What is a Variable Capacitor

Author: Release time:2023-05-05 Source: Font: Big Middle Small View count:223

If you are not familiar with variable capacitor, today our team will introduce it completely for you. There is also a tutorial about how to make the air variable capacitor. Let's get starting.

What is a Variable Capacitor


A variable capacitor is a capacitor whose capacity can be adjusted within a certain range. When the relative effective area between the poles or the distance between the poles changes, its capacitance changes. It is commonly used as a tuning capacitor in radio reception circuits. Its two main types are air dielectric variable capacitors and solid dielectric variable capacitors. It is widely used in tuned amplification, frequency selective oscillation and other circuits.


1. Structure of Variable Capacitors


No matter which type of variable capacitor is used, its electrodes are composed of two sets of mutually insulated metal sheets. In the following, we use the earliest air dielectric variable capacitor (a kind of variable capacitor) to illustrate its structure and working principle: As shown in the figure, the fixed one of the two groups of electrodes is the stator. The rotatable group is the rotor, and the air is used as the medium between the moving plate and the fixed plate.

When the moving piece of the air dielectric variable capacitor is rotated so that all the moving pieces are rotated into the fixed one, the capacitance is maximum; otherwise, when the moving piece is completely turned out of the fixed plate, the capacitance is minimum.


structure of variable capacitors.jpg


In practical application, multiple moving tabs of variable capacitors can be mounted on the same rotating shaft to form a coaxial variable capacitor. Variable capacitors have a long handle that can be adjusted by pulling a wire or a dial. Therefore, air dielectric variable capacitors are divided into air single-connected variable capacitors and air double-connected variable capacitors.


2. Classification of Variable Capacitors

Air dielectric variable capacitor

The electrodes of air dielectric variable capacitor are composed of two sets of metal sheets. The fixed group of the two sets of electrodes is the stator and the rotatable group is the rotor. Air is used as the medium between the moving plate and the fixed plate.

When the moving plate of the air dielectric variable capacitor is rotated so that all the moving plates are rotated into the fixed plate, the capacitance is maximum; otherwise, when the moving plate is completely rotated out of the fixed plate, the capacitance is minimum.

Air dielectric variable capacitor is divided into air single connection variable capacitor and air double connection variable capacitor.

Solid dielectric variable capacitors

Solid dielectric variable capacitor is a mica sheet or plastic (polystyrene and other materials) film as a medium between the moving and fixed sheets (the moving and fixed sheets are irregular semi-circular metal sheets). The case is transparent plastic. Its advantages are small size and light weight; its disadvantage is that it is noisy and easy to wear.

Solid medium variable capacitor is divided into sealed single connection variable capacitor, sealed double connection variable capacitor (it has two groups of rotor, stator and dielectric, which can rotate coaxially and synchronously) and sealed four connection variable capacitor (it has four groups of rotor, stator and dielectric).

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


3. What is the Purpose of Variable Capacitor?


The main function of variable capacitor is to change and adjust the resonant frequency of the loop. It is widely used in circuits such as tuned amplification and frequency selective oscillation.


(1) Resonant circuit

resonant circuit.png


As shown in the figure, the LC resonant circuit can change the resonant frequency by changing the capacity of the variable capacitor C. The resonant frequency is proportional to the square of the capacitor. The resonant frequency is inversely proportional to the square of the capacitance, and the formula is as follows.


resonant circuit 1.png

(2) Selected frequency oscillation

Capacitance should be added to the oscillator so that the oscillation frequency is continuously adjusted within a certain range. In the high frequency signal generator circuit, adjust the single connection variable capacitor C. The output signal frequency can be changed as required.


selected frequency oscillation.jpg

(3) Adjustment

Commonly used in the tuning cycle of the radio to play the role of selecting the radio station. As shown in the figure below, the circuit is a super outlier type RF conversion stage circuit. One C1a of the double variable capacitor C1 intervenes in the antenna output circuit, and the other C1b is connected to the local oscillator circuit. Adjusting the capacity of the two lines of C1 can change the reception of synchronous frequency. C2, C3 is a trimmer capacitor for the antenna input circuit and the frequency calibration of the local oscillator circuit.



