A capacitor (formerly known as condenser) is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric (insulator); for example, one common construction consists of metal foils separated by a thin layer of insulating film. Capacitors are widely used as parts of electrical circuits in many common electrical devices.


Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass, in filter networks, for smoothing the output of power supplies, in the resonant circuits that tune radios to particular frequencies and for many other purposes.

Most capacitors have numbers printed on their bodies to indicate their electrical characteristics. Larger capacitors like electrolytics usually display the actual capacitance together with the unit (for example, 220 μF). Smaller capacitors like ceramics, however, use a shorthand consisting of three numbers and a letter, where the numbers show the capacitance in pF (calculated as XY x 10Z for the numbers XYZ) and the letter indicates the tolerance (J, K or M for ±5%, ±10% and ±20% respectively).

A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporary battery.

Groups of large, specially constructed, low-inductance high-voltage capacitors (capacitor banks) are used to supply huge pulses of current for many pulsed power applications.

Reservoir capacitors are used in power supplies where they smooth the output of a full or half wave rectifier. They can also be used in charge pump circuits as the energy storage element in the generation of higher voltages than the input voltage.

In electric power distribution, capacitors are used for power factor correction. Such capacitors often come as three capacitors connected as a three phase load. Usually, the values of these capacitors are given not in farads but rather as a reactive power in volt-amperes reactive (VAr). The purpose is to counteract inductive loading from devices like electric motors and transmission lines to make the load appear to be mostly resistive.

Because capacitors pass AC but block DC signals (when charged up to the applied dc voltage), they are often used to separate the AC and DC components of a signal.

A decoupling capacitor is a capacitor used to protect one part of a circuit from the effect of another, for instance to suppress noise or transients.

When an inductive circuit is opened, the current through the inductance collapses quickly, creating a large voltage across the open circuit of the switch or relay. If the inductance is large enough, the energy will generate a spark, causing the contact points to oxidize, deteriorate, or sometimes weld together, or destroying a solid-state switch. A snubber capacitor across the newly opened circuit creates a path for this impulse to bypass the contact points, thereby preserving their life; these were commonly found in contact breaker ignition systems, for instance.

In single phase squirrel cage motors, the primary winding within the motor housing is not capable of starting a rotational motion on the rotor, but is capable of sustaining one. To start the motor, a secondary “Start” winding has a series non-polarized starting capacitor to introduce a lead in the sinusoidal current.

The energy stored in a capacitor can be used to represent information, either in binary form, as in DRAMs, or in analogue form, as in analog sampled filters and CCDs.

Capacitors and inductors are applied together in tuned circuits to select information in particular frequency bands.

Most capacitors are designed to maintain a fixed physical structure. However, various factors can change the structure of the capacitor, and the resulting change in capacitance can be used to sense those factors.

Changing the dielectric
Changing the distance between the plates
Changing the effective area of the plates

Capacitors may catastrophically fail when subjected to voltages or currents beyond their rating, or as they reach their normal end of life.

Capacitors used in RF or sustained high-current applications can overheat, especially in the center of the capacitor rolls.

Capacitors used within high-energy capacitor banks can violently explode when a short in one capacitor causes sudden dumping of energy stored in the rest of the bank into the failing unit.

High voltage vacuum capacitors can generate soft X-rays even during normal operation.

Proper containment, fusing, and preventive maintenance can help to minimize these hazards.

The capacitor plague (also known as bad capacitors ) was a problem with a large number of premature failures of aluminium electrolytic capacitors with non solid or liquid electrolyte of certain brands especially from Taiwan manufacturers . The first flawed capacitors were seen in 1999, but most of the affected capacitors failed in the early to mid 2000s. They failed in various electronics equipment, particularly motherboards, video cards, compact fluorescent lamp ballasts, LCD monitors, and power supplies of personal computers. News of the failures (usually after a few years of use) forced most manufacturers to repair the defects. The problem seems to be ongoing; faults are still being reported as of 2010

The most common method of identifying capacitors which have failed because of bad electrolyte is visual inspection. The capacitance may degrade to 4% of the original value, as opposed to an expected 50% capacity degradation over the lifetime.

Bulging of the vent on the top of the capacitor.

Sitting crooked on the circuit board as the bottom rubber plug is pushed out.

Electrolyte (a crusty brown substance) leaked onto the motherboard from the base of the capacitor or vented from the top, visible as rust-like brown deposits, or a visible hole in the vent.

High equivalent series resistance (ESR) can be detected with an ESR meter, but this test sometimes cannot be performed in-circuit without disconnecting the capacitor. Motherboard capacitors are typically in parallel with other capacitors, and cannot be measured individually while in-circuit. However, a high in-circuit ESR reading unequivocally indicates failure of the measured capacitor or capacitors.

As the capacitor ages, its capacitance decreases and its ESR increases. When this happens, the capacitors no longer adequately serve their purpose of filtering the direct current voltages on the motherboard, and system instability results. Some common symptoms are:

* Not turning on all the time; having to hit reset or try turning the computer on again
* Instabilities (hangs, BSODs, kernel panics, etc.), especially when symptoms get progressively more frequent over time
* Memory errors, especially ones that get more frequent with time
* Spontaneous reboots
* In case of on-board video cards, unstable image in some video modes
* Failing to complete the POST, or rebooting before it is completed
* Never starting the POST; fans spin but the system appears dead
* Capacitors with high ESR can make power supplies malfunction, sometimes causing further circuit damage. CPU core voltage or other system voltages may fluctuate or go out of range, possibly with an increase in CPU temperature as the core voltage rises.

Unlike the physical signs which are conclusive evidence the capacitors are failing, many of the operational signs may be caused by other factors, such as a failing power supply, dust clogging a fan, bad RAM, or other hardware problems. Instability, once the operating system has loaded, may indicate a software problem (such as some types of malware, poorly-written device drivers or software), and not a hardware problem at all. If any of these symptoms are experienced, removing the system’s case and inspecting the capacitors, especially those around the CPU, may immediately identify capacitors as the cause. If there are no physical signs, an oscilloscope may be used to examine the AC ripple voltage across capacitors during operation, or an ESR meter to measure ESR when powered down; excessive ripple or ESR is a sign that the capacitors are faulty.
















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