A coilgun is a circuit comprising basically of a powerful capacitor bank, a high
current switch and a coil of wire (or solenoid) from which the projectile is
lunched. The capacitor is charged and then discharged
through the solenoid by the switch.
Going into a bit more detail, a ferromagnetic projectile is usually used. A ferromagnetic material is one that
interacts with a magnetic field but cannot be magnetised.
This projectile is placed within the solenoid at some distance from the centre.
The capacitor bank is usually comprised of a few smaller capacitors combined in
a parallel and series combination to obtain the voltage and capacitance values
desired. The capacitor bank is charged with a DC current. The capacitor bank is
then discharged through a high current switch usually a semiconductor switch in
the form of an SCR (Silicon Controlled Rectifier) or thyristor.
A pulse generator circuit is often needed to activate the SCR by sending a low voltage
signal to the SCR with a particular pulse width which will then allow a
particular pulse of current to flow from the capacitor bank through the SCR to
the solenoid. This is necessary because if the pulse of current through the
solenoid is too long the projectile will oscillate at the centre of the
solenoid. This is because the projectile is being attracted to the centre of the
solenoid in the presence of the magnetic field produced by the high rate of
change of current flowing through the solenoid. The magnetic field must be
turned off after a particular time to stop the projectile being attracted back
to the centre of the solenoid.
This however cause another problem in that once the current is switched off the solenoid has a strong magnetic field
present and the solenoid being an inductor likes to keep current constant or
moving and as there is no current is passing through the solenoid the inductor's
magnetic field collapses into a high current pulse which can then easily damage
the capacitor bank and SCR, so a diode and resistor bleeder circuit is required in parallel
with the solenoid to bleed this high current pulse away. The solenoid needs to have a low
inductance but requires as many turns as possible to increase the magnetic field
produced, this dilemma is discussed with the force equation below.
The circuit diagram below is a slightly simplified version of my circuit diagram and shows the layout of components described
above which should help with making the description easier to understand. Feel
free to contact me via my contact page if you have any questions.
There are a couple of equations that you might want to use when
designing your coilgun. I found and purchased my capacitors first and built the rest
of the coilgun after. This enabled me to calculate what size of thyristor and solenoid
I would need. I bought sixteen capacitors rated at 200V DC at 2200µF. I
arranged two sets of eight capacitors in parallel, in series giving me a
combined rating of 400V DC at 8800µF. The equations needed to calculate
arrangements of capacitors are shown below.
Designing the solenoid requires another two equations that have many design variables
to allow you to create an efficient solenoid. The equations have
many design variables that allow you to investigate what factors of its design
will improve its performance. You want to obtain the highest force possible but
keeping in mind you also want the lowest inductance possible, one variable may
increase its force yet also increase the inductance - you need to make a bit of a compromised
guess as to what is best or to try out different coils for yourself and seeing
which gives you the best result.
Force in Newtons, Inductance in Henries, A is the end area of the
solenoid in m
2, μ
0 is the permeability of free space, μ is the permeability of
the solenoid core (in this case it is air: 1), N is the number of turns of wire,
I is the current in Amperes, l (lower case L) is the length of the coil in
meters.
A really useful tool is
Barry's
RLC modeling simulation. This enables you to enter your solenoid inductance,
solenoid resistance, capacitor bank capacitance, and capacitor bank voltage to
simulate the resonance of the circuit and its damping. It is useful when finding
out what pulse is needed when operating the thyristor, this time should be the
time taken when the current is at its first peak, in my case it is about 1.5ms.
I started off by connecting up my capacitors into formation. I laid out the
capacitors into two sets of eight capacitors in parallel, in series. I stripped
some lengths of single core copper mains wire and straightened them out. I then
soldered each capacitor terminal to a length of this wire until they were all
connected.
Your capacitor bank should have a relatively high voltage
rating around 100 to 400 volts, and also have a capacitance value as high as possible,
somewhere around 5,000 to 30,000µF. The higher the capacitance the more firings you
can get with a single charge, enabling pulsed or rapid firing. The problem with getting
very high capacitance rated capacitors at these particular voltages is that they
become very large and rather expensive. My capacitor bank cost
about £20 in total, as I got the capacitors really cheap from ebay. The bank is rated at 400V
DC and 8800µF.
I made a switching terminal where the charging switch and
firing switch would be located. I designed a net and cut it out on a sheet of
ridged polystyrene sheet. I then used a strip heater to bend the net at the
marked fold lines to form the terminal. I then cut the holes where the switches
were to be inserted. I also screwed a set of chop block connectors to the top of
the terminal.
I decided that I would separate the charging of each side of the capacitor bank
so that I could first charge one set of eight capacitors at 200V and then the other side.
This was because I preferred the idea of using a lower rated power supply to
charge the capacitor bank.
For this I needed a four gang three pole switch. I
wired the switch up so that a DC line input could be switched off, charge the left bank
or charge the right bank. Basically, in position one the switch would not charge the
bank at all, position two it would charge the left bank of capacitors and in the
third position it would charge the right bank. Its a little confusing but just
requires a little thinking.
This switch comprises of four inner terminals and twelve outer
terminals. Each inner terminal is associated with three consecutive outer
terminals. The switch has three positions, each switch position connects all
inner terminals to one of their three associated outer terminals. The inner
terminals are labeled A, B, C and D. The outer terminals are labeled 1 to 12. So
terminal A is associated with outer terminals 1,2 and 3 and the continuity
between A and one of the other three terminals depends on the position of the
switch.
There are two choc block terminals screwed to
the top of the switching terminal. The DC input block has two inputs for
positive and negative. The other block terminal has three inputs, however I have
wired it with the four gang three pole switch (described above) such that the DC
input charges one set of capacitors and then either the two sets of capacitors.
