What are Capacitors?

Capacitors are universally-used passive electrical components that serve to store a defined amount of electrical energy and release it when needed. Originally called condensers, capacitors have been around in one form or another since 1746 and the invention of the Leyden jar. This early capacitive device stored a static charge on metal foil that lined the inside and outside of a glass jar. While many other capacitor types have since been developed, the principle behind their function has essentially remained the same.     

In a capacitor, two electrical conductors, or plates, are separated by an insulator (dielectric). When a power source is connected to the conductors, a current will try to flow between them, but will be hindered by the insulator. This means that the plates will begin accumulating electrons, or electrical charge, until basically no more space is available for additional electrons.

At this point the capacitor is fully charged. Each plate has either a positive or negative (opposite) charge, creating an electric field between them. This potential difference between the plates is voltage, and defines the capacity of the individual capacitor to store charge per unit of voltage. Adding a load to the capacitor (like a camera flash) starts the current flow and leads to discharge for use in a circuit.

Capacitance is the capability of a device to store electrical charge and thus a measure of how much charge a capacitor can carry. The unit for measuring capacitance is the farad (F), which is defined as the capacity to store one coulomb of charge with a potential of one volt. A coulomb (C) is the amount of charge conveyed by a current of one amp in one second. Most capacitors used in electronic devices are rated at just micro fractions of a farad. Higher rated capacitors are used in power electronics.

Capacitors and batteries are very similar in that they both store electrical energy, but they work in very different ways. A battery can both store and produce energy via a chemical reaction producing electrons that flow between terminals and can be released slowly through a circuit. A capacitor does not have the capability to produce electrons. It can only store electrons and release them rapidly, on demand, as electricity. This makes them uniquely suited for powering devices that require an immediate release of energy or continuous cycling.

The power-holding capability of a capacitor can be increased by making the plates larger, by moving the plates closer together, or by improving the properties of the insulator used between plates. Different materials for both the plates and the insulator are used in the manufacture of modern capacitors, giving rise to a variety of designs for different applications. 

Capacitors can be grouped into several categories, depending on materials and function. The broadest categories are fixed capacitors, where the power capacity is not adjustable; and variable capacitors, where the power holding capacity can be altered. Capacitors that are polarized must be oriented correctly in a circuit (DC only). Non-polarized capacitors can be oriented in any direction in a circuit (AC or DC). 

Some of the different types and features of capacitors available today include:

• Air  – use air as the insulator, adjustable, rugged and durable
• Film - most common in use today, uses plastic film as insulator, reliable, stable and low-cost
• Ceramic - use ceramic as the insulator, good frequency response, non-polarized, widely used for high frequency applications
• Mica - use silvered mica as the insulator, low power loss, reliable and stable
• Glass - use glass as the insulator, low power loss, high operating temperature, high voltage applications
• Electrolytic - use electrolyte as the insulator, large capacitance volume
• Super or Ultra - use nanotechnology and ultrathin materials to store large amounts of energy, lower voltage limit, similar to batteries but with fast charge/discharge rates
• Rotary - offers variable capacitance that can be repeatedly adjusted
• Trimmer - offers variable capacitance usually set when initially used in a device and then not varied 

Other criteria that arise in selecting properly-matched capacitors for a circuit design include: device size, manufacturability, operating voltage, frequency response, maximum operating temperatures, aging factors, etc. Usable capacitance of a device can also be affected by outside factors like: temperature, humidity, AC and DC voltage, capacitor age, etc.

Capacitors are used in countless electronic and electrical applications and most frequently in active electronic circuits. They are good for noise filtering, signal filtering, energy storage, and timing. Their ability to charge and discharge rapidly also makes them important components in hybrid and electric motor vehicles.