What Is Capacitor? What are The Functions of A Capacitor?

What is a capacitor?

Capacitor, a electronic component to hold charges, represented by the letter C. It composes of two metal electrodes between a layer of insulating dielectric. When a voltage is applied between the two metal electrodes, the charge is stored on the electrode, so the capacitor is an energy storage electrical part. Any of two conductors that are insulated and close to each other form a capacitor. In addition, the parallel plate capacitor consists of the electrode plate and the dielectric of the capacitor. 

Capacitor is one of the widely used electronic components in electronic equipment. It is widely used in stopping DC and alternating AC, coupling, bypass, filtering, tuning loop, energy conversion, control and so on. Capacitor is different from capacitance. The capacitance is the basic physical quantity, the symbol C, the unit is F (Farah).

A video introducing basic knowledge of capacitors


I. Capacitor characteristics

II. Functions of capacitor in electrical circuits

III. How to use capacitors?

IV. Capacitor types

V. Capacitor volume

VI. Charge and discharge of a capacitor

VII. Matters needing attention when using capacitors

VIII. Common fault of capacitor and treatment method


I. Capacitor characteristics

 - It has the ability of charge and discharge, preventing DC current from passing through, allowing AC current to pass through.

 - During the charge and discharge process, the charge on the bipolar plate accumulates, that is, the voltage is set up, therefore, the voltage on the capacitor will not change abruptly.

Charging: two plates with the same amount of dissimilar charge, each plate with the absolute value of the charge is called capacitor volume.

Discharging: positive and negative charges at both ends of capacitors are neutralized through conductors. During discharge, there is a transient current on the wire.

Capacitor charge

Capacitor charge

 - The capacitive reactance of capacitors is inversely proportional to frequency and capacity. When analyzing the capacitance, the frequency and capacity of the contacting signal must be analyzed.

Calculating formula

Formula of parallel plate capacitor

The dielectric constant of vacuum εr=1, k is a constant of hydrostatic power, s is the positive area of two plates, and d is the distance between two plates.

Explanation: the electric field in the parallel plate capacitor is uniform electric field.

II. Functions of capacitor in electrical circuits

In DC circuits, the effect of a capacitor is equivalent to a open circuit. Capacitors are one of the most commonly used electronic components to store charge.

Capacitors are used in electronic circuits as low-pass, high-pass and band filters. A filter is a circuit that allows current and voltage of a specified frequency and waveform to pass through. A capacitor's reactance is inversely proportional to frequency. By controlling or changing the reactance, you can control the frequency allowed through the circuit. Capacitors also play a significant role in high-speed switching logic circuits. Such circuits' voltage level, which should be steady, can change with current fluctuation, thereby introducing noise or error signals. Decoupling capacitors are built into circuits to stabilize the current, minimizing noise signals.

The effect of capacitor links with the structure of itself. The simplest capacitors are made up of polar plates at both ends and insulating dielectric (including air) at the middle. After electrification, the plate is charged, forming a voltage (potential difference), but the entire capacitor is non-conductive because of the intermediate insulation. However, the condition is that the critical voltage (breakdown voltage) of the capacitor is not exceeded. We know that any substance is relatively insulated, and when the voltage at both ends of the material increases to a certain extent, the material can conduct electricity. We call this voltage a breakdown voltage. When the capacitor is broken down, it is not an insulator. However, in AC circuits, the direction of the current changes with time, that is, this change has functional relation. The charging and discharging process of capacitors is time-dependent, and at this time, a varying electric field is formed between the plates, and this electric field is a function of the change with time. In fact, the current passes between capacitors in the form of an electric field.

III. How to use capacitors?

As a relatively common electronic component, capacitors have a wide range of uses. The following content gives you a brief introduction to the 9 most common scenarios where capacitors are used: Stopping DC, bypass (decoupling), coupling, filtering, temperature compensation, timing, tuning, rectifier, and energy storage.

1.Stopping DC: the function is to prevent the passage of DC and allow the AC to pass through.

DC blocking capacitor

DC blocking capacitor

2. Bypass (decoupling): it provides a low impedance path for some parallel components in AC circuits.

Signal input and output

Signal input and output

3. Coupling: as a connection between two circuits, AC signals are allowed to pass and transmitted to the next stage of the circuit.

