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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: 12 Volt, Strobe Project (Coil Powered)
Date: Sun, 10 Nov 2002 23:24:51 GMT
NNTP-Posting-Date: Sun, 10 Nov 2002 15:24:51 PST
John Jardine wrote:
> Chuck Simmons wrote in message
> > John Jardine wrote:
> > >
> > > Neat strobe circuit!. Nice departure from the usual, lethal rectified
> > > voltages.
> > I always find statements like the second sentence above a bit puzzling.
> > I will try to explain. The strobe in question is said to consume about
> > 12 joules per second (1 amp at 12 volts). I'm guessing that the flashes
> > are about 1 joule each (more or less) limiting the device to about 12
> > flashes per second more or less. Perfectly good for some purposes but
> > not useful for others. For example, if 200 flashes per second are
> > required, the minimum input power is 200 joules per second or about 12
> > volts at 17 amps. A nicely designed lethal mains power supply is very
> > attractive don't you think?
> > What is a "lethal voltage?" It is winter. I routinely (many times per
> > day) get a shock in the 15,000 to 30,000 volt range. I do not keel over
> > and die on the spot (not often anyway). The other guy in the lab can
> > hear it across the room and shouts "gotcha" while I curse the furniture
> > for the 20th time that day. When I was married, I used my ring to catch
> > the arcs because that way, I could not feel them. It does seem like
> > 30,000 volts is not itself dangerous.
> > A few hundred volts has a worse reputation than some high voltages. Why
> > is this? I know the reason. Voltage has little to do with electrocution.
> > It is current and the path it takes that is important. Hence the ancient
> > advice to keep one hand in a pocket while working with high voltage.
> > The fact is that shocks from 100 volts to several hundred thousand volts
> > (lightening) are survivable for various reasons. To understand why
> > voltage is irrelevant, you have to understand the physiology of electric
> > shock. The one caveat here is that in the 100 to perhaps 500 volt range,
> > the muscular spasms can make it impossible to let go of the terminal or
> > wire.
> > Tektronics published an excellent safety bulletin in the early 1960's
> > explaining the main issues in electrocution. Voltage is a factor by
> > ohm's law but current is by far the most important factor.
> The line was meant to suggest that this 12V powered strobe is quite a
> novelty in itself (portability ect) as compared to most other designs
> offered for construction that make easy use of (yes! lethal) rectified mains
But likely has 300 or so volts on the discharge capacitor during normal
operation. This could be fatal because of the possibility of induced
fibrillation which is the usual cause of death in electrocution.
> What I'm saying is that there is no safe acceptable levels of 'electricity',
> sufficient that we are clear of health hazards. The simple mental model I
> carry of the danger aspect, is first to say that DC is worse than AC (DC
> *will* lock muscles up).
> Secondly, look at the voltage level as meaning nothing in itself until
> the pressure builds up sufficiently for some body current to flow. The
> current is the actual killer but requires a voltage to drive it through the
> body. If conditions are right then a lethal voltage may be 10V or 100kV.
> Thirdly, to cause damage, some current actually needs to be available
> to back up the voltage (i.e low impedance source).
In the early days of electrification, Edison claimed that DC was safer
than AC. Edison needed something to save himself after Westinghouse, a
major manufacturer of, in particular, air brakes for railroads, invested
heavily in Tesla's ideas. Edison rightly feared the combination because
Tesla had worked for Edison. Actually, You can find yourself locked by
muscle spasm to AC as well as DC.
> A mains powered strobe scores highly on all these points. I'd never tell new
> starters not to build this kind of strobe or a 20kV laser ect, cos' that's
> where we all start, we also need a few (safe) jolts to build a sense of
> respect. I do say though, that the safety aspects always need clearly
> pointing out in the first instance. Something like a skull and crossbones
> can work wonders.
I will say it is curious what mistakes can lead to accident. In spite of
an early career around high voltage, I very rarely got bit. This is
probably because I really hate shocks. The worst in my memory was from a
grid bias supply in a 30kW linear amplifier. I rather stupidly left my
belt on and it had a metal buckle. Believe the awkward position I was in
caused the buckle to contact my skin and when I slipped so that my hand
touched the grid ring I got the granddaddy of all shocks. Without the
belt, the shock would have been rather mild. It was ironic that aside
from the mistake with the belt, I had taken every precaution including
posting a guard in front of the amplifier to make sure no one pushed the
high voltage button. I really do hate shocks. I remember every one that
"woke" me up.
> I reckon we've both always had a basic (healthy!) respect for this
> elektrickery stuff which is a one reason why we've survived. I know the
> chances of electrocution are low, but death is a bit final and not really
> like a crushed finger.
My exwife always just about had a cow when I would ask her to hold a
flashlight for me while I replaced a light switch on a hot circuit. I
kept having to remind her that I would damned sure get a shock if she
didn't hold the light steady. I never did figure out why she was so
shakey with the light. After all, I'm such a wimp about shocks that I
wouldn't do anything dangerous.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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