Welcome
to one of the scariest and most dangerous sections
of the world of electronics. Consequently, this is
also one area which I enjoy thoroughly!
So everyone
knows that electricity goes 'fizz' and 'crackle' and
'bzzzzzzzzzzzzz', but something go 'KABOOM'
isn't something you hear everyday. Well electricity
has that potential, and it's just a matter of converting
that stored energy into sound.
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Capacitors...
realise the energy!
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Typical
photoflash capacitor
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This
is actually rather simple - it utilizes the principles
of one of the most common electronic components: the
capacitor.
Capacitors
store electric charge. The amount of charge they can
store depends on their value of capacitance and their
rated voltage. In short, the energy stored in a capacitor
(Joules) = (1/2) x Volts^2 x Capacitance (Farads).
To
the left is a typical capacitor found in a camera flash
unit.
These capacitors have extremely low ESR (equivalent
series resistance), meaning they can discharge at very
high currents. That's why we use them in these experiments...
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People
who work in the electricity industry will tell you
that 16 joules is enough to kill a human. And that's
very easily achievable with capacitors. Don't be surprised
if your wife asks you to break out the annuity
calculator and review the life insurance policies
if you consider trying these experiments!
The capacitor
bank that I use in these experiments consists of 5
paralleled photoflash capacitors, giving a total of
550uF at 330V. This rounds off to about 27 joules,
so technically very dangerous.
Initially,
I had put together a much larger capacitor bank,
however the resultant explosions were difficult to
capture on camera due to the sheer intensity of light
involved. Hence I have found this setup to be the
best demonstration circuit.
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My collection of photoflash
capacitors |
The
schematic... click on the picture for a higher resolution
version.
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Although
27 joules is nowhere near enough to do can crushing
and railgun work, some interesting results are achievable.
The aim of this simple, ultra small scale experiment
was to see what happened when a couple hundred amps
was passed through strips of different metals.
The
very simple circuit to the left was designed. As you
can see, I am being rather safe in that there are no
direct mains voltages present - rather a 240/12V transformer
run in reverse off a 12VAC supply. Switching is performed
through a large relay that (in a past life) was probably
used on a railway switch circuit.
Of
course, a much neater alternative would be to use some
kind of electronic means of switching, such as an SCR.
But since we are here to destroy things anyway... |
For
safety's sake, the whole setup was enclosed in a tough
industrial plastic case, just in case the capacitors
decided to blow on me. The important features of the
circuit are labelled on the image to the right. This
is as crude as a pulse discharge circuit can get!
I
decided to test the effects of the circuit on a few
different metals; aluminium, copper, and steel. Strips
of each were secured to the discharge terminals, then
the capacitors shorted straight through the poor little
strips. |
The
setup - messy, but functional.
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The
discharge terminals alight! |
This is
a strip of aluminium being vapourised on the discharge
terminals. Vaporising aluminium produces a very intense
light, and in fact it is commonly used as an ingredient
for this purpose in flash grenades.
I like
this image because it just looks like some kind of
atom collision. The way the molten particles spread
appears rather chaotic.
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This is
again a strip of aluminium being vapourised, but during
an earlier test run of the circuit.
The multimeter
serves no purpose other than tell me how many volts
are left in the capacitor bank after discharge, potentially
saving me from a nasty bite afterwards. Also barely
visible in the foreground is the battery (12V, 7AH
SLA) for energizing the relay.
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Another
strip of aluminium. Notice how the spark trails of aluminium
are relatively straight, indicating that the residual
aluminium is travelling at high velocity. |
In
comparison, this is steel wool vapourising
off the discharge terminals. Very clearly this produces
a different effect. The combustion is not as bright
as aluminium, and the steel particles are relatively
heavier as clearly evident in the parabolic spark trails.
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This is
a single strand of steel wool placed across the discharge
terminals. Again, the molten projectiles are heavy
and fall very quickly with a highly pronounced parabolic
effect.
A good
replacement for fountain fireworks, perhaps?
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A
copper strip over the discharge terminals. Not very
interesting is it... probably due to the higher conductivity
of copper (I couldn't find a thinner strand at the time).
Also note how the explosion has a green shade... Copper
oxide burns green. |
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I
managed to capture some of these discharges on video,
and these are the frames captured during two explosions.
The
first is of a strand of steel wool. Clearly evident
in frame 1 is the green copper tinge of the stressed
relay contacts uncontrollably arcing over. |
1
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2
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The
second video involves a setup of a strip of aluminium
of roughly the same thickness as the steel wool above.
Frame 2 is seen to be completely saturated (hmm I'm
not sure if this is great for the camera), whilst the
ghostly artifact in frame 1 also gives the feeling that
the camera is about to undergo some kind of serious
torment. |
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2 |
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