What does variable capacitors do

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 or distance between the electrodes is changed, the capacitance changes accordingly.Usually used as a tuning capacitor in radio receiving circuits.

Variable Capacitor, hereinafter referred to as "Variable capacitance, adjustable capacitance" (English: Variable/adjustable Capacitor), is a kind of capacitance can be within a certain range of continuous adjustment, Variable Capacitor.The principle is that, by changing the relative effective area or distance between the plates, its capacitance changes accordingly.Variable capacitors are widely used as tuning, compensating/correcting capacitors in various tuning and oscillating circuits of radio (such as radio, television) or NFC/RFID card readers.

Variable capacitor structure and working principle

The electrodes of any variable capacitor are composed of two sets of metal plates insulated from each other.Below, the structure and working principle of the earliest air-dielectric variable capacitor (a kind of capacitor in a variable capacitor) is illustrated: as shown in figure 2, the fixed set of two sets of electrodes is the fixed plate, and the rotating set is the moving plate. Air is used as the medium between the moving plate and the fixed plate.When the moving plates of the air-dielectric variable capacitor are rotated, the capacitance is the maximum.On the contrary, when rotating all the moving plates out of the fixed plate, the capacitance is the minimum.

In practical application, the moving plates of several variable capacitors can be assembled on the same rotating shaft to form coaxial variable capacitors (commonly known as double, triple and even N series).Variable capacitors have a long handle that can be adjusted with a pull wire or dial.Therefore, air dielectric variable capacitors are divided into air single-connected variable capacitors (referred to as air single-connected) and air double-connected variable capacitors (referred to as air double-connected, it consists of two groups of moving plates, fixed plates, can be coaxial synchronous rotation).

What does a variable capacitor do

The main function of the variable capacitor is to change and adjust the resonant frequency of the atmega324p-20mu circuit.

1. Resonance circuit

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

2. Frequency selection oscillation

Capacitors should be used in oscillators to continuously adjust the oscillating frequency within a certain range.In the high frequency signal generator circuit, adjust the variable unrequited love capacitor C, and its output signal frequency can be changed as needed.

3. The tuning

Often used in a tuning loop of a radio to select a station.As shown in the figure below, this circuit is the frequency conversion stage circuit of superheterodyne radio. One of the two connected variable capacitors C1, C1a, is connected to the antenna output circuit, and the other one is connected to the oscillator circuit of the machine. The receiving frequency can be changed by adjusting the synchronous change of the two connected capacity of C1.C2 and C3 are fine-tuning capacitors, which are used for frequency calibration of antenna input circuit and local oscillation circuit respectively.