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.

Well this is what I've thought up so far. I'd really be interested in getting
some input on how to solve these issues.

BAJ

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.