"Wayne Chirnside" wrote to "Mike Ross" (31 Oct 02  12:01:00)
 --- on the topic of "Re: BATTERY PACKS"

 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.


 WC> Ah, disagree, perhaps 40 - 70 amps if the surge is brief however if
 WC> the internal resistence is high as likely once the short in the
 WC> cell clears, discharged cell, any excess charge on that cap may very
 WC> well last long enough to fry the junction.

I remember using a 6 amp scr on 30,000 uF at 40 volts and the scr never
even got warm, as it should be BTW. Remember an scr is supposed to
behave as a dead short or switch once triggered.

How long does it take to discharge 80,000 uF at 50 volts into .1 ohms?
Let's see... using the approximation t=CV/I, gives about 8 milliseconds.

Now translating that into power dissipation at say .7 volts across the
scr that's just shy of 3 watts averaged out over 1 second. Of course the
approximation assumes a constant current, so in reality it would be an
exponential decay and thus even less power dissipation.


 WC> Anyway I'm not a fan of stressing solid state components by
 WC> continually exceeding their ratings, leads to reliablity issues.

You should appreciate that semiconductor ratings are usually meant for
steady continuous duty at a conservative level. However, "time" is a
factor that is not often given in the specs and "time" is very important
when dealing with very brief pulses. Semiconductors can and do withstand
really huge current surges if the timing is kept very brief.


 WC> I don't "believe" I *know*, both from literature and from personal
 WC> experience. You still have to get those milliamperes to do the
 WC> damage and that takes low skin resistance, higher voltage or both.
 WC> I've checked my typical dry skin resistence and it's in the 50K
 WC> range and at that you're not even going to feel 50 volts.

I don't doubt you believe you know... well, in all evidence to the
contrary you should since you're still alive here to reply about it!


 WC> BTW the reason that AC is so much more dangerous is there's a
 WC> relaxation cycle the heart passes through which is especially
 WC> subject to electrically induced fibrillation and the standard
 WC> 60 CPS AC current standard assures that that WILL occure in a very few
 WC> cycles of AC current.

Doesn't this seem like the case between Edison and Tesla all over again?
Edison went to great lengths to fry elephants with AC and sponsoring
electric chair execution of sentenced murderers. A lot of that (excuse
the pun) Edison generated hype still survives to this day.


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

One should always worry when a table top experiment starts to exhibit
the phenomena of St.Elmo's fire. 

 Mike
 ****

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