Date: Fri, 2 Jun 2000 03:06:35 -0400
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: Power for PCs (eg Mac) in Vanagon
In-Reply-To: <v01550107b55cf7dd1378@[139.80.94.12]>
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At 01:35 6/2/2000, Andrew Grebneff wrote:
>2-6-2000
>How about running a full-size computer in the van, say when on holiday?
Calculate the power requirements, and add a hefty percentage of surge
rating because both PC and monitor tend to suck a lot of power as they
start up. To ease the starting surge you can start the monitor and PC
separately.
There are three basic types of inverter, with reference to the power they
put out. First type puts out a square wave. This was the only cheap
method back when inverters had a lot of iron in them. However, some types
of gear don't like it, and I wouldn't use it for a computer myself. You'd
have trouble finding a new one of this type, I imagine. The ones I knew
were heavy, expensive, and drew a lot of current just idling.
Second type puts out a "modified sine wave" which is market-speak for a
little square wave sitting on top of a big one, so that if you have a real
good imagination it looks like a sine wave. This is the common cheap type
today, available in sizes from 30 watts or so to over a kilowatt. Not that
the big ones are exactly cheap, but still.... The battery draw when idling
is small -- the little ones up to a couple of hundred watts prolly draw
less than 100 ma idling. Peak efficiency (?over 60% load?) is up around
80% or better. These will power most anything without hurting it, but they
may introduce a buzz in audio gear, and will probably cause minor but
irritating artifacts on a monitor. Perfectly usable, but not pleasant to
look at for long. If you plug a line filter (not simply spike protector)
into one of these, it (the filter) might smoke -- it's not really meant to
handle stuff with sustained power.
Third type actually constructs a nice sine wave and amplifies the heck out
of it. The output is impeccable, probably better than what comes out of
the wall, but AFAIK they don't make little ones, and the big ones are quite
expensive. I suspect they're not as efficient as the others, but don't
know. This is the way to go, though, if you want really clean power. In
fact, the thing to do is get a "full-time" UPS that generates a clean sine
wave for the computer no matter what the line is doing, and power it from
your house battery instead of the internal one. Make sure it uses a 12v
battery, though -- I used to have one that worked on 120v DC -- ten 12v
batteries in series. Or alternatively, use a cheap inverter to drive the
AC input. Since it's generating a new sine wave from scratch it shouldn't
mind being supplied by the "modified sine wave" of the inverter. More
efficient without the extra step, though.
Some of the really fancy inverters of this type will synchronize to the
mains frequency so you can use them to feed power back into the grid. The
power companies weren't best pleased, but they have been required to buy
the power from you. However they are *not* required to pay you retail for
it, and they don't. They use two watt-hour meters in parallel but oriented
oppositely, each one with a ratchet in it so it will only go "forward"
Nota Bene -- some (maybe all? dunno) inverters use both "hot" and "neutral"
as hot wires. If you ground the "neutral" wire you can smoke the
inverter. The ones I've looked inside have a three-wire outlet on the
front, but the third pin doesn't go anywhere.
david
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
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