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What is a variable capacitor and its function?

Post Date：2024-09-11，

What is a variable capacitor?

A variable capacitor is a capacitor whose capacitance can be adjusted within a certain range. When the relative effective area between the poles or the distance between the pieces changes, its capacitance changes accordingly.

It is generally composed of two sets of mutually insulated poles: a set of fixed poles is called a fixed set, and a set of movable poles is called a dynamic movable. The dynamic chips of several variable capacitors can be combined on the same hinge to form a coaxial variable capacitor (commonly known as double, triple, etc.). Variable capacitors all have a long handle, which can be adjusted with a pull wire or dial. The shape is as shown in the figure:
What is a variable capacitor and its function?

It is usually used as a tuning capacitor in the radio receiving circuit.

How to change the capacitance of a variable capacitor?

The specific way to change the capacitor of variable capacitors

Mechanical capacitor: Change the capacitor by mechanically adjusting the distance between the two metal plates. For example, a single variable capacitor changes the relative position of the movable chip and the fixed chip by rotating the axis, thus adjusting the capacitor.

Piezoelectric capacitor: Use the piezoelectric effect to make the capacitor have variable capacitance characteristics.

Reverse controllable capacitor: change the capacitance value by changing the reverse bias voltage.

Electronic capacitor: Use semiconductor components or ferroelectric materials to change the charge distribution and polarization degree in the substance under the influence of an additional electric field or voltage, so as to realize the adjustment of the capacitance value.

Gas regulation: It is a widely used method of variable capacitors. It changes the capacitance value by changing the gas medium between the two poles of the capacitor. Gas regulation is divided into two ways: dynamic gas regulation and static gas regulation.
What is a variable capacitor and its function?

What are the two types of variable capacitors?

Variable capacitors can be divided into air-dielectric variable capacitors and solid-dielectric variable capacitors according to the dielectric materials they use.

1. Air dielectric variable capacitor

The electrode of the air-dielectric variable capacitor is composed of two sets of metal sheets. In the two groups of electrodes, the fixed set is the fixed sheet, and the rotable set is the dynamic sheet. Air is used as the medium between the moving sheet and the fixed sheet.

When the dynamic chip of the air medium variable capacitor is rotated into the fixed chip, its capacity is the largest; conversely, when all the dynamic chip is rotated out of the fixed chip, the capacity is the smallest.

Air-dielectric variable capacitors are divided into air single-connection variable capacitors (referred to as air single-connection) and air dual-connection variable capacitors (referred to as air double-connection, which are composed of two sets of moving chips and fixed chips, which can rotate synchronously on the same axis). Air-media variable capacitors are generally used in radios, electronic instruments, high-frequency signal generators, communications In equipment and related electronic equipment

2. Solid dielectric variable capacitor

A solid-dielectric variable capacitor is to add a mica sheet or a plastic (polystyrene and other materials) film as a medium between its dynamic sheet and a fixed sheet (dynamic and fixed sheet are all irregular semicircular metal sheets), and the shell is made of transparent plastic. Its advantages are small size and light weight; its disadvantages are loud noise and easy to wear out.

Solid-dielectric variable capacitors are divided into sealed single-connection variable capacitors (referred to as sealed single-connection), sealed double-connection variable capacitors (referred to as sealed double-connection, which has two groups of dynamic chips, fixed-chips and dielectrics, which can rotate synchronously on the same axis) and sealed four-connection variable capacitors (referred to as sealed four-connection, which has four-connection, fixed-chip and dielectrics).

Sealed single-connection variable capacitors are mainly used in simple radios or electronic instruments; sealed double-connection variable capacitors are used in transistor radios and related electronic instruments and electronic equipment; sealed four-connection variable capacitors are commonly used in AM/FM multi-band radios.

What are the advantages of variable capacitors?

Wide adjustment range: Variable capacitors can continuously adjust the capacitance value within a certain range, which can meet the needs of different circuits.

Small size: Variable capacitors are usually smaller and suitable for use in compact circuit boards.

Good continuity: Variable capacitors can maintain good continuity in continuous adjustment, and the adjustment process is stable.

What are the disadvantages of a variable capacitor?

Poor stability: The stability of variable capacitors is not as good as that of fixed capacitors, and there may be changes in capacitance values caused by drift or temperature changes.

Temperature sensitivity: Variable capacitors are more sensitive to changes in temperature, and temperature changes will lead to changes in capacitance values.
High cost: Compared with fixed capacitors, the price of variable capacitors is higher.

