Jinx,

On Mon, 15 Oct 2007 22:49:40 +1300, Jinx wrote:

> > I don't know how frequent it is that a battery goes below 5V during
> > starting. I'm not sure, but I gather that there isn't much starting torque
> > anymore at this voltage, so maybe you can design with a workable
> > minimum voltage
> 
> For ordinary 12V automotive I doubt you'd get any turnover from a
> battery at that voltage. Below 10.6V they're considered flat

Right, but during turning-over the voltage can typically fall to around 8V even with a battery in good condition.  With a 3V headroom requirement on most 5V 
3-terminal regulators, and diode-drops on the way there, you may find that a 5V system without a bit of a reservoir may fall below its designed voltage.

Something that surprised me when I had a DMM connected to my car for a while, was that when I switched off the ignition, as the engine is stopping there was a 
momentary rise in battery voltage to about 15.5V - I presume the regulator is shut off by the key so the alternator gives its all as it's slowing down.  This is from 
idle, so revving the engine up and then turning off the ignition is a Bad Thing for this reason among others!  My father always used to do this, I think based on how 
he was taught to drive in the army, but it's not a habit I ever got into.  Since my past two cars have had turbochargers, which need a while at idle to settle down 
gently before stopping, I'm glad I didn't.
 
> Diode isolation between the battery and a reservoir cap should be enough
> 
> Someone did post an automotive 5V PSU a while ago. IIRC it included
> a pi filter comprising a couple of coils, with additional diodes and caps,
> and perhaps a zener too, all before a 5V regulator
> 
> I have an idea it was one of the Bobs

As long as the 5V regulator never sees anything above its top voltage rating - this can happen with load-shedding in a car - presumably the Zener would be to 
prevent that.  People see that a 7805 is rated for 35V input and assume that's plenty of headroom on a "12V" car system - it may not be!

Cheers,


Howard Winter
St.Albans, England


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