Coupling Capacitor Circuit Model

Coupling capacitor circuit model

Capacitor as Coupling component

Capacitor as coupling component

The purpose of using capacitor as coupling part is to transmit the front stage signal to the next stage, and to separate the influence of the DC of the former stage on the latter stage, so that the circuit is simple to debug and its performance is stable.

The amplification of AC signal without capacitor will not changed, but the work points at all levels need to be redesigned. Because of the influence of the front and back stages, to debug at working points is very difficult and can hardly be realized at multistage.

4. Filtering: this is very important for the circuit, the capacitor behind the CPU is having this function basically.

Impedance formula(filtering circuit)

Impedance formula(filtering circuit)

That is, the larger the frequency f, the smaller the impedance Z of the capacitance. At low frequency, the capacitance C can pass smoothly because of the large impedance Z, and at high frequency, the capacitance C is very small because of the impedance Z, which is equivalent to shorting the high frequency noise to the GND.

5. Temperature compensation: it improves the stability of the circuit by compensating for the influence of other components on the temperature adaptability.

Temperature Compensation

Temperature compensation

Analysis: because the capacity of the timing capacitor determines the oscillation frequency of the horizontal oscillator, the capacity of the timing capacitor must very stable and does not change with the humidity in the environment. Therefore, the capacitors with positive and negative temperature coefficients are used for temperature complementation.

When the operating temperature increases, the capacity of Cl is increasing, while the capacity of C2 is decreasing, and the total capacity of two capacitors is the sum of the two capacitors after parallel connection. Because one capacity is increasing and the other is decreasing, the total capacity is basically stable.

Similarly, when the temperature decreases, the capacity of one capacitor decreases while the other increases, and the total capacity is basically unchanged, which stabilizes the oscillation frequency and realizes the purpose of temperature compensation.

6. Timing: the use of capacitors in conjunction with resistors to determine the time constant of the circuit.

Capacitor and resistor(timing)

Capacitor and resistor(timing)

Inputting signal from low to high, after buffer 1 then input RC circuit. The characteristics of capacitor charging make the B point signal not change immediately with the input signal, but there is a gradual process of increasing. When it becomes larger to a certain extent, the buffer 2 flips over, resulting in a delay jump from low to high at the output end.

7. Tuning: having systematic tuning to circuits which related to frequency, such as cell phones, radios, and televisions.

System tune

System tune

Because the resonant frequency of the oscillation circuit is a functional relation of lc. It is fond that the ratio of maximum to minimum resonant frequency varies with the square root of capacitance ratio. Here the capacitance ratio refers to the ratio of the capacitance at the minimum reverse bias voltage to the capacitance at the maximum reverse bias voltage. Therefore, the tuning characteristic curve (bias voltage and resonant frequency) is basically a parabola.

8. Rectifier: switch on or off a semi-closed conductor component at a predetermined time.



Filtering wave form

Filtering wave form

9. Energy storage: storage of electrical energy for release when necessary. For example, camera flashlights, heating devices, etc. (some capacitors now store energy at levels close to lithium batteries; a capacitor can store electricity as one-day power for a mobile phone.

IV. Capacitor types

According to the analysis and statistics, capacitors are divided into the following 10 categories:

1. According to the structure: solid capacitor, variable capacitor and fine-tuned capacitor.

2. Classified by electrolytes: organic dielectric capacitor, inorganic dielectric capacitor, electrolytic capacitor, electrothermal capacitor and air-spaced capacitor.

3.According to the usage: high-frequency bypass  capacitor, low-frequency bypass  capacitor, filtering capacitor, tuning capacitor, high-frequency coupling capacitor, low-frequency coupling capacitor, small capacitor.

4. According to the different materials: ceramic capacitor, polyester capacitor, electrolytic capacitor, tantalum capacitor, advanced polypropylene capacitor etc.

5. High frequency bypass: ceramic capacitor, mica capacitor, glass film capacitor, polyester capacitor, glass-glazed capacitor.

6. Low frequency bypass: paper capacitor, ceramic capacitor, aluminum electrolytic capacitor, polyester capacitor.

7. Filter: aluminum electrolytic capacitor, paper capacitor, composite paper capacitor, liquid tantalum capacitor.

8. Tuning: ceramic capacitors, mica capacitors, glass film capacitors, polystyrene capacitors.

9. Low coupling: paper capacitor, ceramic capacitor, aluminum electrolytic capacitor, polyester capacitor, solid tantalum capacitor.

