A capacitor is an electronic device that is used to store electrical charge. It is one of the most important electronic devices in circuit design. A capacitor is a passive component that is able to store both negative and positive charges. This is the reason why it can temporarily behave as a battery. Depending upon the design, construction, size, and storage capacity of a capacitor, it can be used in a variety of applications. The property of storing charges associated with the capacitors is known as capacitance. The capacitance is defined as the ratio of electric charges accumulated across the conducting plates of the capacitor and the potential difference existing between them. The capacitance is measured in Farads, which is named after English physicist Michael Faraday.
Construction of a Capacitor
A parallel plate capacitor has the simplest structure of all the capacitors. It consists of two conducting plates that are placed parallel to each other and are separated by a dielectric. The dielectric material present between the two plates acts as an insulator, which resists the flow of current between the plates. The size and shape of the plates of the capacitor vary as per the application. The dielectric medium used in between the two plates of the capacitor can be air, ceramic, polymer, paper, etc.
Working of a Capacitor
Initially, the conducting plates of the capacitor consist of an equal number of positive and negative charges; therefore, the plates are considered to be electrically neutral. When a battery is connected across a capacitor, the plate connected to the positive terminal of the battery accumulates a positive charge on it and an equal amount of negative charge gets deposited on the other plate that is connected to the negative terminal of the battery. The dielectric material present between the two plates acts as a barrier that obstructs any further flow of charges. Due to the charges present on both the plates, an electric field is created around the capacitor, which is directly proportional to the potential difference and inversely proportional to the distance between the two plates. When the capacitor develops a potential that is equal to the potential developed by the battery connected to it, it is considered to be fully charged. The time taken by the capacitor to accumulate the maximum amount of charge across its plates is known as the charging time. When the battery is removed, the capacitor acts as a source of energy. After connecting the charged capacitor to the load, the charges leave the capacitor plates, causing the flow of current in the circuit. This process continues till the capacitor plates gain the electrically neutral state and is known as the discharging of the capacitor.
Every country has its own way of denoting capacitors symbolically. Some of the standard capacitor symbols are given as:
1. Fixed Capacitor
As the name indicates, a fixed capacitor is a type of capacitor that produces a fixed amount of capacitance. This means that it is able to store only a predetermined amount of charges in it. Further fixed capacitors can be classified as per the dielectric material used between the conducting plates, for example, paper capacitor, plastic capacitor, ceramic capacitor, etc.
1. Polarized Capacitors
Polarized capacitors are capacitors that have a predefined polarity of the pins. It is essential to keep the polarity of the capacitor pins in mind before connecting a polar capacitor to the circuit. The most common polarized capacitors are electrolytic capacitors.
2. Non-Polarized Capacitors
Non-polarized or non-polar capacitors are the capacitors that can be connected in a circuit irrespective of the polarity of the pins. This signifies that the non-polar capacitors do not have any assumed polarity of the pins. They are also known as bipolar capacitors.
2. Variable Capacitors
The capacitors whose value of capacitance can be varied either electronically or mechanically are known as variable capacitors. A variable capacitor consists of a fixed plate and a variable plate. By varying the distance between the two plates, the capacitance can be varied. These capacitors are used in antennas for impedance matching.
1. Tuning Capacitors
The tuning capacitor or the tuning condensers consists of a stator, a rotor, and a frame. The stator is a stationary part, and the rotor moves with the help of a movable shaft. When the rotor blades move into the stator slot, they act like the capacitor plates. The capacitance value is maximum when the rotor blades fit the slots of the stator, and the capacitance value is minimum when the blades are away from the slots. The capacitance value of tuning capacitors ranges from a few Pico Farads to a few tens of Pico Farads. They are mostly used in the radio receiver LC circuits.
2. Trimmer Capacitors
Trimmer capacitors consist of three pins; one is connected to a stationary plate, one to a rotary plate, and the other one is the common pin. The capacitance of a trimmer capacitor can be varied using a screwdriver. The movable plate of the capacitor is semi-circular in shape. The capacitance depends upon the area opposite the movable semi-circular disk and the fixed plate. When the opposite area is bigger, the capacitance value will be higher, whereas with the reduction in the opposite area, the capacitance decreases accordingly.
3. Electrolytic Capacitors
The first electrode of an electrolytic capacitor is made up of a thin metal film, whereas the second electrode or the cathode consists of a semi-liquid electrolyte solution, which is in jelly or paste form. A thin layer of oxide gets developed between the two electrodes, which acts as the dielectric medium. An electrolytic capacitor is used in applications where high capacitance values are required.
4. Ceramic Capacitor
Ceramic capacitors are the capacitors that make use of ceramic as a dielectric medium between the two electrodes. They typically have a low value of capacitance and are non-polar capacitors. A ceramic capacitor is generally round-shaped and orange in colour.
5. Film Capacitor
The film capacitors make use of plastic film as a dielectric material. They are most commonly used in applications where stability, low inductance, and low price is desirable. Further, film capacitors can be classified as polyester film, metallized film, polypropylene film, PTE film, and polystyrene film capacitors.
6. Mica Capacitor
Mica is a mineral that is naturally present in the rocks present on the earth’s surface. Due to its excellent insulation properties, mica is used as a dielectric medium in capacitors. Mica capacitors have high inductive and resistive losses, which is why they are able to exhibit high-frequency properties. The construction of a mica capacitor comprises a thin sheet of mica layered over a thin sheet of silver placed between the two electrodes. The range of mica capacitors lies between a few pF and a few nF. They have high precision and are quite stable in nature.
7. Paper Capacitor
A paper capacitor consists of two aluminium electrode plates separated by the paper as a dielectric medium. Paper capacitors offer high leakage currents and have a capacitance value in the range of 500pF to 50nF. These capacitors are most commonly used in applications such as car audio systems, analogue equalizers, radio receivers, etc.
Applications of Capacitor
You must have observed that during troubleshooting a fan, the technician approaches a cylindrical electronic device connected to the internal mechanism of the fan. This cylindrical device is actually the capacitor. A capacitor is used in ceiling fans to help the fan get started, and it also helps the fan to spin. The magnetic flux developed by the capacitor is used to produce torque. The torque further helps to rotate the fan.
2. Signal Filtering
One of the major applications of capacitors is found in noise filtering. The signal filtering circuits have a specific time response that helps in rejecting frequencies above or below a certain threshold level. The prime application of signal filtering lies in loudspeakers, woofers, tweeters, etc.
3. Energy Storing Devices
Capacitors are able to temporarily act as an energy source. The energy supplied by a capacitor is lower than a battery of similar specification; however, they have a comparatively long life span. Also, a capacitor supplies energy at a faster rate, which makes it best suited for applications where a burst of power is desired.
4. AC to DC Converter
Diode rectifiers are basically used to perform the task of converting AC to DC; however, the working of such circuits depend a lot on capacitors. The output of a rectifier is a pulsated waveform. Hence, the charging and discharging of a capacitor can be used to convert the pulsating signal into a steady DC.
5. Timing Devices
The charging and discharging time of capacitors can be easily determined by calculating the RC time constant. Hence, they can be easily used as timing devices. Circuits like time delay circuits also make use of capacitors.