Re: ATM Powering a PC from a 12v battery

[Andrew Grisdale <roadwarrior@techieguy.com>]

Hi Philip,

On Mon, 13 December 1999, "Philip Lardner" wrote: 
> Using power regulator chips, I can get +5v and +12v without difficulty, but
> how do I get the negative voltages? Are there similar power regulator chips
> that will do this, or is there a way to canabalise the PC's switching PSU to
> do this for me... sort of rip out the mains voltage side, and just plug in
> 12vdc?
There are some chips that will do negative conversion for you, however, you might not 
need the negative voltages. As far as I know, most boards only need the -12v for the 
serial port, and -5v is a legacy thing mostly for some old (pre AT) memory chips that 
needed + and - 5v, I do hear tell that some soundcards and modems have availed themselves 
of these though too. So, you might get away with just the +5 and +12, however, if your 
system checks the negative lines at boot, it may fail to boot. If it won't boot, a simple 
way to implement the voltages would be to make a battery pack with dry cells, tapping it 
at -6v and regulating that to -5v and taking -12v from the top. It should last a good 
long while unless the serial port is heavily used.

> Once I get that going, I'll tackel the screen which I think (hope) also runs
> off +12vdc
Nope, you are out of luck here, a CRT screen needs 10's of thousands of volts to drive 
the tube which it derives from transformers driven by AC line voltage. You either need a 
power inverter or to think of something different.

Something different....
A 12v portable monochrome TV might be converted into a monochrome VGA monitor by using a 
small circuit, a driver and getting a tv technician to help you find the sync and video 
inputs inside. Ask me for more details if this interests you. Small Color TVs intended 
for 12v use for camping with scart or other RGB inputs would also be able to be used as a 
monitor with a small circuit and driver. Be warned however, the display quality is rather 
fuzzy on a TV. More details here....
http://www.hut.fi/Misc/Electronics/circuits/vga2tv/cindex.html

You could build your own power inverter....
http://www.hut.fi/Misc/Electronics/epanorama/psu.html#dctoac

If you get a monochrome VGA monitor or a small (sub 12inch) color one then the 45W 
circuit there might cover that.

Power usage estimation
386 motherboard +CPU+Mem, 5A @ 5v, 0.5A @ 12v
Drives 1.5A @ 12v, 0.75A @ 5v
Expansion Cards 1A @ 5v, 0.5A @ 12v

Simpler is better, try to make do with just the vid card, the i/o card (if these are not 
built in) and one Hard drive and one 3.5 inch floppy drive. 
That would be about 10A @ 5v and 3A @ 12v, total consumption around 86 Watts, this is a 
bit of an overestimation, I have 386 boards running in cases with only 60W PSUs, but by 
the time you allow for inefficiencies in conversion, this is probably around what it will 
pull from a 12v battery.

5.25 inch disk drives are thirsty as are full height hard drives, try to get a 1/3 height 
IDE hard drive in there, one of those will probably be young enough to be able to be 
powered down.

If you need any more alternatives for the video, I can tell you where to find 5-9 inch 
monitors and LCD displays at economical prices.

regards, steady hands and steady skies!

Andrew G.


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