Re: digital packet in the car
> 1. Use a 300W inverter. This sounds like a real waste, going from DC
> to AC and back to DC again.
> 2. Run 3.3 and 5 V DC directly into the motherboard, hard drive,
> etc. This will require regulators and a good filter. This sounds
> risky, but way cool.
Three hundred watts at 12 volts is like 25 amps. It's fairly common
to install 100 watt HF transceivers in cars; these generally draw 20+
amps on transmit. (Duty cycle applies here; transmit time is a
usually a fairly small percentage of total operating time; the PC
doesn't work that way.) A few hams do run amplifiers which draw 60-80
amps by adding additional batteries and using dual or high-output
alternators. You can get by with 25-30 amps without making changes to
the car power system if you are careful, but forgetting to shut the
gear down when you get out of the car will be IMPORTANT -- that kind
of draw will flatten your battery in a matter of an hour or two.
The back panel sticker on a PC indicates the maximum rated load on its
power supply, not its actual draw. The latter depends on what's
really inside. My experience with a batch of servers and a UPS
installation is that the typical real draw is usually substantially
lower.
You can actually get PC power supplies which eat 12 volts DC instead
of 120 volts AC, so it's not necessary to build. They're a little
expensive, since they're only used in industrial applications.
The best approach would be to alter the power requirements of the
computer. Computers designed to be small and to consume far less
power are readily available. One possibility is a laptop (car
adapters are available for nearly any brand). Another option is to
use hardware designed for embedded applications, which often need a
small footprint and low power consumption. There's even the option of
using something like a Palm device, if the software exists, or you're
up to writing it.
When it comes time to do your wiring, remember that since
thump-mobiles are fairly common these days, there's gotten to be a
consumer market for the electrical wiring parts needed to make these
installations. The stuff works just as well for ham gear.
Don't forget that the display will eat power too. Since you're in
California, the fact that LCD displays don't like temperature extremes
much may not be too much trouble.
> Ultimately, I'll want the radio running as well. Are there
> car-mounted radios, or am I looking at an inverter for that?
There are lots of choices in radios designed for mobile use. They run
on 12 volts, are smaller than their desktop counterparts, and often
have detachable front panels, so you can mount the display/controls in
easy reach, and the rest of the radio under a seat or in the trunk or
some other out-of-the-way place.
Commercially available TNCs (modems for radio use, but with additional
functionality) often need fairly little power. The Kantronics KPC
line, for example, only draws like 80 mA, and current models are
pretty small.
Dennis Boone
KB8ZQZ
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