What is the difference between a fixed and variable capacitor?

Fixed capacitor: Fixed capacitor is a capacitor with fixed capacitance, and its capacitance cannot be changed once it is made. The deviation between the capacitance of a fixed capacitor and the nominal capacitance is called error, and the allowable deviation range is called accuracy.

Differentiate
Capacitance adjustment: Variable capacitors can adjust the capacitance within a certain range, while the capacitance of fixed capacitors is fixed and cannot be changed.
Application scenario:
1. Fixed capacitors are widely used in various circuits. For example, fixed capacitors need to be used in power supply filtering, coupling and decoupling, frequency division, oscillation circuits, etc
2. Variable capacitors are mainly used when the circuit frequency needs to be controlled. For example, the tuning circuit in radio stations and television sets, the tuning device in the radio, the tone adjustment in the electronic piano, etc. all need variable capacitors.
Stability

In the process of use, the fixed capacitor is more stable because there is no conductive software and other problems. Because the internal mechanism needs to slide to adjust the capacitance value, the variable capacitor will cause stability problems due to high use or violent operation.

Price

Compared with variable capacitors, the price of fixed capacitors is low due to its simple structure and fixed capacitance values. The variable capacitor needs to design a complex mechanical structure, which is mainly adjusted by mechanical and electrical components such as motors, and the price is also relatively high.

Different ways to use:
1. How to use variable capacitors
When using a variable capacitor in a circuit, it is necessary to adjust the capacitance value through an external circuit or adjusting the device to achieve the frequency response adjustment of the circuit.
2. How to use fixed capacitors
The capacitance value of the fixed capacitor has been fixed, and it can be directly connected to the circuit.
What is a variable capacitor and its function?

What do variable capacitors use for dielectric?

There are two types of media for variable capacitors: air and polystyrene. The air medium variable capacitor has a large volume and small loss, and is mostly used in tube radios. Polystyrene medium variable capacitors are made of sealed, small in size, and mostly used in transistor radios.

How does a variable capacitor tune a radio?

Variable capacitor is one of the important components in the radio. Its function is to adjust the reception frequency.

The radio adjusts the frequency of the circuit by adjusting the capacitance of the variable capacitor. Because different audio reception rates require different circuit frequencies, radios need to adjust the capacitance of variable capacitors to achieve signal reception at different frequencies.

When the capacity of the variable capacitor is large, the lower the frequency of the circuit, the low-frequency signal can be received; when the capacity of the variable capacitor is smaller, the frequency of the circuit is higher, and high-frequency signals can be received.

ls an air capacitor a variable capacitor?

An air capacitor is a capacitor that uses air as a dielectric, which can be designed as a fixed or variable capacitor. The fixed capacitor type is not often used, because there are different types of fixed capacitors whose characteristics are much better than those, so the variable capacitor form is used more frequently due to its simple structure.

How to check the variable capacitor?

A Gently rotate the hinge with your hand. It should feel very smooth, and should not feel sometimes loose, tight or even stuck. When pushing the axle forward, backward, up, down, left, right and other directions, the hinge should not be loose.

B Use one hand to rotate the hinge and gently touch the outer edge of the moving piece with the other hand. You should not feel any looseness. The variable capacitor with poor contact between the hinge and the moving disc cannot be used again.

C puts the mulmeter in R×10k gear. With one hand, the two pens are connected to the moving chip and the lead end of the fixed chip of the variable capacitor respectively. With the other hand, the hinge slowly rotates back and forth several times, and the meter pointer should not move in the infinite position. In the process of rotating the hinge, if the pointer sometimes points to zero, it means that there is a short circuit point between the moving chip and the fixed chip; if it touches a certain angle, the mater reading is not infinite but a certain resistance value, indicating that there is an electric leakage phenomenon between the variable capacitor moving chip and the fixed chip.

Function of variable capacitors

Radio receiving circuit: Variable capacitors are used as tuning capacitors in radio reception circuits to select and adjust the received signal frequency by adjusting their capacitance.

Tuning circuit: In the tuning circuit, the variable capacitor is used to adjust the resonant frequency of the circuit to match a specific signal frequency.

Oscillator: In the oscillator circuit, the variable capacitor is used with the inductor to change the oscillation frequency by adjusting its capacitance.

Filter: In the filter circuit, the variable capacitor is used to adjust the cut-off frequency of the filter to filter out the unwanted frequency components.

Volume control: In the audio amplifier, the variable capacitor is used to control the volume and adjust the gain of the audio signal by changing the capacity.

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