An Introduction to Capacitors

Like batteries, capacitors store electrical energy. However, unlike batteries they do not produce electrons. Capacitors consist of two terminals, each connected to a metal plate which is separated by a non-conducting substance referred to as a dielectric. The plate that is attached to the:

• Negative terminal of the battery that accepts the electrons that the battery is producing.
• Positive terminal of the battery that loses electrons to the battery.

Once fully charged, capacitors have the same voltage as their associated batteries or other power source.

Application Determined by Dielectric Material

Specific capacitor applications range from small plastic capacitors in hand-held calculators to large ones that power commuter buses to glass capacitors that power space shuttle circuitry and space probes. In fact, the materials used for the dielectric determines each capacitor’s function. For example, the following materials are used as the dielectric in to the various applications:

• Air  in radio tuning circuits,
• Mylar in clocks, alarms and counters,
• Glass in high-voltage applications, and
• Ceramics in antennas, x-ray, and MRI machines.

Capacitor Storage Potential

The storage potential of a capacitor is referred to as capacitance and it  is measured in units called farads. A 1-farad capacitor, typically the size of a one-liter soda bottle, can store one coulomb of charge at one volt; and a coulomb is equal to 6.25 billion-billion electrons). Thus, capacitors are typically measured in microfarads or millionths of a farad.

Capacitor Applications

The primary characteristic of a capacitor over a battery is that a capacitor can release its entire charge virtually instantaneously, where as a battery typically takes minutes to hours to complete the discharge. For example, the electronic flash on a camera uses a capacitor where the battery charges up the flash’s capacitor over several seconds, and the capacitor releases the full charge into the flash tube instantaneously. With this concept in mind, the following summarizes the various ways capacitors are used in circuits:

• Store charge for high-speed uses (flash)
• Eliminate fluctuations in lines carrying DC voltages, smoothing the voltage by absorbing the peaks and filling in the valleys
• Block DC voltage while facilitating the flow of any AC signal unimpeded