10. Small capacitors: metallized paper capacitor, ceramic capacitor, aluminum electrolytic capacitor, polystyrene capacitor, solid tantalum capacitor, glass-glazed capacitor, metallized polyester capacitor, polypropylene capacitor, mica capacitor.

V. Capacitor volume

Since capacitors are a container for storing charges, there is a problem of capacity. In order to measure the capacity of capacitors to store charges, the capacity is determined. A capacitor must store a charge under the action of an applied voltage. The amount of charge stored in different capacitors under voltage may also different. According to the international standard, when the capacitor is subjected to a 1V DC voltage, the value is the charge that can store in the the capacitor (that is, the electric quantity per unit voltage), which is expressed by the letter C. The basic unit of capacitance is the Farah (F). At 1V DC voltage, if the capacitor stores the charge is 1 Coulomb, the capacitance is set at 1 farah, and Farah is represented by the symbol F, 1 F=1 Q/ V. In practical application, the capacitance of capacitors is often much smaller than that of 1F, and is often used in smaller units, such as mF, μF, nF, pF, etc. The relationship between them is as follows:

1F=1000mF 1mF=1000μF 1μF=1000nF
1nF=1000pF 1F=1000000μF 1μF=1000000pF

VI. Charge and discharge of a capacitor

When the capacitor is connected to the power supply, under the action of the electric field force, the free electron connected with the positive electrode of the capacitor moves through the power supply to the plate connected to the negative electrode of the power supply. The positive electrode is positively charged because of the loss of the negative charge; the negative electrode is negatively charged because of gaining negative charge. The positive and negative plates have the same charge size and the opposite sign, so the charge moves in a fixed direction to form a current. Due to the repulsive effect of the same charge, the initial current is maximum, and then decreases gradually. During the process of charge movement, the charge stored on the electrode plate of the capacitor increases continuously. When the voltage Uc between two poles of capacitor is equal to the power-supply voltage U, the charge stop moving. The current I=0, switch closed, through the wire connection, the capacitor plate charge neutralized. When K is closed, on the one hand, the positive charge of the capacitor C can be neutralized on the negative electrode; on the other hand, the negative charge of the negative electrode can also be moved to the positive electrode. The charge gradually decreases, the apparent current decreases and the voltage decreases to zero.

VII. Matters needing attention when using capacitors

Because the two poles of the capacitor have the residual charge, it is necessary to release the charge at first, otherwise the electric shock will occur easily. When dealing with the faulty capacitor, the circuit breaker and the upper and lower disconnector of the capacitor set should be opened first, and if the fuse protection is adopted, the fuse tube should be removed first. At this time, although the capacitor set has discharged  itself, there will still be part of the residual charge, therefore, it is necessary to carry out manual discharge. When discharging, the grounding end of the ground wire and the grounding grid should be fixed first, then the capacitor should be discharged with the grounding rod several times until there is no spark and discharge sound, and finally the ground wire is fixed again. Meanwhile, it should also be noted that if the capacitor has internal breakage, fuse failure or poor lead contact, there may be residual charges between the two poles, which will not be released during automatic discharge or manual discharge. Therefore, the operation or maintenance personnel should wear insulating gloves before contacting the faulty capacitor, and use a short line to connect the two poles of the fault capacitor to make it discharge. In addition, the capacitor with series connection should be discharged separately.

VIII. Common fault of capacitor and treatment method

(1) When the capacitor explodes, the power should cut off immediately and extinguish the fire with sand and dry-firefighter.

(2) When the capacitor fuses, it shall report to the dispatch and open the circuit breaker of the capacitor after obtaining the consent. When the power supply is cut off to discharge it, external checks are carried out, such as whether there are flashover marks on the outside of the casing, whether the casing is deformed, the oil leakage and the short circuit of the earthing device, etc., and the insulation resistance between the poles and the ground is measured. Check whether the capacitor set connection is complete, firm, lacking of phase phenomenon. If not found fault phenomenon, it can be replaced after the investment. If the insurance still melts after power transmission, the faulty capacitor should be withdrawn and the rest should be power on. If the circuit breaker tripped at the same time as the fuse, at this time, don’t connect power supply. After the above inspection has been completed, the insurance must be replaced.

