On Thu, Jan 10, 2002 at 02:22:30PM +0200, Vasile Surducan wrote:
> 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

At 11V the gel cell is dead anyway. The minimum input voltage is 12V.

I agree on the 16V.

>   input spikes from 0 to hundred of volts

That's a seperate issue that has been addressed in many forums. A combination
of a fuse, MOV/TVS, and a shunt regulator can handle the spike issues.

>   ambient temperature from -35 to +70 C ( sorry for Farenheit speakers )

The unit is in the cabin. Ambient is more like -5 to 40 C. Hasn't been a
problem up to now.

>   plenty of vibrations

I haven't succeeded in getting the unit to glitch yet.

>
>   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).

I think the gel cell UPS battery eliminates a lot of the environmental issues
you raise. It creates a high power, low impeadance 12V source that can be
depended on. Isolation from the main battery will prevent the voltage dips
you refer to.

>  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. )

I may agree with that assessment if one can show a reasonable discrete design.
But those designs always end up being "well you need a Max something or other
or a linear tech part, plus some specialized flyback transformer that has to
be hand wound using a core that you can get from a scrap microwave..."
They end up being complex, laden with specialized, expensive, hard to get
parts.

A PNP transistor, even a low saturation one, isn't a specialized hard to get
part.

> 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.

The only problem I have with this approach is that you have to specify some
specific component. I'd prefer a supply that works for all standard MB, HD,
CDROMs, and ZIPs.

And certainly if they work with 8V, then 11.4V should be absolutely no problem.

BAJ

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