Function of capacitors
Energy storage
When the capacitor is separated from its charging line, the capacitor stores energy, so it can be used as a battery to provide short-term power. Capacitors are commonly used in electronic devices that are used with batteries. They provide power when replacing batteries to avoid the loss of stored data due to lack of power.
Capacitors are also commonly used in power supplies to mitigate the output of full-bridge or half-bridge rectifiers. Capacitors can also be used in charge pump circuits to store energy to generate a voltage higher than the input voltage.
In many electronic devices and larger power systems such as factories, capacitors and power circuits are often connected in parallel to provide a "clean" power supply for signal or control circuits. For example, the audio system uses several capacitors to remove the 60 Hz signal coming from the power line. Capacitance can store DC power, and make the AC current generated by the power circuit a bypass path. In vehicle audio system, capacitors are often used to compensate for the shortage of instantaneous output power of batteries.
Power Factor Correction (Improvement)
Capacitors can be used in situations where power factor correction is required. In this case, three capacitors are often used in conjunction with three-phase loads. In this case, the unit of capacitor does not need farad calculation, but uses Reactive Power, the unit of capacitor is VAr. The purpose of adding capacitors is to offset the influence of inductance loads such as motors or fluorescent lamps and make the loads as close as possible to resistive loads.
Filtration and filtering
Signal Coupling
Because capacitors block the passage of DC signals, capacitors are often used to filter the DC part of signals, leaving only AC signals, called AC coupling (sometimes transformers are used to achieve similar purposes). The capacitor used for AC coupling will have larger capacitance, and its capacitance value does not need to be very precise, but when the AC component of the signal flows through, the capacitance needs to have a low inductance value. For this purpose, capacitors designed to pass through a metal control panel are called piercing capacitors, which differ slightly from the symbols of other capacitors on a circuit diagram.
Noise filter, motor starter and shock absorber
When the inductor has current flowing through and the instantaneous switch opens, the inductor current drops to zero instantaneously because the switch cannot flow through the current, which will generate high voltage at both ends of the switch or relay. If the inductance is large, the energy will generate sparks, which will make the contacts oxidize or melt, or cause damage to the solid-state switch. If the Snubber capacitor is connected in parallel beside the switch, the inductance current path can be provided when the switch is open, and the service life of the switch can be prolonged. For example, the circuit breaker of the automobile ignition system will be connected in parallel with a buffer capacitor.
In low-power systems, sparks will not cause switch damage, but high voltage will produce radio frequency interference (RFI). If buffer capacitors are installed, the interference caused by open-circuit switching can be reduced. Buffer capacitors are usually connected in series with low resistance resistors, which can consume energy and reduce radio frequency interference.
The induction motor needs a rotating magnetic field whose angle varies with time to work properly. Three-phase induction motor can generate rotating magnetic field directly from three-phase power supply. If single-phase induction motor is used, a capacitor should be installed when starting. The phase difference between capacitor and motor inductance is used to generate rotating magnetic field to start the motor. This capacitor is called starting capacitor.
signal processing
Energy stored in capacitors can be used to express information, such as binary form in computers, or analog form in switched capacitor circuits and bucket-brigade delay lines. Capacitors can be used in analog circuits as integrators or more complex filters, as well as in negative feedback loop stability. Signal processing circuits also use capacitors to integrate circuit signals.
Tuning circuit
Capacitors and inductors are used in tuning circuits to select signals within a fixed frequency range. For example, the receiver of a radio uses a variable capacitor to adjust the frequency of reception.
Sensor application
Most applications of capacitors do not change their physical structure, but use the characteristics of capacitors to change voltage or current. However, capacitors can also be used in sensing applications if the physical or electronic properties of dielectrics are changed at a fixed voltage. If air can penetrate into the dielectric of capacitors, the humidity of air can be measured by capacitors. Capacitors made of flexible plates can measure stress or pressure. In a capacitive microphone, one end of the capacitor can be displaced with the air pressure, the other end is fixed, and the capacitor can be used as a sound sensor.
Some accelerometers use microelectromechanical capacitors etched on the chip to measure the direction and magnitude of acceleration. This is used in tiltmeters or car airbag sensors to measure acceleration changes.
Pulse Power and Weapon Application
Capacitor banks with low inductance and high voltage resistance are often used to provide high current for pulse power applications. These applications include electromagnetic forming, Marx pulse generator, pulse laser (especially TEA laser), pulse forming network, radar, nuclear fusion research and particle accelerator.
Large capacitance packs are used as bridge explosives, detonation devices in nuclear weapons and other special weapons. Experiments are under way to use capacitance packs as power sources for electromagnetic armour, railguns and track-coil hybrid launchers.
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