On the subject of batteries and safety; i recently accidentally shorted out
a 6v (4xAA cells) battery pack when a screwdriver in my bits box slipped
across the contacts,the resulting smoke,noise,heat,and stained underwear
where something else...
----- Original Message -----
From: "Olin Lathrop" <olin_piclist@embedinc.com>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Monday, November 14, 2005 7:19 PM
Subject: Re: Safety of 12V dc to 110v ac using D cells


> John Waters wrote:
> > It is hard to imagine 8 D cells could kill someone, right?
>
> Wrong.  It doesn't take long at much of a current accross the heart or
thru
> the brain to kill a human.  In other words, the total energy to kill a
human
> by electrocution isn't that high.  I'm sure 8 D cells contain way more
than
> that amount of energy.  Of course it has to be converted to the
appropriate
> voltage/current, but that's exactly what your inverter is doing.
>
> Do the math.  I'm not up on the amp-hour rating of D cells, but 2 AH for a
> primary AA cell is quite conservative.  Therefore 10 AH for a D cell must
at
> least be in the ball park.  10A x 3600 seconds x 1.5V = 54KJoules, so
let's
> say we've got about 50Kj to start with, and 40Kj after conversion to a
> different form.  And that's just one D cell.
>
> So let's look at some examples of 40Kj.  40Kj = 40000 N-m = 4082Kg-m at
1g.
> That's about 1 ton (2000 pounds) lifted 4.5 meters or about 15 feet up.
> Imagine that crashing down on you.  Or let's say you weigh 100Kg.  40Kj is
> how much energy gets dissipated when you fall 41 meters.  In other words,
> 40Kj is about the splat you'd make after jumping out a 13th floor window.
>
> But back to an electrical form of the energy.  40000 joules is 1000V and
1A
> for 40 seconds.  A D cell couldn't release all its energy that fast, so
this
> is only meaningful if it were stored then quickly released.  But let's say
> each cell can deliver 3A for long enough to kill a person.  3A x 1.5V x 8
> cells = 36 watts.  That's 300mA at 120V.  Even if it's 200mA to account
for
> conversion losses, that means the D cells can sustain a 400 ohm load at
> 120V.  Without special electrodes that make good contact, you're body is
> going to present a lot more than 400ohms, so the D cells will have no
> problem maintaining the 120V.  And, this is using rather conservative
> numbers.  In reality a 8x D cell to 120VAC converter should be able to
> sustain a considerably lower resistance load for long enough to kill you.
>
>
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