Can Crushing
Aluminum soda pop cans that have been electromagnetically crushed, using
1750 joules each.
It is possible to crush an aluminum soda pop can simply by surrounding
the can with a heavy coil of wire and passing an enormous surge of
electricity through the wire. In order for this to work, however, a
extremely low inductance / low impedance high voltage capacitor must
be used. Standard electrolytic and filtering capacitors just can't
discharge fast enough to provide the narrow pulse required for this
application.
For these experiments, I used a General Electric 12,500 volt
22.5 uF low inductance capacitor, pictured below. (Careful examination
of the picture will reveal my friend
Howard Tayler
hiding behind the capacitor.
Watch those fingers, Howard!)
My coil of wire consisted of
6 or 7 turns of 1.5" copper strap, salvaged from a pulse transformer.
The turns of the coil are insulated from each other with 4 mil high
density polyethylene plastic sheet (sold for winterizing storm windows).
The coil fits the can snugly, but is not electrically connected to the
can.
Low Impedance 12500 volt 22.5 uF capacitor and can-crusher coil
Click on either of the pictures for a larger version.
When the capacitor is discharged, an enormous surge of current flows through
the coil, which generates a huge magnetic field around the coil (and through
the can). This induces a similar surge of current in the can, with it's
accompaning magnetic field. Since the two magnetic fields (the coil's and
the can's) are in the same direction, they oppose each other, pushing outward
on the coil of wire and inward on the can. This makes the coil of wire want to expand
to a larger diameter and makes the can want to contract to a smaller diameter.
Since the coil of wire is much stronger then the can, the can is what gives,
as demonstrated in the pictures.
If less energy is used (by charging the capacitor to less than 12,500 volts), the can
will not be crushed as much...
This can was crushed with an 800 joule pulse.
The thick aluminum used for the top and the bottom of the can provides a lot of
support to the can while it is being crushed. If both the top and the bottom
of the can are cut off (using scisors) before the can is crushed, then the can will be much more
thoroghly crushed...
This can was crushed with a 1000 joule pulse (with the top and bottom removed first).
The power supply used to charge the capacitor consists of a full-wave rectified 15 Kv
neon sign transformer regulated with a variac and a 50 Kv volt meter, with a 1K resistor
on each output leg to protect the diodes from spikes. The high current switch used to
fire the device consists of a spark gap with an electrode spacing of about 1/20 inch.
The space between the electrodes is filled with about 10 sheets of
the 4 mil polyethylene plastic sheeting mentioned above, hooked to a long string.
The sheets (which hang over the edges of the gap by 2 or 3 inches in each direction)
prevent the high voltage from prematurely closing the switch). The string allows
the experimenter to close the switch from a distance, simply by yanking the plastic
out of the spark gap - at which point the high voltage easily breaks down the gap
and closes the switch.
To operate the device, the capacitor is shorted with a shorting bar, an aluminum can
is slipped into the coil, and the shorting bar is removed. The capacitor is then
charged to the desired voltage, and the power supply
is disconnected from the capacitor (to protect the diodes from the enormous surge of firing).
High quality ear muffs are then put on (the spark gap makes a horrendous noise)
and the device is fired by tugging on the string that is hooked to the plastic in the
spark gap. The capacitor is then completely discharged by bridging the terminals
with a shorting bar, and the can is removed from the coil.
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