Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!pasteur!ames!amdcad!weitek!practic!vlsisj!davidc
From: davidc@vlsisj.VLSI.COM (David Chapman)
Newsgroups: sci.electronics
Subject: Re: DC power supply for IBM PC?
Summary: Don't bother
Keywords: PC, power supplies
Message-ID: <15241@vlsisj.VLSI.COM>
Date: 1 Jun 89 02:44:10 GMT
References: <4682@drivax.UUCP> <1989May28.214142.26384@utzoo.uucp> <30@bartal.CRLABS.COM> <1989May29.235155.26128@utzoo.uucp>
Reply-To: davidc@vlsisj.UUCP (David Chapman)
Organization: VLSI Technology Inc., San Jose, CA
Lines: 41

In article <1989May29.235155.26128@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>In article <30@bartal.CRLABS.COM> phillip@bartal.CRLABS.COM (Phillip M. Vogel) writes:
>>... While it may be slightly tricky to get +12V regulated out of a 
>>car battery (ie. you can't just use a 7812), there's those of us who know 
>>how to design supplies with something other than 3 terminal regulators.
>>I'm not saying the job is trivial, but it certainly is not unreasonably 
>>difficult.  
>
>Depends on who's doing it.  Unless I'm much mistaken, the original inquirer
>basically doesn't know anything about power supplies.  What's "simple"
>(note that that's the word I used) for such a person isn't the same as
>what's simple to somebody who designs switching regulators for fun. :-)
>Yes, if you're willing to spend a little while learning about power-supply
>design, it's no big deal -- but that's not the question that was asked.

Having built a switching power supply or two, I think I'm qualified to say
"it's not that easy."  Yes, there are easy-to-use chips that step up, step
down, and invert.  I've used them (TI's TL497 comes to mind).  But look again
at the original post:  he wants 5V at 20 amps (plus others, but you get the 
point).  That is _not_ a trivial power supply, can _not_ be constructed easily 
using most of the chips available, and probably _will_ require some fancy 
construction, cooling, etc.

The original poster (sorry, forgot your name :-) may be better off with a
12VDC-120VAC inverter.  Most of the switching power supplies should be able
to handle the usually trashy AC (square waves) that these things output.
Anyone know this for sure?  (Be careful when you use one of these that you
don't drain your battery in an all-nighter.  It's hard to push-start a PC :-)

As for me, my portable weighs 2 pounds, has 512K RAM and removeable 128K EPROM
cartridges, and runs on batteries (4 AA) for most of a day.  Of course, I just
use it for writing and don't need MS-DOS compatibility on the road (runs a
Z-80 with a proprietary operating system).  It's called the Z-88 and is made by 
Cambridge Computers in the UK (Clive Sinclair's latest venture, I believe).  
Costs about $1K as described.  None of this 120 VAC stuff for me (no car 
batteries on my bike :-).
-- 
		David Chapman

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