The outer connections on the larger block terminal are always the same, the top
one being negative and the bottom one being positive. the central connection
changes depending on the position of the switch, positive in conjunction with
the negative top connection and positive in conjunction with the positive bottom
connection. The top and bottom connections also have wires leading out to the
thyristor and solenoid (for discharge/firing). The photo may help with this
explanation.
I needed to start mounting the components
onto a solid base, so I cut a base board from plywood. The capacitor bank and
switching terminal were connected together and placed on the board and screwed
into place. I used four small wood screws to screw the switching terminal to the
base, and for the capacitor bank I cut large washers from a sheet of ridged polystyrene
and screwed long screws directly to the base board clamping the capacitors down
with the large washers.
The next thing was to mount my thyristor which was another great find on ebay and cost about £35. It is defiantly over rated
for my coilgun but I decided I could always use it for bigger experiments later
on. It is a Westcode P300KH08EJ0 rated at 300A (average current), 550A (RMS
current), 10,450A (surge current), 800V (average voltage), 25µS (turn-off
time), 300mA (gate current). There were holes in the corners of the base of the
thyristor so I simply screwed it down to the base board with wood screws and
washers. The base of the thyristor is also the anode and the positive wire
from the capacitor bank is connected to the base here as well. The cathode of
the thyristor (the very thick red wire coming out of the top of the component)
was bolted to an L bracket which was also screwed to the base board.
The all important solenoid was now needed. The coil needs to be wound on a tube
called a coil form. I have found many different coil forms on the internet
ranging from glass, brass and plastic. Metallic coil forms are not advised as eddie
currents are created when in operation that oppose the magnetic field that
created them, reducing their efficiency. Glass has a tendency to shatter under coil compression when fired
so I would advise against this too. I think plastic coil forms are the easiest
to work with as they can be easily worked and obtained.
My coil form has an internal measurement of 10mm Ø and is a piston tube from a science kit. The tube had grooves at
each end that allowed me to include end guides that were large external cir
clips that coincidently fitted perfectly in the grooves on the tube. I have read
that metal end guides also improve the magnetic field strength of the coil in
operation but I am unsure about this. I fixed them in place using epoxy resin.
I have used single core copper mains wire for the windings. It is important
to work out the length of wire needed before cutting a certain length and before
winding. My coil is made up of four layers, each layer consisting of about 30
turns. The windings were secured by tightly wrapping the outer windings with
insulation tape.
I cut two 'V' blocks from some wood and screwed them
to the base board. This allowed me to mount the solenoid but to also allow other
different sized coils to be mounted on the same 'V' block mounting that may be used in the future.
The picture only shows the temporary fixing of the solenoid using insulation tape, but
I have now drilled holes in the
sides of the 'V' blocks and threaded cable ties through and over the coil to permanently
fix it into place securely. It is important to fix the coil down securely
before firing as there is a surprisingly strong recoil when the coilgun is fired
which can fire the coil in the opposite direction which could be dangerous.
A nearby choc block was screwed to the base board and the solenoid wires were
connected to it along with the wires from the thyristor cathode and negative wire from the
capacitor bank. Refer to your circuit diagram carefully when wiring up the
circuit - you don't want to get it wrong and blow up your nice new thyristor or expensive
capacitor bank!
The diode and resistor bleeder circuit protection was
to be built next. I was unable to work out the exact maximum current that this circuit
would need to handle so I over compensated just to be safe. I soldered three
P600J rectifier diodes rated at a total of 1200A surge current together in parallel.
I also bought a power resistor rated at 2.2 ohms at 26W. This was soldered in
series with the diode network. I then screwed the diode to a ridged polystyrene
sheet which was also screwed down to the base board. This simple circuit was then
wired in parallel with the solenoid via the choc block terminal that was
connected to the solenoid.
The last part to add to the coilgun was the pulse
generator circuit which operates the thyristor. This circuit activates the thyristor
for a certain amount of time set by this circuit. I have used a variable timer
circuit kit based on a 555 timer chip purchased from Maplin Electronics. The circuit had to be customised
do what I wanted. The original circuit diagram of the timing circuit is shown
below with red notations notifying the changes to be made that are discussed
below.
Firstly, I wanted the pulsed signal to go to the thyristor and not
operate the relay supplied, so I didn't use the relay at all, I just
added two connections from Co2 and Co1 on the circuit board which go to the thyristor.
Co2 goes to the thyristor gate and Co1 goes to the thyristor cathode. In those connections
I included a couple of diodes just for safety's sake for this circuit. The second
modification was to do with the minimum output pulse of the circuit. It was
originally 0.5 seconds, I needed about a 1.5ms pulse. So to change this I needed
to change a capacitor value of the pulse generator circuit. I replaced C3 in the
circuit diagram above with a 0.1µF capacitor. This changed the RC characteristics
of the circuit and with a bit of testing and RC modeling I could calibrate the variable
potentiometers to give me an output pulse of about 1.5ms. The circuit is
operated by a 9V battery (PP3) which has a series push button switch added, that
is located on the switch terminal (this is the firing button). This circuit board was then screwed to the case
board as well.
Everything should be now be complete, so some low powered tests can be carried out, however
a check of the wiring should be carried out first. There is nothing more annoying in trying to
find an illusive misconnection that is stopping the device from working or worst
still blowing up some vital component.
For the ferromagnetic projectiles I used lengths of ferrite rod which are
easily obtainable from electrical shops, the internet and old radios. Try
experimenting with different lengths and sizes of projectile to get the best
results.
This is just a short guide to operating the device.
• Make sure the circuit is correctly set up.
• Connect the power supply and charge the capacitor bank.
• Insert a projectile and fire.