-=> MIKE ROSS wrote to WAYNE CHIRNSIDE <=-

 MR> "Wayne Chirnside" wrote to "Mike Ross" (30 Oct
02  13:05:00)
 MR>  --- on the topic of "Re: BATTERY PACKS"

 MR> A rather heavy knife switch, I might add, like the kind you see in
 MR> Frankenstein movies. 

 WC> Got mine at Radio Shack long time ago.

 MR> Was that back when they still had only half a dozen stores?

Pretty near ;-)

 WC> Yu don't get an SCR that can handle THAT surge at Radio Shack but they
 WC> are available from DigiKey.

 MR> You're dealing with surges. So even a relatively low rated 6 ampere SCR
 MR> will pass a full score of 100's amperes without a sweat.


Ah, disagree, perhaps 40 - 70 amps if the surge is brief however if
the internal resistence is high as likely once the short in the
cell clears, discharged cell, any excess charge on that cap may very 
well last long enough to fry the junction.
Anyway I'm not a fan of stressing solid state components by 
continually exceeding their ratings, leads to reliablity issues.

 WC> I'd agree about safety issues under
 WC> certain cercumstances, you're sweating and well grounded.
 WC> Much of those concerns could be eliminated by adding some capacitor
 WC> filtering after the rectifier thus removing the pulsed DC
 WC> component which is far more dangerous to causing heart fibrillation
 WC> than is DC.

 MR> Oh, boy! If you believe what you just wrote, you believe anything. No
 MR> insult intended. 
 
I don't "believe" I *know*, both from literature and from personal 
experience. You still have to get those milliamperes to do the
damage and that takes low skin resistance, higher voltage or both.
I've checked my typical dry skin resistence and it's in the 50K
range and at that you're not even going to feel 50 volts.
BTW the reason that AC is so much more dangerous is there's a 
relaxation cycle the heart passes through which is especially
subject to electrically induced fibrillation and the standard 
60 CPS AC current standard assures that that WILL occure in a very few
cycles of AC current.
I can make a case for a the hazards of a few picoamperes
as there's a case where a pacemaker patient was placed on a grounded
hospital bed and the leakage current from that device offed him.
BEWARE changing the battery in your watch!!!

 MR> In the shock effects literature it can be found that
 MR> heart rhythm problems occur within a narrow current range of a few
 MR> milliamperes. Less than this range there is no effect, higher and flesh
 MR> burns result but the heart survives.

Genrally the lethal level is around 17 -20 MA. but than there's
that matter of skin resistence again.
Now if you pierce the skin with a charged line that's another matter
entirely however even then AC is more dangerous than DC but you will
feel a jolt.

 WC> I once was redoing my phone line at the outside box, neighbor
 WC> had tapped into my line and reversed my line :-(
 WC> It was raining and while I was connecting the phone line properly
 WC> the damn thing rang... and jumped me quite a bit.

 MR> It's the surprise that gets you most. One time I nearly died laughing
 MR> when I handed my paw the secondary of a filament transformer while I
 MR> applied a "D" cell to the filament. He nearly jumped out
of his shoes!

I rewound an external winding on a salvaged T.V. flyback and drove it
with a  very stiff 12 volt source, 12 amps draw, through a push-pull 
oscillator and got a wicked belt just from the low turns primary when 
disconecting the  source... 
now the secondary threw  a wicked spark and would ignite
cardboard in about 1 second.
Now that puppy was really deadly.
 

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