MiniSport Laptop Hacker - Vol 12, 6 Apr 1993
Copyright(C) 1993 by Brian Mork.

>>> PORTABLE POWER OPTIONS
Twelve volt DC universality has again raised its head.  A number of peo-
ple seem to be interested in running their computer from 12v DC, whether
from the cigarette lighter or from portable batteries.  No problem!  The
pinout (pinin?) of the power connector on the left side of the computer
is very simple.  The Zenith manufactured power supply meant to plug into
this connector is a switching power supply; be careful measuring its
voltages, if you do that to compare with my numbers.  Switching power
supplies only regulate voltages well when they're under load.
  The round shield of the connector is ground.  It's more than a
shield--it actually carries the return current.  It is electrically con-
nected to the forward contact on the batter pack, becoming the BLACK pow-
er line inside the computer.
  The single smaller hole is a 5v DC (TTL) input that indicates when the
AC adapter is being used.  If you're interested in running off a portable
battery, leave this unconnected.  I'm not sure how (or if) this actually
gets into the computer.  Apparently the battery pack itself has some cir-
cuitry that requires the 5v signal.
  The bottom of the two large holes is a 9v DC input providing power di-
rectly to the computer.  This 9v doesn't electrically connect to the bat-
tery pack at all.  It is mechanically routed to the center conductor on
the bottom of the battery pack, becoming the RED power line soldered to
the main CPU board inside.
  The upper, rearward large hole is a 6v DC, used to recharge the battery
pack.  Directly, or indirectly through a few components, it makes its way
to the rearward contact of the battery pack and subsequently the YELLOW
power lead inside the computer.
  I claim ignorance on the inside of a battery pack.  Does anybody have a
torn-open pack?  What exactly is the 5v signal used for?  Are there any
compenents built into the pack or is it just batteries and wires?
  In any case, I've had complete success running the computer from a 9v
wall-wart transformer.  Large size paper clips happen to be exactly the
correct size to make the 9v contact and rub in place next to the ground
connection.  The Zenith power supply indicates that maximum of 2A is tak-
en from the 9v supply.  Use an adjustable 3-terminal regulator to bring
the 12v DC down to 9v.  If you're running straight from the cigarette
lighter, put some huge inductance in line (a lamp ballast works fine) and
a large capacitor on the car side of the inductor. A 1 amp diode put
backwards (banded end toward the car side) will clamp any high voltage
spikes from arcing into the connector if you pull the paper clip out,
making the magnetic field of the inductor collapse.

73, Brian Mork (Opus-OVH)          KA9SNF@wb7nnf.#spokn.wa.usa
                                   Internet ka9snf@jupiter.spk.wa.us
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