Applications of capacitors


Capacitors have many uses in electronic and electrical systems. They are so ubiquitous that it is rare that an electrical product does not include at least one for some purpose. Capacitors allow only AC signals to pass when they are charged blocking DC signals.
The main components of filters are capacitors.
Capacitors have the ability to connect one circuit segment to another.
Capacitors are used by Dynamic Random Access Memory devices to represent binary information as bits.

Energy storage

A capacitor can store electric energy when it is connected to its charging circuit and when it is disconnected from its charging circuit, it can dissipate that stored energy, so it can be used as a temporary battery. Capacitors are commonly used in electronic devices to maintain power supply while batteries are being changed.
Conventional electrostatic capacitors provide less than 360 joules per kilogram of energy density, while capacitors using developing technology can provide more than 2.52 kilo joules per kilogram.
In car audio systems, large capacitors store energy for the amplifier to use on demand.
An uninterruptible power supply can be equipped with maintenance-free capacitors to extend service life.

Pulsed power and weapons

Groups of large, specially constructed, low-inductance high-voltage capacitors are used to supply huge pulses of current for many pulsed power applications. These include electromagnetic forming, Marx generators, pulsed lasers, pulse forming networks, fusion research, and particle accelerators.
Large capacitor banks are used as energy sources for the exploding-bridgewire detonators or slapper detonators in nuclear weapons and other specialty weapons. Experimental work is under way using banks of capacitors as power sources for electromagnetic armour and electromagnetic railguns or coilguns.

Power conditioning

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.
Capacitors are connected in parallel with the DC power circuits of most electronic devices to smooth current fluctuations for signal or control circuits. Audio equipment, for example, uses several capacitors in this way, to shunt away power line hum before it gets into the signal circuitry. The capacitors act as a local reserve for the DC power source, and bypass AC currents from the power supply. This is used in car audio applications, when a stiffening capacitor compensates for the inductance and resistance of the leads to the lead-acid car battery.

Power factor correction

In electric power distribution, capacitors are used for power factor correction. Such capacitors often come as three capacitors connected as a three-phase Electrical load. Usually, the values of these capacitors are given not in farads but rather as a reactive power in volt-amperes reactive. The purpose is to counteract inductive loading from devices like Induction motors, electric motors and transmission lines to make the load appear primarily resistive. Individual motor or lamp loads may have capacitors for power factor correction, or larger sets of capacitors may be installed at a load centre within a building or in a large utility electrical substation. In high-voltage direct current transmission systems, power factor correction capacitors may have tuning inductors to suppress harmonic currents that would otherwise be injected into the AC power system.

Suppression and coupling

Capacitors used for suppressing undesirable frequencies are sometimes called filter capacitors. They are common in electrical and electronic equipment, and cover a number of applications, such as:
Because capacitors pass AC but block DC signals, they are often used to separate the AC and DC components of a signal. This method is known as AC coupling or "capacitive coupling". Here, a large value of capacitance, whose value need not be accurately controlled, but whose reactance is small at the signal frequency, is employed.

Decoupling

A decoupling capacitor is a capacitor used to decouple one part of a circuit from another. Noise caused by other circuit elements is shunted through the capacitor, reducing the effect they have on the rest of the circuit. It is most commonly used between the power supply and ground.
For higher frequencies an alternative name is bypass capacitor as it is used to bypass the power supply or other high impedance component of a circuit.

High-pass and low-pass filters

A high-pass filter is an electronic filter that passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency depends on the filter design. A high-pass filter is usually modeled as a linear time-invariant system. It is sometimes called a low-cut filter or bass-cut filter. High-pass filters have many uses, such as blocking DC from circuitry sensitive to non-zero average voltages or radio frequency devices. They can also be used in conjunction with a low-pass filter to produce a bandpass filter.
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter design. The filter is sometimes called a high-cut filter, or treble-cut filter in audio applications. A low-pass filter is the complement of a high-pass filter.

Noise filters and snubbers

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 an electric spark, causing the contact points to oxidize, deteriorate, or sometimes weld together, or destroy 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. Similarly, in smaller scale circuits, the spark may not be enough to damage the switch but will still radiate undesirable radio frequency interference, which a filter capacitor absorbs. Snubber capacitors are usually employed with a low-value resistor in series, to dissipate energy and minimize RFI. Such resistor-capacitor combinations are available in a single package.
Capacitors are also used in parallel to interrupt units of a high-voltage circuit breaker to distribute the voltage between these units equally. In this case, they are called grading capacitors.
In schematic diagrams, a capacitor used primarily for DC charge storage is often drawn vertically in circuit diagrams with the lower, more negative, plate drawn as an arc. The straight plate indicates the positive terminal of the device if it is polarized.

DC motor suppression

Ceramic disc capacitors are usually used in snubber circuits for low voltage motors for their low inductance and low cost.

Switched mode power supply filtering

Low ESR electrolytes are often required to handle the high ripple current.

Mains filtering

Mains filter capacitors are usually encapsulated wound-plastic-film types, since these deliver high voltage rating at low cost, and may be made self-healing and fusible. Mains filter capacitors are often ceramic RFI/EMI suppression capacitors. The additional safety requirements for mains filtering are:
  • Line to neutral capacitors are flame retardant, and in Europe are required to use class X dielectrics.
  • Line or neutral to earth: Must be flame retardant; also, the dielectric must be self healing and fusible. In Europe these are class Y capacitors.

    Power rail filtering

s are usually used due to high capacity at low cost and low size. Smaller non-electrolytics may be paralleled with these to compensate for electrolytics' poor performance at high frequencies.
Computers use large numbers of filter capacitors, making size an important factor. Solid tantalum and wet tantalum capacitors offer some of the best CV performance in some of the most volumetrically efficient packaging available. High currents and low voltages also make low equivalent series resistance important. Solid tantalum capacitors offer low ESR versions that can often meet ESR requirements but they are not the lowest ESR option among all capacitors. Solid tantalums have an additional issue which must be addressed during the design stage. Solid tantalum capacitors must be voltage derated in all applications. A 50% voltage derating is recommended and generally accepted as the industry standard; e.g. a 50V solid tantalum capacitor should never be exposed to an actual application voltage above 25V. Solid tantalum capacitors are very reliable components if the proper care is taken and all design guidelines are carefully followed. Unfortunately, the failure mechanism for a solid tantalum capacitor is a short which will result in a violent flaring up and smoking on a PCB capable of damaging other components in close proximity as well as completely destroying the capacitor. Fortunately, most solid tantalum capacitor failures will be immediate and very evident. Once in application solid tantalum capacitor performance will improve over time and the chances of a failure due to component mis-manufacturing decrease. Wet tantalums are a type of the electrolytic capacitor, using a tantalum pellet in an electrolytic material sealed in a hermetic package. This type of tantalum capacitor does not require the same derating that a solid tantalum does and its failure mechanism is open. A 10% to 20% voltage derating curve is recommended for wet tantalums when operating from 85C to 125C. Wet tantalums are not commonly referred to as just 'electrolytics' because usually 'electrolytic' refers to aluminium electrolytics.