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 pole pieces or the distance between the pieces is changed, its capacitance changes accordingly. Usually used as a tuning capacitor in a radio receiving circuit.

A variable capacitor is a capacitor whose capacitance can be adjusted within a certain range. When the relative effective area or distance between the poles is changed, its capacitance changes accordingly. Usually used as a tuning capacitor in a radio receiving circuit.

Chinese name

Variable capacitor

Foreign name

Variable capacitor

Types of

Capacitors adjustable within a certain range

Features

Used as tuning capacitor in radio receiving circuit

Variable capacitor definition

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

Generally consists of two sets of pole pieces insulated from each other: a fixed set of pole pieces is called a fixed piece, and a movable set of pole pieces is called a moving piece. The moving blades 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: Variable capacitors are capacitors whose capacitance can be adjusted within a certain range. They are usually used as tuning capacitors in radio receiving circuits.

Variable capacitor related content

When you need to adjust the value of the capacitor manually or automatically, the AD5300BRTZ-REEL7 uses a variable capacitor in the circuit. The capacitance of these capacitors is usually less than 300pF, but it can also have some larger values in some special applications. Figure 9.16 shows the circuit symbol for a variable capacitor.

Generally, adjustable capacitors with slotted screw-type adjustments for making fine adjustments in a circuit are called trimmers. Commonly used dielectrics in these types of capacitors are ceramics or mica. The capacitance is usually changed by adjusting the distance of the plates. In general, the capacitance of the trimmer capacitor is less than 100pF. Figure 9.17 shows some typical devices.

A varactor diode is a semiconductor element that changes the capacitance by changing the voltage on its pins, exhibiting the properties of a capacitor. Details of this component are available in the course on electronic components. ^[1]

Identification of variable capacitor capacitors

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, its units are defaulted from the values given and are identified empirically. For example, one is marked as. 001 or. The unit of 01 ceramic capacitor is Hong F, because the pF value is too small to be used. As another example, the unit of a ceramic capacitor labeled 50 or 300 is pF, because if the unit is Hong F, this type of capacitor usually does not have such a large capacitance value. In some cases, 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. In some cases, pF or pF is used as the unit; sometimes pF units are labeled as MF or MFD.

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 and negative temperature coefficients are O, so that the capacitance does not change with temperature. Some types of capacitors are marked with colored ribbons. ^[1]

Variable capacitor classification

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

Variable capacitor 2

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 fixed piece, and the rotatable one is a moving piece. Air is used as a medium between the moving piece and the fixed piece. ^[2]

When the moving piece of the air-dielectric variable capacitor is rotated into the fixed piece, the electric capacity is the largest; otherwise, when the moving piece is completely rotated out of the fixed piece, the electric capacity is the smallest.

Air medium variable capacitors are divided into air single-link variable capacitors (referred to as air single-links) and air double-connected variable capacitors (referred to as air double-links, which are composed of two sets of moving and fixed blades that can rotate coaxially and simultaneously) Air dielectric variable capacitors are generally used in radios, electronic instruments, high-frequency signal generators, communication equipment and related electronic equipment.

Common domestic air single-link variable capacitors are CB-1- ××× series and CB-X- ××× series, and common air double-link variable capacitors are CB-2- ××× series and CB-2X- ××× series.

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 piece and the fixed piece (the moving piece and the fixed piece are irregular semi-circular metal pieces). The shell is transparent plastic. Its advantages are small size and light weight; its disadvantages are large noise and easy to wear.

Solid dielectric variable capacitors are divided into sealed single-connected variable capacitors (referred to as sealed single-connected), sealed double-connected variable capacitors (referred to as sealed double-connected, it has two sets of moving, fixed and dielectric, which can rotate coaxially and synchronously) And sealed four-connected variable capacitor (referred to as sealed four-connected, it has four moving and fixed pieces and dielectric).

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

Variable capacitor detection method

1. Gently rotate the shaft by hand, it should feel very smooth, and it should not feel tight or sometimes stuck at times. When the load axis is pushed forward, backward, up, down, left, right and other directions, the shaft should not be loose.

2. Rotate the shaft with one hand, and touch the outer edge of the film group with the other hand, and you should not feel any looseness. Variable capacitors with poor contact between the rotating shaft and the moving blade can no longer be used.

3. Set the multimeter to R × 10k. Connect two test leads to the moving and fixed parts of the variable capacitor with one hand, and slowly rotate the shaft a few times with the other hand. Should not move at infinity. In the process of rotating the shaft, if the pointer sometimes points to zero, it means that there is a short-circuit point between the moving piece and the fixed piece; if it encounters an angle, the multimeter reading is not infinity but a certain resistance value, indicating that the variable capacitor moves There is a leakage phenomenon between the chip and the stator. ^[3]