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From: "John Jardine"
Subject: Re: 12 Volt, Strobe Project (Coil Powered)
Date: Sun, 10 Nov 2002 22:01:56 -0000
NNTP-Posting-Date: 10 Nov 2002 21:50:56 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
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
> 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 times have been,
> That, when the brains were out,
> the man would die. ... Macbeth
> Chuck Simmons email@example.com
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
The first and only mains powered strobe I built as a kid, I remember very,
very, clearly because of the number of times I got a nastly 'belt' from the
HT supply. This is a UK 240Vac supply, so peak capacitor voltages are about
+340V as compared to the rest of the world's wussy +150V. I know what 440Vac
feels like. I've had RF burns from home brewed kit. I've had shocks from
valve (tube!) HT's and TV final anodes. Like everyone else I've also know
the sting of spark plug voltages (>30kV) and static discharges >80kV?).
As someone wisely mentioned elsewhere, I can only tell this tale, because I
survived. Over the years and with hindsight I've also learned that I've had
a little bit of luck.
I started young with a healthy respect for this electricity stuff (small
kids fingers fit nicely into sockets etc) and the respect grew as the years
went by. Despite all my experience and high regard for these electrics, I
still took (and take!) a hammering.
17 years ago I saw the results of a workmate being 'stuck' to some faulty
110Vac rectified to 150Vdc, equipment. He'd taken all reasonable precautions
but his hand still required lots of plastic surgery.
I also particularly remember 6 years ago a Health and safety inspector
telling me he'd just investigated a case of a (shipbuilding) welder who'd
died from the 30Vac from his welding set. The poor bastard had just been
doing his day to day job and died cos of it. 30Vac is safe innit?. He took
hours to die. Explain this to his wife and kids.
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).
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 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.
I'm a bit wary that this 'respect' doesn't always seem to be the case
nowadays with new starters and considering some of the bold adventurers who
ask starter questions on building Xray power supplies I feel the more
supplied safety info the better. (don't stop 'em, just inform 'em). It's
their fault if they then end up as stiffs!.
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