A variable capacitor is a capacitor whose capacitance can be adjusted within a certain range. When the relative effective area between the pole metal plate or the distance between the plates is changed, its capacitance changes accordingly. It is usually used as a tuning capacitor in a radio receiving circuit. Its two major types are Air dielectric variable capacitor and Solid dielectric variable capacitor. It is widely used in tuning and amplification, frequency selective oscillation, and other circuits.

I Variable Capacitor Introduction

Capacitors whose capacitance can be adjusted within a certain range are called variable capacitors.

A variable capacitor generally consists of two sets of pole plates insulated from each other: a fixed set of pole plates is called a stator, and a movable set of pole plates is called a rotor. The rotors of several variable capacitors can be combined on the same shaft to form a coaxial variable capacitor (commonly known as double, triple, etc.). Variable capacitors have a long handle, which can be adjusted by pulling wires or dials. The shape is as follows:

Figure1. variable capacitor

II Capacitor Identification

The value of the capacitor is identified on the capacitor body by a number or a combination of alphanumeric codes, and sometimes it is identified by a ribbon. The capacitor label gives the various parameters of the capacitor, including the capacitance value, rated voltage, and tolerance.

Some capacitors do not have a unit of capacitance. In these cases, their units are defaulted from the given values and are identified empirically. In some cases, the 3-digit notation is used. The first two digits are the first two digits of the capacitance value, and the third digit is the multiplier or the number of Os after the second digit. For example, 103 means 10000pF.

Some types of capacitors use WV or WVDC to indicate the rated voltage, and other types of capacitors are omitted. When omitted, the rated voltage can be determined based on the information provided by the manufacturer. Capacitor tolerances are usually expressed as a few percent, such as ± 10%. The temperature coefficient is expressed in parts per million (ppm). This type of mark consists of P or N and the following numbers. For example, N750 means a negative temperature coefficient of 750 ppm /°C, while P30 means a positive temperature coefficient of 30 ppm /°C. The NPO mark indicates that both the positive temperature coefficient and the negative temperature coefficient are O so that the capacitance does not change with temperature. Also, certain types of capacitors are marked with color ribbons.

Figure2. circuit symbols for a fixed capacitor and a variable capacitor

(a) shows a graphic symbol representing a fixed-value capacitor in a circuit. Both types are commonly used. In some types of capacitors, the left curve in the figure usually represents the outer plate (that is, the end near the outer package). This end is usually identified by a colored stripe near the wire connected to the plate.

(b) shows the symbol of the variable capacitor. They add an arrow through the plate to the fixed capacitor. Small trimmer capacitors are usually represented by the symbol on the right. The arrows indicate the variable plates.

III Classification of Variable Capacitors

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

1. Air dielectric variable capacitor

The electrode of the air dielectric variable capacitor is composed of two sets of metal sheets. The fixed one of the two groups of electrodes is a stator, and the rotatable one is a rotor. Air is used as a 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 screwed into the fixed plate, the capacitance is the largest; otherwise, when the moving plate is completely rotated out of the fixed plate, the capacitance is the smallest.

Air medium variable capacitors are divided into air single-connected variable capacitors and air double-connected variable capacitors. Air dielectric variable capacitors are generally used in radios, electronic instruments, high-frequency signal generators, communication equipment, and related electronic equipment.

Figure3. (a) air single-connected variable capacitors (b) air double-connected variable capacitors

2. Solid dielectric variable capacitor

The solid dielectric variable capacitor is a mica sheet or a plastic (polystyrene and other materials) film as a medium between the moving plate and the fixed plate (the moving piece and the fixed piece are irregular semi-circular metal plates). The shell is transparent plastic. Its advantages are small size and lightweight; its disadvantages are large noise and easy to wear.

Figure4. mica sheet 

Solid dielectric variable capacitors are divided into sealed single-connected variable capacitors, sealed double-connected variable capacitors (it has two sets of the rotor, stator, and dielectric, which can rotate coaxially and synchronously), and sealed four-connected variable capacitor (it has four sets of the rotor, stator, and dielectric).

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

IV Structure and working principle of variable capacitors

1. The structure of a variable capacitor

Regardless of the type of variable capacitor, its electrodes are composed of two sets of metal sheets insulated from each other. Below, we use the earliest air-dielectric variable capacitor (a kind of variable capacitor) to illustrate its structure and working principle: As shown in Figure, the fixed one of the two groups of electrodes is a stator. A group that can be rotated is a rotor, and the air is used as a 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 pieces are screwed into the fixed plate, the capacitance is the largest; otherwise, when the moving piece is completely rotated out of the fixed plate, the capacitance is the smallest.

