What Is a Capacitor?

What Is a Capacitor?

A capacitor is an electronic device that can store energy in the electric field between a pair of closely-spaced conductors (called 'plates'). When voltage is applied to a capacitor, charges of equal magnitude but opposite polarity build up on the plates. Capacitors are used in many different circuits for many purposes, including filtering, energy storage, decoupling, and timing circuits.

When you plug a battery into a capacitor, electricity instantly accumulates on the plate. This is called 'charging up the capacitor', and it happens when the voltage of the battery goes from zero to the operating voltage of the capacitor. Once the capacitor is charged up, it will no longer allow any additional electric current to flow through it, as the electronics accumulated on each plate repel new electronics from entering the plates. This is the fundamental principle of a capacitor, and it was discovered by Benjamin Franklin in his Leyden jar experiments. This phenomenon also inspired Michael Faraday to develop the first practical capacitors out of large oil barrels. The capacitance of a capacitor is measured in farads, named in honor of the great English physicist.

One of the most familiar applications of capacitors is in power supplies, where they help to smooth out fluctuations in the output voltage. However, they are also used to block the flow of direct current (a steady one-directional flow of electrons) and let alternating current pass through unimpeded. This effect is exploited by electronic devices such as radio receivers and speakers to tune them to specific frequencies.

In other applications, capacitors can detect mechanical changes such as air humidity or fuel levels. They can also be used to separate low frequency signals from higher-frequency ones, for example in crossover circuits within speaker systems. Capacitors can even be used to encode data as binary numbers, an effect exploited by Dynamic Random Access Memory (DRAM) devices.

The voltage required to charge a capacitor depends on the size of the electrodes, the material between them, and the temperature of the surroundings. The larger the capacitor, the more electrical energy it can hold. Smaller capacitors are usually rated in microfarads (uF), nanofarads (nF), or picofarads (pF). High-voltage DC capacitors are often pre-charged to limit the in-rush current at power-up, thus prolonging their life and mitigating high-voltage hazards. This is especially important for HVDC capacitors, as the sudden release of the stored charge can cause dangerous flashes and explosions if it happens too quickly. Moreover, the rapid discharge can damage sensitive components. To prevent this, a resistor or other component can be placed in series with the capacitor to slow down its rate of discharge.