Hi Byron, I think you have already hit on the
cleverest solution, using a gell cell battery
allows you to use a linear series regulator,
and because of the storage capacity of the
battery you can regulate the output voltage up
to the PEAK input voltage. Not the MINIMUM
input voltage like a normal series regulator.
:o)

You should aim for your battery to be kept at
about 12.8v, and full, so it can be charged
anytime the car is above 13.0v with a simple
series regulator transistor. The only times the
car will be under 13.0v will be when you start
it, or maybe idling with the headlights on. Anytime
the motor is "driving" the car will produce about
14.2v to 14.5v.
-Roman



Byron A Jeff wrote:
>
> I think I've done my homework on this one folks. I googled. I checked
> the archives for this list. I checked the news archives.
>
> Here's the situation:
>
> I want to power a PC in the car. 5V has multiple solutions so it isn't
> of concern here. I'm particularly interested in the 12V supply.
>
> The supply will have a 12V 7Ah gel cell to act as a mini UPS. So the
> input voltage will be from 12 to 14 volts.
>
> I want to get a regulated supply of 12V +- 5% @ 4A.
>
> Lastly and most importantly to me: It needs to be a discrete solution with
> components that can be purchased at the local parts store. Think Radio Shack
> or Radio Spares. This requirement has a twofold purpose: I want to learn
> the process, and I want to be able to have readily available replacement
> components.
>
> >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.
>
> So 12V in and 11.4V output screams low dropout regulator. So I started
> researching them. NPN and NMOS are immediately depricated. PMOS and PNP
> are the way to go with the series pass element. This leaves the problem of the
> control/error element. The obvious, a regular linear regulator, is out
> because the are amost universally NPN devices, which means high dropout
> voltages.
>
> The next choice is a zener. Simple, clean, and reasonable regulation. But
> each and every circuit I've seen that uses a zener control has an NPN pass
> element. Again high dropout voltages.
>
> So I was wondering if anyone had any ideas on how a zener can be coupled with
> a PNP/PMOS pass element so that reasonable regulation and low dropout is
> still possible.
>
> On a parallel path I did take a look at the buck switching regulators that
> have been discussed in the last year. However they seemed to need more voltage
> headroom than I was able to provide.
>
> One last issue to raise again was the input voltage spike protection. Most
> of the discussions that have been proposed in the past have been rather low
> current. I was wondering if a low valued resistor with a power shunt before
> the supply could work. For a shunt I was thinking along the lines of Fr.
> Thomas' power shunt here:
>
> http://www.infosite.com/~jkeyzer/piclist/2001/Jan/2382.html
>
> (login and password are the name of this list in lowercase)
>
> or a simple crowbar using a high valued zener driving an NPN shunted to
> ground. This would be helpful because I really wanted to regulate the UPS
> batteries input voltage to 13.8V anyway.

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