This is a fun and non-dangeros project for those people who like to
throw projectiles magnetically. It simply works by placing a
ferromagnetic projectile at one end of a coil and pulsing some power in
it. The trick is to switch off power when the projectile is at the
middle of the coil, there are some ways to do it but it isn't important
now. The second trick is to use a coil as close as possible to
projectile to waximize coupling and the third to avoid saturation, that
means keeping the current not to high.
I've been messing with coil launchers for a year. The first model was a
straw with some wire wrapped on it and an electrolityc capacitor (200V)
pulsed in it. Lots of sparks and metal flying but was able to shoot a
nail accross my room. I started esperimentation wrapping wire on glass
and using more litycs charged by mains power, very bulky and
disappointing (100J of energy where only able to blast through 2
sheets). After holydays I started more mature experimentation employing
SCRs (solid state switching) photoflash electrolytics and optical
sensors, and built a bi-coil launcher (2 stages), complicated but
poweful ( blasted through a can) but the timing is critical, so
transient simulations were a must (and a L-C-R meter). The next
launcher will be mosfet-switched with 3 coils and optical sensors, but i
don't have the funds/parts yet to build such an expensive launcher, but
already designed plans and models. For now i decided to build a small
funny coil launcher using one scr and coil on glass. Using my LCR meter
and multisim simulator i designed it for max efficiency.
For first you need some disposable camera flashes, so try to search at
your local photo shop. You need 4 of more of them. Desolder the caps and
collect them. They have ratings of 330V 120-160uF and can survive pulses
up to 300A (each), or even dead shorts (but don't do it because it is
quite a bang). Paralleling them reduces the ESR (internal resistance)
and ESL and highers peak current capability. I used 4 of them and my
coil launcher has a pulse current of 400A (about 100A for cap). The more
caps , the more energy
but more longer pulse, so the additional energy may slow down the
projectile instead of fastening it so I would advice to keep them only
4. I trusted the simulations and have almost broken a window with this
thing ;-) fortunately the curtains slowed it down .
Concerning the charging circuit, use the largest you got, and try to use
more batteries than originally if you want a fast charge. I used a max
kodak disposable flash and it works fire with 4 1.5V batteries
(originally it was designed to use one) and doesn't burn out. The caps
where soldered on a small breadboard with parallel tracks made with lots
of solder (otherwise the solder will blow).
The coil was made with 200 turns distributed on 10 layers (rembember to
insulate eachover) of insulated magnet wire with 1mm diameter on a glass
pipe with 6 mm external diameter and 3 mm internal diameter. Also
plastic works good as barrel but must be hard , metals must be
NON-Ferromagnetic and must have a cut along the barrel lenght (to limit
eddy currents).
The resistance is 350 milliohm and inductance is 165 uH (according to my
LCR meter) giving a pulse lenght of 1 ms (Multisim simulation) and peak
current of 400A, limited by inductance. Using more caps would increase
pulse current because the resistance is low and the current is
Inductance-limited, so i advice to use only 4 caps (maximum 5).
Concerning the switching i used 2N6509 SCRs (Onsemi) (25A 800V) which
have a pulse rating of 300A x 6 ms (450 x 1ms). They are very good cheap
and small and requires small signal to drive. A reverse diode was added
to the caps because the simulations showed
a peak reverse voltage in capacitors (it is an undamped LCR circuit)
that could damage them. This solution limits reverse charging (to -0.5 V
to 1V) and makes the current decay better reducing the such-back effect.
The projectile is a 2.5 mm diameter nail with the same lenght as the
winding.
When placing the projectile the projectile starting position is
critical, make various shots at different positions to see which is
better.
Rembember: this circuit is low power but propels metal projectiles at
speeds of 20-25 m/s and although can make no harm (to skin)
it would break fragile things, also dangerous for eyes so don't point to
peoples's or animals's head . Use it to shoot down small things
or to shoot people's back (but watch who you shoot, some peoples don't
like such jokes and you would be in trouble...)
There is alse electrical shock hazard. This circuit stores 30 or more
Joules at 310V and can shock you badly, use only one hand
and insulate high voltage sections. Be wise, have fun, learn, and be
safe.
Disclaimer: I don't accept any responsibility for the damages done with
this device to peoples, things, animals, fragile things etc , if you
build it, you must accept this condition.
