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In article <1233@ccnysci.UUCP>, sukenick@ccnysci.UUCP (SYG) writes:
> >>Does anyone out there have a circuit for producing the
> >>necessary +12, -12, +5, -5 for powering an IBM PC from
> >>a single lead acid battery?  The supply should provide
> >>enough current to run a loaded PC with a hard disk drive,and a 12 volt monitor.
> 
> If the -5, -12 sections don't draw much current, then why not wire
> in a -12 & -5 volt inverter (either the integrated circuit or forreal
> fun, the diode/capacitor combos ;-)) to the power bus ?
> 
> PC's are pretty  modular; to get it to work off of straight DC,
> just remove the connector from the power supply and plug into your
> DC supply.  

A cleaner more modular method is to get a DC input open frame switching
power supply.  There are switching power supplies that input low voltage
DC and output the regulated voltages.  See the EEM or similar 
buyers guides.  If you want to run a monitor and floppy disks, you'll
need at least 2 amps at 12 volts.  Your battery isn't going  to last
very long.

This is request that shows on the net with great regularity.
Here is an excerpt from a posting that showed up last time this came up.
(I tried to mail this but you know how that goes)

--------------------------------------------------------
:From: sgt@dukeac.UUCP (Stephen G. Tell)
:Newsgroups: sci.electronics
:Subject: Re: DC to DC powersupplys
:Summary: Commercial unit; not cheap but good
:Message-ID: <1113@dukeac.UUCP>
:Date: 20 Nov 88 07:39:16 GMT
:
:
:I have used some switching power supplies from Converter Concepts, Inc.,
:and noticed that they have some that run on a DC input of 10-40 volts.
:(I used the models that ran on 100-380VDC or 90-265VAC, and liked them.)
:
:Their VT/VX-50 (50-watt) series has one that can take 10 to 40 volts
:DC input and produce that following outputs:
:+5 @ 1.2 to 6 amps
:+12 @ 0.05 to 1 amps
:-12 @ 0.05 to 1 amps
:
:as for input current; the requirements
:you specified of 5(5)+1(12)+.1(12) = 38.2 watts; call it 40.
:They specify efficiency of typicaly 70%, so input power is 57 watts, or just
:under 5 amps at 12 volts.  The other 17 watts comes out as heat, which
:doesn't look too bad.
:
:...
:I would caution against connecting too directly to the vehicle's battery
:(say using that 13.8 as the +12 supply directly.)  The regulation
:won't be good enough, and automotive electrical systems have lots of
:noise of all kinds.  A switching regulator of this sort will help
:insulate you from a lot of that.
:
:My only connection with these guys is as a satisfied customer:
:Converter Concepts Inc.
:Industrial Parkway, Pardeeville WI, 53954
:800-253-5227  or  608-429-2144
:-- 


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