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How to Make Air Variable Capacitor

1. What is Air Variable Capacitor


The air variable capacitor is composed of a set of fixed and immovable plates and a set of movable plates that can be rotated, and the insulating medium between the movable plates and the plates is air, as shown in the figure below. As the rotating shaft is connected with the moving tabs, the angle between the moving tabs and the fixed tabs can be changed by rotating the rotating shaft, so that the size of the capacitance can be changed. When the moving piece is rotated out from the fixed piece position, the capacity is the smallest; when the moving piece is rotated into the fixed piece position, the capacity is the largest. The capacity of the capacitor depends on the distance between the two sets of pole pieces and the size of the pole piece area.

air variable capacitor.png

Characteristics of air variable capacitor


Air medium variable capacitor has air as the medium between the moving piece and the fixed piece, and the variable capacitance of air medium variable capacitor is 100~1500pF; air medium variable capacitor has many advantages of simple design, accurate capacity adjustment, small dielectric loss, good stability, long life, large volume, high insulation resistance, small loss and high efficiency. They can be made into linear, linear wavelength, linear frequency and logarithmic type according to the requirements.


In order to meet the requirements of different applications, the movers of air variable capacitors are often made into different shapes, such as linear capacitor type, linear wavelength type, linear frequency type and logarithmic capacitor type. 

Applications of air variable capacitor


Air dielectric variable capacitors are widely used due to their simple design and power. Their most important use is in the RF field, for example, by setting resonant frequencies to tune radios. They are also important components in RF matching networks, magnetic resonance imaging (MRI) equipment, and precision capacitance meters. In addition, they are used for testing the performance of air and other gases.


2. How to Make Air Variable Capacitor

Air variable capacitors are very easy to make. The following steps need to be followed to make an air variable capacitor at home.


Step1. Designing the Layout


The first step before proceeding to the hardware step is to design the structure layout as well as the precise measurements. The layout can be designed with Cadvance or any other open source software.


Step2. Mark the metal sheet


Place the aluminum sheet on the table and draw a 5cmx5cm box. Mark the center of the first piece of aluminum and use it as a reference to mark a hole every 5cm. Place a circle gauge at the marked holes and draw circles of equal diameter on each square piece of aluminum. Then, a vertical line is drawn on the arc of each circle with the center point of the arc as the tangent point up to the end of the aluminum sheet. The number of moving pieces should be one more than the number of fixed pieces.


Step3. Cut the aluminum sheet


After the drawing is completed, cut the aluminum sheet. Take out one aluminum piece and draw a line to connect the center of the circle to the left corner. Mark the midpoint of the line between the arc and the left corner. Mark another hole in the right corner at the corresponding location. Using this as a reference, mark the same holes on all the aluminum pieces.


Step4. Separate the moving piece from the fixed piece


Draw a smaller inner circle from the center of each circle and cut off the upper half of the inner circle. Drill the holes to the diameter. Make sure there is no arching, twisting or curling on the aluminum tabs.


Step5. Drill holes for the rear end plate


Take one moving piece and one fixed piece and place them in a full mesh position on the end plate. Mark the position of the holes on the end plate according to the position of the holes on the two aluminum sheets. Place one of the two end plates on top of the other and drill holes at the marked holes.


Step6. Assemble the aluminum tabs


The next step is the assembly of the moving and fixed plates. To assemble the movable plate, remove the bolt and remove the bolt head. Place the nut on the end of the bolt and screw it to 5cm from the end of the bolt. Then, insert the first tab, two washers, the second tab, two washers, and so on, until all tabs are inserted. Finally, insert the stop nut. Repeat the same process to install the tabs on the rear end plate.


Step7. Capacitor general assembly


The capacitor housing consists of two end plates or covered with some insulating material. Remove the moving piece set and insert the swivel into the rear end plate (attached to the stator set). Place the spacers on the spindles on both sides of the end plates. Turn the spindle to the full web position and fasten the other end plates. Use stop nuts and spacers for fastening.



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