> series resistors with a zener diode clamp to ground after each > >> resistor. Add a few caps and you have something which will survive > >> most "events". We did something similar in a -50V Telecom > >> environment > >> long ago. The two stages of zener clamping do wonders to eliminate > >> transients which are almost unstoppable with a single stage. > > >> The beautiful LM2936 would be easier, but its far from cheap. > > > Because of the -50V to +60V input transient survival protection ? > > Because of all the things in the LM293x data sheets that they say they > are protected against which they put there because of what they must > have learnt the hard way over the years in automotive environments. > Looking at the list in the data sheets gives an excellent guide to > what you need to do if you are 'rolling your own'. > > > That can be solved on 7805 too. > > Yes. > As I noted. > By using the methods I suggested, or something equivalent. > The user has to decide, as I also noted, whether the end result is > small or cheap by their standards. > A single zener and resistor clamp will NOT remove really energetic > fast rise time spikes well enough to let an older style voltage > regulator such as the 78xx handle them without passing spikes through > to the output. I'm affraid we are talking about the same thing in different ways. I have no enough patience for describing schematics in words like you have. However, as you said, there are different solutions which makes a 0.1$ components running exactly as a $2 component if it's used corectly. 7805 is definitely one of those. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist