HomeRoast Digest

Topic: ...an adapter? Yes...there is! (9 msgs / 307 lines)
1) From: Steve

2) From: Paul Goelz
Unfortunately, this is incorrect.  If you allow for the less than 100%
conversion efficiency, you actually need MORE input power (watts) than you
get out.  Unless you have discovered >100% efficiency conversion, that is.  
While 500W inverters may be commonly available, I think that if you examine
the ratings you will find that they require more than 500W input.  At a
nominal 12V, that would be at least 41 amps.  Check a 500W inverter
carefully and you will see that is true unless it is rated for less than
continuous duty.  And even then, at 500W out, it will require 500W in at
And of course, 500W is insufficient for most coffee pots.  
There are, however, small and really cute travel coffee makers available
that run on 12V or 120V.  They don't make much coffee, but they can be used
in a car. 
Paul Goelz
Rochester Hills, MI
pgoelz at eaglequest dot com
Videoastronomy. electric helicopter and music web site:http://lists.sweetmarias.com/mailman/listinfo/homeroast">http://www.eaglequest.com/~pgoelzhomeroast mailing listhttp://lists.sweetmarias.com/mailman/listinfo/homeroast

3) From: Thom Mackris
I think you should rig up a generator to a bicycle trainer   Sip your
latte & crank away.
Seriously, a butane camping stove seems to be the way to fly, IMHO.  A whole
lot less complexity as well as more independence.  The backpacking models
are designed for compactness.

4) From: Michael Allen Smith
As a California resident subject to "rolling blackouts", I have thought
about doing this.
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5) From: owen cox
From a list lurker,
Agree that a camp stove is ideal for self generated hot H2O. Have had great
fun in a construction delays by getting mine out and brewing up a cup while
waiting. Some folks root for me to get "caught with my stove out;" others
cheer when I get to finish and then there are those so caught up in feeling
sorry for themselves that they don't get it.
I find that the hot water dispensers in convenience store coffee urns is
acceptable for a presspot or swiss gold single cupper. In which, by the way,
I use a fine grind and think the taste is quite full. Quite possibly I don't
know how bad "over extraction" is.
This is a great list, but would like to hear from more stove toppers (I use
my great grandmother's cast iron skillet) and am partial to virtually all
the dry processed coffees I have tried. Perhaps the variability that one
gets in an open skillet is a good match for the variation in the wild beans!
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6) From: Steve

7) From: Spencer W. Thomas
Steve wrote:
Well, I guess I'll have to go back and tell my college Physics profs that.
They'll be happy to know that it's possible to get more energy out of a system
than you are putting in. I wonder who can be the first to get the patent for a
perpetual motion machine approved?
With regards to your AC to DC power supply, let's do a little math here:
output: 25A * 13.8V = 345W
input: 120V * 5A = 360W
No problem here, the input draws more power than the output produces.
As to your friends inverter, we do have an apparent problem, in that 12V * 15A =
180W, which is significantly smaller than 300W. I can only speculate as to what
is happening here, but I can be quite sure that if he was actually pulling 300W
out of the output, he must be drawing more than 300W from the input.  I can
think of a few possibilities. Possibility 1: the fuse is larger than 15A.
Possibility 2: the truck battery supplies 20+V instead of 12V. Possibility 3:
the 300W rating of the inverter is a lie. Possibility 4: he's never plugged in
something that actually draws 300W.
Because if he's really getting 300W out of 12V * 15A input, all he's got to do
is to plug your power supply into the output of his inverter, and to plug the
inverter into the output of your power supply, and he's got a free source of
energy.  He can pull 15A from the output of your power supply, leaving 10A free
for other purposes.  Somehow, I don't think so.
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8) From: Monty Harris
Here is your answer guys:http://www.skingcompany.com/portable_power/calc2.htmMonty
At 11:24 AM 3/28/01 -0500, you wrote:
for a
15A =
plugged in
to do
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9) From: Henry C. Davis
Well, for one thing, most 12 volt batteries are considered all but dead when
they get down to an actual 12vdc output. Usually a fully charged 12vt
battery produces somewhere between 13.4vdc and 15vdc. Also, most consumer
electronics are not using real watts to measure power. When they market a
300 amp inverter, it won't usually be capable of producing 300 watts of
available power. (Take a look at the "watts" of power available from some
battery backup/lineconditioner products. Some advertise real watts, some
don't, at least the last time I looked.) In addition, most inverters list
peak and continuous ratings, but only the continuous ratings actually make
much sense in terms of performance. Further, your numbers are off for the
variation in AC power that is acceptable to most AC units and as is produced
by most inverters. Usually you get somewhere between 108 volts and 117 volts
on most of the units I have seen. (electric motors and some electronics are
far more picky about the range of usable power.) You should also recognize
that in many places in the US "120" volt home systems are actually either
115 or 110 volts, not 120.
More importantly, you are not, so long as the vehicle is running, limited to
the output of the battery, but rather the output of the alternator, which
generally produces about 15+ volts DC and more amps available than the
battery can produce. Except for protected leads, which the cigarette lighter
port is not, you should be able eat DC voltage as high as the alternator
will put out, and convert that.
You really have to look at your actual numbers to see how much raw current
you are dealing with before discussing conversion results. Not to mention,
you have to look at actual available output from the inverter (say 115vac
rather than 120vac) because that also has an effect on the numbers.
However, DC current will, as a practical matter, produce a heated element
using resistance faster and more efficiently than an AC unit will, so if you
get a hot pot to heat water and intend only to use it in the car/RV/boat, a
DC unit without having to convert to AC is probably the best way to go.

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