Motion System Design

All charged up

A capacitor is an electrical circuit element that consists of a pair of metal plates separated by a dielectric, an electrically insulating material. Capacitors are so named because of their capacity to store charge.Forcing electrical current through an uncharged capacitor initiates a process whereby the metal plates fill with charge, thus building up an electric potential or voltage. Think of a deep hole in a dry riverbed. As water begins to flow, the hole must fill completely before it releases additional water downstream. If upstream flow stops abruptly, the hole continues to drain until it is empty. Such is the effect of a capacitor on current transients, or fluctuations, in an electrical circuit.

The key to understanding how a capacitor works is the electric field that develops in the space between the metal plates, which separates positive charge to one side and an equal amount of negative charge to the other. The electric field and corresponding voltage potential reach their maximum when the capacitor is fully charged. In the sense that a capacitor is a voltage-storing device, it also functions somewhat like a battery — one that charges and discharges in milliseconds rather than hours.

Capacitor construction

After resistors, capacitors are the most commonly purchased passive components. They are classified as either electrostatic or electrolytic. Electrostatic capacitors contain dielectrics made of plastic film, ceramic, glass, or mica. Electrolytic capacitors have thin, oxide dielectrics made by electrochemical processing.

A typical electrostatic capacitor is made by rolling a plastic dielectric film and metal foil into a cylinder, and then affixing leads. Another common manufacturing method is to cut and stack metallized foil, and attach leads to that. After rolling or stacking, the foil is uniformly coated or dipped in insulating plastic that serves as the dielectric.

A wet-anode tantalum construction — common in electrolytic capacitors — is made from a porous tantalum pellet that forms by pressing finely ground tantalum powder and binder in a mold and firing into a vacuum furnace at 2,000°C. Heat welds (sinters) the powder into a solid sponge-like pellet. Then, a film of tantalum oxide is grown electrochemically on the pellet and electrolyte added. Metal foils are acid etched to make them porous.

Calculating capacitance

Capacitance is defined mathematically as C = Q/V, where C is expressed in farads (after Michael Faraday), Q in coulombs, and V in volts. For parallel plates in a vacuum, C = 0(A/d), where 0 = permittivity constant of free space (8.85 × 10-12F/m)

A = total plate area

d = distance between them

Ultimately, a conductor's shape, size, and dielectric material determine capacitance.

What's a dielectric?

Dielectrics are the electrically insulating materials that separate a capacitor's metal plates. They can sustain force from an applied electric field and store it as energy for use once the field is removed. Their main jobs are to maximize the capacitor's charge and energy storage, increase capacitance (more than a vacuum), and prevent the two metal plates from touching.

Their ability to store energy is expressed as K = C/C0 = V/V0 where K is a dielectric constant, C and V are capacitance and voltage with a dielectric present, and C0 and V0 are values before adding a dielectric.

Glass 5.0 to 10.0
Mica 2.6 to 9.2
Mylar 3.1
Paraffin 2.0 to 3.0
Porcelain 5.0 to 6.5
Titanium dioxide 100.0
Vacuum about 1.0
High dielectric constants are useful in high-value capacitors with small physical volume. Note that all dielectric constants depend on temperature.

Picture this

One application where capacitors are sure to draw smiles is in disposable cameras. Here, capacitors help deliver the huge burst of charge needed to ignite the flash tube. Remarkably, the capacitor-based circuit can charge to hundreds of volts from a 1.5-V battery.

The circuit, which also employs an inductor, moves energy from one component to another, each time accumulating more stored charge. Unlike capacitors, which store power in electric fields, inductors store energy in magnetic fields. In successive cycles, the inductor absorbs power from the battery then dumps it (in the form of current) into the capacitor.

Diodes and transistors act as switches and valves, making sure current flows exactly where it's supposed to during each phase of the cycle. They also isolate the capacitor from the battery, which is why the capacitor is able to charge to voltage levels several hundred times that of the battery. When the picture button is pressed, the floodgates open, and energy stored in the capacitor rushes into the flash tube.

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