On Feb 10, 2:25*am, (Dave Martindale) wrote:
Smitty Two writes:
A dozen 10000 UFd caps charged up to the voltage of a motorcycle battery
will do ... absolutely nothing. *12V DC is harmless.
Is that so? I'd invite you to try it yourself, with just *one* cap. Then
tell me how harmless it is.

What do you *think* it will do?

I have here an 82,000 uF capacitor, charged to 12 V. *I can touch it
with my fingers and feel nothing. *In fact, I just did. *12 V does not
cause enough current flow through (dry) skin resistance to be felt at
all.

Now, if I dropped a piece of fine wire across the terminals of the
capacitor, there would be a bang and a flash as the wire evaporated.
There's enough energy in the capacitor to do damage when connected to a
low-resistance load. *But that's not true of skin contact.

And in most circumstances, this capacitor doesn't store enough energy to
cause much damage to low-resistance things either. *If I dropped a piece
of heavy copper wire or a wrench across the terminals, there would be a
spark and some visible arc damage, but the wire or wrench would not get
noticeably warmer. *A 12 V lead-acid battery is *much* more dangerous to
get a low-resistance load across, since there's enough energy to get
substantial-sized things very hot.

* * * * Dave

If I dropped a piece of heavy copper wire or a wrench across the
terminals, there would be a spark...

When I was in the Coast Guard back in the late 70's we used to have
"safety meetings" for the newbies to show them why they shouldn't
enter the transmitter building without a trained transmitter tech.

First, we'd explain to them that the transmitters used a 15KV power
supply. Then we'd take a 5 farad, oil filled cap and hook the strap of
a grounding stick to one side. (A grounding stick has a long wooden
handle with a metal rod sticking of it and grounding strap with a
large clip. Before you worked on the transmitters, you'd clip the
strap to a ground and tap the big caps with the rod to make sure they
were fully discharged. The metal rod was hooked so it could be used as
a dead-man stick if needed.)

Next we used a high-pot to charge the cap up to 6 KV or so. Finally
we'd turn off the lights and tap the other side of the cap with the
grounding rod. The resulting noise, spark and smoke was usually enough
to convince even the most grizzled Boatswain's Mate that the
transmitter building was a dangerous place to hang out.

One time the "lesson" blew the metal rod out of the wooden handle,
essentially stripping the 3 inches of threads inside the handle. That
one even scared us!