On Thu, 10 Jan 2002, Byron A Jeff wrote:

> > > >From the specs the problem is obvious: How in the heck do you get a regulated
> > > 12V output from a possible 12V input? It's impossible. However with the
> > > error term regulation down to 11.4V is acceptable.
> > >
> >   No is not imposible. How is possible to have regulated +5V from +3V ?
>
> But it's a different problem. I'm not trying to generate a higher voltage.
> If I were then I'd be looking at a boost converter.
>
> But neither of these are the case. I can pretty much guarantee that the
> input voltage will be at or above 12V. I am willing to have a regulated
> voltage that is below that input voltage.

  You have much heavily design condition that you imagine, Byron:

  input voltage from 11 to 16 V
  input spikes from 0 to hundred of volts
  ambient temperature from -35 to +70 C ( sorry for Farenheit speakers )
  plenty of vibrations

  You can't do it with a linear regulator ( or maybe yes with some
malfunctions or choosing right the output voltage at 9 or better 8V).
 A simple isolated DC-DC
converter will solve all your headache regarding isolating spikes. The
same one will gave you +5V and any other voltage  ( including 5000V for
ionisation of the habitacle etc. )
By the other hand if this is only for supplying the motherboard, HDD
and CDrom, then
probably you know there are boards which are running well at +8V instead
of +12 and without -12 and/or -5V. Also newest CDroms  doesn't need +12V
at all, only +5V.


> Let me try to pull Roman and Russell back into the conversation. IIRC Russell
> said in one of his posts of his 2 transistor switching PS that it required
> 1V of headroom. Was there a particular reason for that design spec? What
> happens if the input voltage goes into the dropout range of within 1V of
> the regulated output voltage? Especially when that voltage is right at
> the breakdown knee of the zener control element?
>
  Then the stabiliser will have the same function like a resistor.

  best regards, Vasile

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