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I suppose the first thing to consider is what the DC in rate is for the computer that you will be using. Most laptops will have a plug in to use power from the transformer that comes with the laptop. This SHOULD be DC in, but I'm not sure. You'll want to check the rate for this DC in plug.
Another thing to consider is, because vehicles are connected to an alternator, you'll probably experience voltage spikes and/or drops, anywhere between 15 volts, down to 10 volts. Some people have stated spikes greater than 15 volts, but I'm not too sure on that one. The bottom line to this voltage spike issue is: Most sensitive equipment (Computers) wont be tolerant of these spikes. Thus, there is a necessity to have a power supply that will regulate the output voltage to stay at the predetermined value, sort of like a surge protector.
If you can give us the Voltage and the amperage that your computer needs, I'm sure we can provide a circuit or somewhat. I think your best bet is to look on the transformer that comes with the laptop. It should give you the specifications there. It should say something along the lines of:
" Input 120 VAC / 60 Hz / 4.8 VA
" Output 3.7 VDC / 340 mA
The output figures are what is important here. Given the output values, you can create a power supply to match those values, with voltage regulation to protect your laptop from those surges.
You'll also need a +3.3 volt output and a +5volt standby line.And unless you are using old ISA cards, the -5 volt is not needed.I have personally seen spikes greater than 20 volts during sudden RPM changes, I have heard spikes up to 60 volts are possible, from various places like SAE and National Semiconductor.