What is a Capacitor?

Capacitors are passive electronic device and used mostly in all electronic circuits. It stores energy in the form of electrical charge and produce potential deference across its terminals or plates. In this article we will discuss about basic definition of capacitor and different uses of capacitor.

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"Capacitor"

Uses of Capacitor

According to their applications or uses, capacitors are widely used in electronic circuits. Below are some of the applications or uses of capacitor;

Storage of Energy

A capacitor stores electric energy when it is disconnected from its charging circuit, so it acts like a temporary battery.

A capacitor line up the dielectric molecules across an electric field to collect or store energy.

Capacitors are usually used in electronic devices to provide temporarily supply of power, when batteries are being changed, which helps to prevent memory loss from device.

Super capacitors are now mostly used in electronics circuit. It can receive and supply charge much sooner than batteries.

Supercapacitor

A supercapacitor line up the charges of an electrolyte (dielectric) on either side of an insulator to collect or store a double-layer charge.

Power Conditioning

Capacitors as reservoir are used in power supplies, where they smooth the output signal of a rectifier.

Also used in the charge pump circuits as the energy storage component and produce higher voltages compare to the input voltage.

Capacitors perform conditioning of output signal from power supply. Capacitor allows AC signals to pass but it blocks DC signals when they are charged.

In audio application capacitors are used to shunt away power line hum before it reach into the signal amplifying circuit.

Capacitor for Power Conditioning

Power Factor Correction

In electric power distribution line, for power factor correction capacitors are used. Normally such capacitors often come as a pack of three capacitors which are connected as a three phase load. Generally, the capacitance ratings of these capacitors are specified as a reactive power in volt-amperes reactive (Var) and not in farads.

Motor or lamp loads may have capacitors for power factor correction.

Capacitor for Power Factor Correction

More details on Power Factor Correction you can get from below mentioned articles;

Suppression and Coupling

Signal coupling

When charged up to the applied dc voltage capacitors can pass AC but block DC signals. They are normally used to separate the AC and DC components of a signal. This process is known as “AC coupling” or in other words we can say it as, "capacitive coupling". Polyester film capacitors are often used as coupling capacitors.

For capacitive coupling, normally a higher capacitance value with small reactance at the signal frequency is used.

Capacitor for Signal Coupling

Decoupling

To protect one part of a circuit from the adverse effect like noise or transients of another circuit of a decoupling capacitor is employed. A circuit element which is generating noise is shunted by the capacitor.

Decoupling capacitor is also called as bypass capacitor.

It is generally used between the power supply and ground connection i.e. we can see it connected in between Vcc and Gnd terminal or pin.

Snubbers

The current through the inductance falls rapidly when an inductive circuit is opened; this creates a high voltage across the open circuit of the switch (MOSFET) or relay.

This high voltage may generate a spark if the inductance is large, which will cause contact points of the switch i.e. MOSFET or relay to oxidize, weaken and finally the switch will fail.

By introducing snubber capacitor across this opened circuit, will create a path for this impulse to bypass the contact points, thereby protecting the switch life.

These snubber capacitors are generally connected with a low resistance resistor in series, to dissipate generated energy and to reduce RFI.

Snubber Circuit in Power Supply

EMI Filtering

Electromagnetic interference (EMI) is a phenomenon where one electromagnetic field interferes with another, resulting in the distortion of both the fields. This may damage the device also.

An EMI filter is is used to suppress interference that is present in a signal. These filters suppress differential and common mode interferences.

As noise is mostly concentrated in high frequency regions of the signal it is a low pass filter.

An EMI filter consists of Capacitors & Inductors.

Capacitor in EMI filtering

Motor starters

The primary winding within the single phase squirrel cage motor housing is not capable of starting a rotational motion on the rotor.

Secondary winding of this motor has a series connected non-polarized starting capacitor which increases the sinusoidal current and generates a rotating electric filed to rotate the rotor. The start capacitor is typically mounted to the side of the motor housing.

When rotating speed achieved a centrifugal switch or current-sensitive relay in series with the main winding cut off the capacitor.

In simple words, these capacitors provide phase splitting and helps in auto start of Single Phase Induction Motors.

These capacitors have high starting torque.

Motor-starting capacitors (Capacitor to Start) are usually non-polarized electrolytic types.

Running capacitors (Capacitor to Run) are paper or plastic film dielectric types.

Motor-starting capacitors are typically non-polarized electrolytic types, while running capacitors are conventional paper or plastic film dielectric types.

Capacitor in Motor Starter

Signal Processing

The energy stored in a capacitor can be used to represent information, either in binary form or in analogue form.

Dynamic Random Access Memory (DRAM) devices use capacitors to characterise binary information as bits. When the capacitor is charged the device reads one value and another when it is discharged.

Dynamic Random Access Memory

CCDs i.e. Charge Coupled Devices use capacitors in an analog form.

In analog circuits capacitors can be used as components of integrators or more complex filters and also in negative feedback loop stabilization.

To integrate a current signal; signal processing circuits also use capacitors.

Tuned Circuits

Capacitors are also used in combination with inductors to tune the circuits to specific frequencies. We can see this in radio receivers and speakers.

Capacitor in Radio Tuning Circuit

The resonant frequency (f) of a tuned circuit is a function of the inductance (L) and capacitance (C) and is given by:

f = 1 / [2π √ (LC)]

Sensing

Generally most of the capacitors have static or fixed physical structure, but in the working environment or in the application their physical structure change, which changes the capacitance, this capacitance can be sense and can be used for measurement of many elements or factors.

The change in the characteristics of the dielectric can be used for sensing purposes.

We can measure a variety of things like; air humidity, mechanical strain and fuel levels.

To measure humidity in air, the capacitors with an uncovered and porous dielectric can be used.

A capacitive humidity sensor measures relative humidity by employing a thin strip of metal oxide between two capacitor electrodes.

Capacitive Humidity Sensor

When the atmosphere’s relative humidity varies, it also varies the metal oxides electrical capacity. This variation is further processed to get humidity value.

In capacitive pressure sensor, one of the capacitor metal plates is allowed to move in and out, due to changing distance between the plates the capacitance changes.

These movable plates are attached to a diaphragm which expands or compresses when it senses the pressure. The movement of the diaphragm affects the attached metal plate’s position and vary the capacitance.

In aircraft capacitors are used for measuring fuel level. A capacitance-type fuel-gauge system consists of a variable capacitor placed in the aircraft fuel tank along with an amplifier and with an indicator.

This circuit forms an electrical bridge which always becomes re-balanced as a result of the differences between the capacitance of the tank capacitor and a reference capacitor.

This signal is amplified to operate a motor which drives the pointer to indicate the capacitance change of the fuel tank capacitor and hence change in the fuel capacity is shown or indicated.

Conclusion

Capacitors are most frequently used component of the electronic circuits. Nearly all types of circuits have its presence. Uses of capacitor decided by their types. There are many types of capacitors and mostly used in electronic circuits for blocking DC while allowing AC to pass and also used for energy storage.

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Capacitor and Uses of Capacitor