Figure5. Air variable capacitor

In practical applications, the moving plates of several variable capacitors can be mounted on the same rotating shaft to form a coaxial variable capacitor. The variable capacitors have a long handle that can be adjusted with a pull-wire or dial. Therefore, the air medium variable capacitor is divided into an air single-connected variable capacitor and an air double-connected variable capacitor.

2. What does a variable capacitor do?

The main role of the variable capacitor is to change and adjust the resonance frequency of the loop. It is widely used in tuning and amplification, frequency selective oscillation, and other circuits.

(1) Resonance circuit

Figure6. Resonance circuit

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

(2) Selected frequency oscillation

The capacitor should be applied to the oscillator so that the oscillation frequency can be continuously adjusted within a certain range. In the high-frequency signal good generator circuit, adjust the single-connected variable capacitor C, and the output signal frequency can be changed as needed.

Figure7. Selected frequency oscillation

(3) Tuning

It is often used in the tuning loop of radio to play a role in selecting a radio station. As shown in the figure below, this circuit is a super-heterodyne radio frequency conversion stage circuit. One of C1a in the double variable capacitor C1 intervenes in the antenna output circuit, and the other C1b is connected to the local oscillation circuit. Adjusting the capacity of the two lines of C1 can change the reception in synchronization frequency. C2 and C3 are trimmer capacitors, which are used for frequency calibration of the antenna input circuit and the local oscillation circuit.

Figure8. Tuning

V Trimmer capacitors

A Trimmer capacitor is a kind of variable capacitor, also called a semi-variable capacitor. It plays the role of micro-adjustment. It is often used to adjust the capacitance accurately, and it is no longer required to change the capacitance during use. In the circuit, the most important requirement for trimmer capacitors is to maintain the reliability of a given capacitance.

There are many types of trimmer capacitors. According to the dielectric material, it can be divided into air trimmer capacitors, porcelain-trimmed trimmer capacitors, organic film trimmer capacitors, and mica trimmer capacitors. It is often used as a compensation or correction capacitor in various tuning and oscillation circuits. Capacitance can be adjusted within a small range, and the capacitor that can be fixed to a certain capacitance value after adjustment is called a trimmer capacitor also called a semi-trim capacitor. When you are adjusting a trimmer capacitor, you should change the distance or area between the two plates.

A trimmer capacitor is made of two or two sets of small metal plates with a dielectric sandwiched between them. As shown, the shape of the variable capacitor. Semi-variable capacitors generally do not have handles and can only be adjusted with screwdrivers, so they are often used in places where frequent adjustment is not required. Semi-variable capacitors are used as compensation or correction capacitors in various tuning and oscillation circuits.

Figure9. The shape of a semi-variable capacitor

Trimmer capacitors can be divided into ceramic trimmer capacitors and organic film trimmer capacitors. Ceramic trimmer capacitors are composed of two plates made of silver porcelain. The bottom plate is a fixed plate and the top plate is a moving plate. The moving plate can rotate with the shaft. Because the area covered by silver on the two plates is less than a semicircle, the capacity can be changed when the shaft is rotated. Organic thin-film trimmer capacitors use polyester film as a medium, and single-layer or multi-layer phosphor copper sheets as fixed and moving plates. The volume is smaller than that of porcelain-based trimmer capacitors.

VI How to test a variable capacitor?

The capacitance of the variable capacitor is generally very small and cannot be measured with a multimeter, but it can be judged whether there is a chip or leakage between the moving and fixed plate, as shown in the figure below.

Figure10. Variable capacitor test

The distance between the moving plate and the fixed plate of the variable capacitor is very small, and it is easy to be short-circuited by touching the plate. Whether the variable capacitor touches the chip can be detected by the multimeter's electric block.

During the test, you should put the two test leads of the multimeter on the rotor and the stator of the capacitor, and slowly rotate the shaft of the capacitor back and forth. If the meter hand is always stationary, indicating that there is no bump. If the hand points to zero ohms when rotated to an angle, it means that the plates are touching here. After the capacitor hits the plate, firstly check whether the distance between the moving plate and the fixed plate is uniform. If individual moving or fixed plates are found to be skewed or distorted, it is generally caused by the impact of external factors, as long as they are straightened with a thin blade. If it is found that one or two sets of fixed plates of the capacitor are all bent or deflected to one side, it may be caused by loosening of the fixed board bracket rubber board or solder desoldering at the fulcrum at both ends of the fixed plate.

Electrostatic noise refers to a series of "chatter" noises that appear in the radio speakers when the variable capacitor shaft is turned during the tuning of the radio station. If the connecting wire of the fixed piece is soldered and no short circuit is found, then we say this is the electrostatic noise caused by the electrostatic effect. When the organic sealed variable capacitor generates electrostatic noise, you can connect the two pins of the rotor and stator of the capacitor to a 12V DC power supply, and then rotate the rotor several times to eliminate the electrostatic noise of the capacitor.