A capacitor is an electrical device that has, in DC circuits, the purpose
of storing energy. In particular, it stores an electrical charge.What is a capacitor? Why do we use them in a coilgun? How can you put some little capacitors together to make a big capacitor?
A capacitor is an electrical device that has, in DC circuits, the purpose
of storing energy. In particular, it stores an electrical charge.
Suppose two flat metal plates are placed close to each other (but not touching) and are connected to a battery through a switch. At the instant the switch is closed, electrons are attracted from the upper plate to the positive terminal of the battery, and the same number are repelled into the lower plate from the negative battery terminal. Enough electrons move into one plate and out of the other to make the voltage between them the same as the voltage of the battery.
If the switch is opened after the plates have been charged in this way, the top plate is left with a deficiency of electrons and the bottom plate with an excess. Since there is no current path between the two, the plates remain charged despite the fact that the battery no longer is connected.
You can do a simple experiment with a large capacitor, such as a large electrolytic capacitor from a power supply (or a coilgun!). Charge it up to ten or twenty volts. Disconnect the power. When you measure the voltage on the capacitor's terminals, you find it may remain for a very long time, perhaps even days or months! I have been shocked (literally) to find a capacitor in a surplus parts store that spit sparks when I shorted the terminals together, and who knows how many months it had been sitting there!
The basic unit of capacitance is the farad, but this is much too large for practical work. (Except possibly for car stereos with thundering bass boom boxes!) Capacitance is usually measured in microfarads (abbreviated uF or mfd) or picofarads (pF).
The microfarad is one millionth of a farad (10-6 F, and the picofarad is one-millionth of a microfarad (10-12 F).
Capacitors
nearly always have more than two plates, the alternate plates being
connected together to form two sets, as shown here. This makes it possible
to attain a fairly large capacitance in a small space, since several plates
of smaller individual area can be stacked to form the equivalent of a single
large plate of the same total area. Also, all pates except the two on the
ends are exposed to plates of the other group on both sides, and so are
twice as effective.
The electrolytic capacitor uses aluminum-foil plates with a conducting semi-liquid chemical compound between them. The actual dielectric is a very thin film of insulating material that forms on one set of plates through electro-chemical action when a dc voltage is applied. The capacitance obtained with a given plate area in an electrolytic capacitor is very large, because the film is so thin. Much thinner than anything practical with a solid dielectric. However, this also causes the breakdown voltage to be much lower than with solid dielectrics. The electrolyte is necessarily an acid; therefore it is extremely dangerous if heat builds up inside an electrolytic capacitor. It can bulge, leak and even explode. Look carefully at your higher quality electrolytic capacitors, and you may see a scoring mark 'X' at the terminal end which is designed to rupture and leak before the can explodes.
The larger the plate area and the smaller the spacing between the plates, the greater the capacitance. The capacitance also depends on the kind of insulating material between the plates; it is smallest with air insulation. Substituting other insulating materials for air may increase the capacitance many times.
The ratio of the capacitance with some material other than air between the plates, to the capacitance with air between the plates, is called the dielectric constant of the insulating material. The material itself is called a dielectric.
The dielectric constants of a number of common materials in capacitors are shown in this table. For example, if a sheet of polystyrene is substituted for air between the plates of a capacitor, the capacitance will be increased 2.6 times.
| Material | Dielectric Constant* | Puncture Voltage** |
| Air | 1.0 | 21 |
| Alsimag 196 | 5.7 | 240 |
| Bakelite | 4.4 - 5.4 | 300 |
| Bakelite, mica filled | 4.7 | 325-375 |
| Cellulose acetate | 3.3 - 3.9 | 250 - 600 |
| Fiber | 5 - 7.5 | 150 - 180 |
| Formica | 4.6 - 4.9 | 450 |
| Glass, window | 7.6 - 8 | 200 - 250 |
| Glass, Pyrex | 4.8 | 335 |
| Mica, ruby | 5.4 | 3800 - 5600 |
| Mycalex | 7.4 | 250 |
| Paper, Royalgrey | 3.0 | 200 |
| Plexiglass | 2.8 | 990 |
| Polyethylene | 2.3 | 1200 |
| Polystyrene | 2.6 | 500 - 700 |
| Porcelain | 5.1 - 5.9 | 40 - 100 |
| Quartz, fused | 3.8 | 1000 |
| Steatite, low loss | 5.8 | 150 - 315 |
| Teflon | 2.1 | 1000 - 2000 |
| *At 1MHz | **In volts/mil (0.001 inch) |
The charge or quantity of electricity that can be held in the electric field between the capacitor plates is proportional to the applied voltage and to the capacitance of the capacitor: Q = C * V where
The energy stored in a capacitor is also a function of voltage and capacitance: W = V2 * C / 2 where
A capacitor stores energy, and we can use it to make a quick and powerful electric discharge to propel our projectile. Why not use a large battery? Because the internal resistance of a battery is much higher than a capacitor. It takes a much longer time to get the same amount of energy from a battery, and we will see that timing is very important. In fact, using a different combination of capacitors can give us the exact control we need over this delicate issue.
Charging capacitors is really not an issue... Just insert them in a circuit with a voltage that does not exceed the capacitor's voltage rating. Wait till current stops flowing, and your caps are fully charged.
Discharge time is the real trick! Here's what we are trying to do: Have as much current as possible until the projectile is halfway down the tube. Anything longer than that is not just a waste of energy, it actually hurts performance! If some current is still in the coil when the projectile goes past the middle it will actually PULL THE PROJECTILE BACKWARDS. (In highly technical terms this is known as "the suckback effect".) Thus it will at least slow it down, if not pulling it back into the coil. On the other side, if current dies down before the projectile is halfway through the coil it will reduce efficiency. If you have to go for a comprise, be sure to opt for the second choice though.
Experiment to find what works best for you, and let us know!
Credits: Many thanks to Filipo for providing a rough draft of this material!