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Coilgun I

Introduction

Originally published: 2007

This page documents the build of my first coil gun. It is a 704 joule coilgun, capable of firing a small projectiles at high speeds. A coil gun is a type of a linear electromagnetic accelerator. It fires ferromagnetic projectiles using electromagnetic coils or solenoids.

I have tried to include some supplementary information regarding the basic theory behind a simple single stage coil gun and how to go about building one. It involves basic knowledge of electromagnetic physics and high current/voltage electronics.
Coil gun basics

A coil gun is a circuit comprising of three main components – 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 coil by the switch.

A ferromagnetic projectile is usually used but any ferrous material could be used. A ferromagnetic material is one that interacts with a magnetic field but cannot be magnetised. This projectile is placed within the coil at specific distance from the centre. The capacitor bank is usually comprised of a many smaller capacitors combined in a parallel and series combination to obtain the voltage and capacitance values desired for the bank. The capacitor bank is charged with a DC current via a charging circuit. The capacitor bank is then discharged through the coil via 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 pulse of current to flow from the capacitor bank through the SCR and coil. This is necessary because if the pulse of current through the coil is too long the projectile will oscillate at the centre of the coil. This is because the projectile is being attracted to the centre of the coil in the presence of the magnetic field produced by the high rate of change of current flowing through the coil. The magnetic field ideally needs to be turned off after a particular time to stop the projectile being attracted back to the centre of the coil.

This however causes another problem known as inductive kick-back. Once the current is switched off, the coil has a strong magnetic field present. Coils/solenoids are inductors which like to oppose a change in current and in this case wants to keep current flowing but cannot as the circuit is broken via the switch. The solenoid’s magnetic field collapses into a high voltage pulse which can easily damage other components of the circuit. The solution is to install a power diode and resistor bleeder circuit in parallel with the solenoid to bleed this inductive pulse away.

The circuit diagram below is a simplified circuit diagram and shows the layout of components described above with some example component values.

coilgun_circuit

At some point during the design of a coil gun you will have to play with some equations. When looking at capacitors these are the three main equations you will need.

eq1 eq2 eq3

The capacitor bank energy is proportional to the square of the voltage so an effective way to increase the bank energy is to have a relatively large bank voltage. However increasing the capacitance will ultimately dictate the maximum surge current and its duration when discharged.

The solenoid design relies on a compromise between different variables. It needs to have a low inductance to reduce Ohmic inefficiencies but also needs many turns to increase the magnetic flux and hence magnetic force induced onto the projectile. Looking at the force and inductance equations below it is possible to see how the different variables affect the total force produced. In general increasing the force will also result in increasing the inductance – a compromise of the two has to be made this can be done experimentally or with the aid of some simulations.

eq5 eq4

Force is in Newtons, Inductance is in Henries, A is the cross-sectional area of the coil in square meters, μ0 is the permeability of free space, μ is the permeability of the coil core (in this case it is air: 1), N is the number of turns of wire, I is the current in Amperes, l is the length of the coil in meters.

A really useful tool is Barry’s RLC circuit simulation (Java required). This tool enables you to enter your coil inductance, coil 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 period should be the time it takes for the current to hit it’s first maximum peak.

Building the coil gun

The first thing I did was look at what things restricted my design. Capacitor values are somewhat fixed in terms of voltage and capacitance. Everything else in the circuit could be built around the capacitor bank such as SCR ratings and power supply. I found a listing for a box of 16 new old stock capacitors with a rating of 200VDC and 2200µF which seemed like a good set for a first capacitor bank. I decided to arranged two sets of eight capacitors in parallel, with each set connected in series giving me a combined bank rating of 400VDC at 8800µF (704J).

I laid out the capacitors into two sets of eight capacitors in parallel, with each set connected 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 correctly.

coilgun2

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 box. 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 where the capacitor bank was connected to the charging circuitry.

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 (200VDC which is what I had available) to charge the capacitor bank.

coilgun3 coilgun4

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.

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 labelled A, B, C and D. The outer terminals are labelled 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.

coilgun5 coilgun6

There are two chop block terminals screwed to the top of the switching terminal. The DC input block has two inputs for positive and negative from the charging PSU. The other block terminal has three inputs which is wired from 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 other connections on the larger block terminal are connected to the capacitor bank with a mid-point tap. The bank also has wires connected here which lead to the thyristor and coil.

coilgun7

I needed to start mounting the components onto a solid base, so I cut a base board from plywood (which was later coated with polyurethane varnish). 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. 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 that only cost £35. It is defiantly over rated for my coil gun 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.

coilgun8

The all important coil 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 the coil’s efficiency. Glass has a tendency to shatter under coil compression when fired so I decided against this too. I believe that plastic coil forms are the easiest to work with as they can be easily worked and obtained.

My coil form has an internal diameter of 10mm and is made from clear polystyrene (acrylic could be used also). 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 haven’t verified this. I fixed them in place using epoxy resin.

coilgun9

I have used single core copper mains wire with a PVC coating for the windings. I think using some enamelled copper wire with extra thick coating would also be viable and also improve the winding turn density but I couldn’t source it easily. 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.

coilgun10

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 since secured the coil more permanently with clamping straps and stops. It is important to fix the coil down securely before firing as there is a surprisingly strong recoil when the coil gun is fired which can throw the coil in the opposite direction to the projectile.

coilgun11

A nearby chop 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.

The power 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 tried to over compensate – 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 chop block terminal that was connected to the coil.

coilgun12

The last part to add to the coil gun 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. 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 sacrificial diodes to attempt to protect the circuit from surges or revered polarity. 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. The original circuit diagram of the timing circuit is shown below with red notations notifying the changes that I made.

pulsecircuit coilgun13

For the ferromagnetic projectiles I used lengths of ferrite rod which were obtained from some old radios. Different lengths and profiles will be experimented with later.

Everything was complete so some low powered tests were carried out to test the device before a full power test was successfully undertaken. I will be looking into tuning the circuit and making some tweaks here and there to try and improve the system better in due course.