(3) The circuit breaker of the capacitor tripped and the shunt safety was not broken, the capacitor should be discharged for three minutes before checking the power cable of the circuit breaker current inductor and the outside of the capacitor. If no anomaly is found, it may be due to external fault bus voltage fluctuations. After inspection, it may be put on trial; if not, a comprehensive test of the protection should be carried out. Through the above inspection, the test, if still can not find the reason, it is necessary to act according to the system, the capacitor is gradually tested. No trial test shall be made until the cause has been found.



1. What is a capacitor used for?

A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store energy electrostatically in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e., insulator).

2. What is capacitor and how it works?

In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. ... Inside the capacitor, the terminals connect to two metal plates separated by a non-conducting substance, or dielectric.

3. When should you use a capacitor?

Power Supply Smoothing. This is the easiest and very widely used application of a capacitor. ...

Timing. If you supply power to a capacitor through a resistor, it will take time to charge. ...

Filtering. If you pass DC through a capacitor, it will charge and then block any further current from flowing.

4. What is capacitor and its types?

The most common kinds of capacitors are: Ceramic capacitors have a ceramic dielectric. Film and paper capacitors are named for their dielectrics. Aluminum, tantalum and niobium electrolytic capacitors are named after the material used as the anode and the construction of the cathode (electrolyte).

5. Are capacitors AC or DC?

When we connect a charged capacitor across a small load, it starts to supply the voltage (Stored energy) to that load until the capacitor fully discharges. Capacitor comes in different shapes and their value is measured in farad (F). Capacitors are used in both AC and DC systems (We will discuss it below).

6. What is the principle of capacitor?

A capacitor is a device that is used to store charges in an electrical circuit. A capacitor works on the principle that the capacitance of a conductor increases appreciably when an earthed conductor is brought near it. Hence, a capacitor has two plates separated by a distance having equal and opposite charges.

7. Are capacitors dangerous?

Capacitors may store hazardous energy even after the equipment has been de-energized, and may build up a dangerous residual charge without an external source. "Grounding" capacitors in series, for example, may transfer (rather than discharge) the stored energy.

8. What type of capacitor should I use?

The general rule is always use a capacitor with a higher working voltage than the circuit it is used in. This is of particular importance in power supply circuits with high value electrolytic capacitors. The working voltage should always exceed the peak working voltage of the circuit by a minimum of 20%.

9. What is capacitor and its applications?

Capacitor is a basic storage device to store electrical charges and release it as it is required by the circuit. Capacitors are widely used in electronic circuits to perform variety of tasks, such as smoothing, filtering, bypassing etc…. One type of capacitor may not be suitable for all applications.

10. Do capacitors change AC to DC?

No, capacitor cannot convert AC to DC. Capacitor can add DC to AC so that zero reference of AC signal can be changed, in other words capacitor works as level shifter.

11. Can Capacitors store AC?

Capacitors do not store AC voltage - it stores voltage. It's rated to handle 450 VAC; that means it can withstand an AC voltage being applied to it. In other words, the capacitor is non-polar (it has no positive or negative lead). Polar (or polarized) capacitors are best known as "Electrolytic" capacitors.

12. What is the difference between a capacitor and a battery?

A battery is an electronic device that converts chemical energy into electrical energy to provide a static electrical charge for power. Whereas a capacitor is an electronic component that stores electrostatic energy in an electric field.

13. How much current can a capacitor handle?

A 3.5V charger will charge the capacitor up to 3.5V only. You need a higher voltage DC source to charge the capacitor to higher potential. Remember, in your case, 100V is the maximum which capacitor can handle.

14. What happens when a capacitor fails?

During a failure, half of the capacitor could fail open, which would result in overall capacitance being lost. Or half of the capacitor could fail short, which would result in the overall capacitance being halved.

15. Does type of capacitor matter?

Yes, the type of capacitor can matter. Different types of capacitor have different properties.

Some of the properties that vary between capacitor types:

a. Polarised vs unpolarised

b. Max voltage

c. Equivalent Series Resistance (ESR)

d. Lifetime (electrolytics are particularly bad in this case)

e. Physical size (e.g. a 100,000 uF ceramic capacitor would be HUGE!)

f. Tolerance of capacitance (again, electrolytics are bad here, often being +/- 20%

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