Components Expert

What is a capacitor? What is the function of a capacitor?

What is a capacitor?

Capacitor, an electronic component that holds electric charge, is represented by the letter C. It consists of two metal electrodes between a layer of insulating dielectric. When a voltage is applied between two metal electrodes, the charge is stored on the electrodes, so the capacitor is an energy storage electrical component. Either of the two insulated conductors close to each other forms a capacitor. In addition, parallel plate capacitors are composed of electrode plates and the capacitor’s dielectric. Capacitors are one of the electronic components widely used in electronic equipment. It is widely used in stopping DC and AC power, coupling, bypass, filtering, tuning loop, energy conversion, control, etc. Capacitors are different from capacitance. Capacitance is a basic physical quantity, symbol C, unit is F (Farad).

Function of capacitor in circuit

In a DC circuit, the function of a capacitor is equivalent to an open circuit. Capacitors are one of the most commonly used electronic components for storing charge.

Capacitors are used as low-pass, high-pass, and band-pass filters in electronic circuits. A filter is a circuit that allows current and voltage of a specific frequency and waveform to pass. The reactance of a capacitor is inversely proportional to frequency. By controlling or changing the reactance, the frequency allowed by the circuit can be controlled. Capacitors also play an important role in high-speed switching logic circuits. The voltage level of this circuit should be stable, but will change with current fluctuations, thereby introducing noise or error signals. A decoupling capacitor is built into the circuit to stabilize the current and minimize noise signals.

The function of the capacitor is related to the structure of the capacitor itself. The simplest capacitor consists of plates at both ends and an insulating medium (including air) in the middle. After being energized, the plates are charged to form a voltage (potential difference), but due to the intermediate insulation, the entire capacitor does not conduct electricity. However, the condition is that the critical voltage (breakdown voltage) of the capacitor is not exceeded. We know that any substance is relatively insulating, and when the voltage across the material increases to a certain level, the material can conduct electricity. We call it breakdown voltage. When a capacitor fails, it is not an insulator. However, in an AC circuit, the direction of current changes with time, that is, this change has a functional relationship. The charging and discharging process of the capacitor changes with time. At this time, a changing electric field is formed between the plates. The electric field is a function of time. In fact, current flows between capacitors in the form of an electric field.


Capacitor charging and discharging

When the capacitor is connected to the power supply, under the action of the electric field force, the free electrons connected to the positive pole of the capacitor move through the power supply to the plate connected to the negative pole of the power supply. Due to the loss of negative charge, the positive electrode is positively charged; due to the negative charge, the negative electrode is negatively charged. The positive and negative plates have the same charge magnitude and opposite signs, so the charge moves in a fixed direction to form a current. Due to the repulsion of the same charge, the initial current is the largest and then gradually decreases. In the process of charge movement, the charge stored on the plates of the capacitor continues to increase. When the voltage Uc between the two poles of the capacitor is equal to the power supply voltage U, the charge stops moving. The current I=0, the switch is closed, and the charge of the capacitor plate is neutralized through the wiring. 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 drops to zero.

Matters needing attention when using capacitors

Since the two poles of the capacitor have residual charge, the charge must be released first, otherwise electric shock may easily occur. When handling a faulty capacitor, the upper and lower isolation switches of the circuit breaker and the capacitor bank should be disconnected. If a fuse is used for protection, the fuse tube should be removed first. At this time, although the capacitor bank has self-discharged, there will still be some residual charges, so manual discharge is required. When discharging, first fix the grounding end of the grounding wire with the grounding grid, and then use a grounding rod to discharge the capacitor several times until there is no spark or discharge sound, and finally fix the grounding wire. At the same time, it should also be noted that if the capacitor is damaged, the fuse is faulty or the lead is in poor contact, there may be residual charge between the two poles, which will not be released during automatic discharge or manual discharge. Therefore, operators or maintenance personnel should wear insulating gloves before touching the faulty capacitor, and use short wires to connect the two poles of the faulty capacitor to discharge it. In addition, series capacitors should be discharged separately.