Variable capacitor: description, device and diagram

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

What is an element such as a capacitor? This is a small radio element with a concentrated electric capacitance formed by two or more electrodes. In some cases, this element is also called a lining. These little parts are separated by something called a dielectric (special paper, a thin layer of mica, ceramic, etc.). The capacitance of this part will depend on indicators such as the size (area) of the plates, the distance between these elements, as well as on the properties of the dielectric itself.

General information

Very important fact. A capacitor has one property that shows up in an AC circuit. For such a circuit, this part will be a resistance, the value of which will depend on the frequency. If the frequency increases, then the resistance will decrease, and vice versa.

There are basic units of measurement with which you can determine the ownership of a particular capacitor. These include Farad, microFarad, etc. The designation on the elements of these units, respectively, is: Ф, μF.

Variable Cells

The variable capacitor incorporates parts such as plate sections of metal material. One of these sections can perform a smooth movement in relation to the second. During this movement, it happens that the plates of the moving part, that is, the rotor, are most often introduced into the gaps that exist between the plates of the fixed part - the stator. Through this movement, the following occurs. The area of overlapping of some plates by others changes, as a result of which the capacitance of the variable capacitor also changes.

The dielectric in such elements is most often air. Although it is worth noting that, if we talk about equipment with small dimensions, for example, about transistor pocket receivers, then they often use variable capacitors with a solid dielectric. As this element, wear-resistant and high-frequency raw materials are used there. Most often it is fluoroplastic or polyethylene.

KPI parameters

The main parameter for such parts, which will help determine the possibility of the device operating in an oscillatory circuit, has become the minimum and maximum capacitance. This indicator is most often indicated next to the variable capacitor itself on the device diagram.

It is worth noting that devices such as radio receivers and radio transmitters use severaloscillatory circuits. In order to set up the operation of several parts at once, capacitor blocks are used. One block most often consists of two, three or more KPI sections.

The rotor part for such units is usually mounted on one common shaft for all variable capacitors. This is done for convenience, since when only one rotor rotates, it becomes possible to change the capacitance of all devices located in this section at once.

KPI schemes

It is important to note that in the diagram, each capacitor that is included in the block is displayed separately. In order to indicate that the capacitance of the variable capacitor from this block and the rest of the elements can be changed with just one knob controlling the entire block, those arrows that indicate regulation must be connected by one dashed line of mechanical connection.

It is worth noting that there are some varieties of such KPIs. One of the types is differential capacitors, which have found their application, for example, in the arms of capacitive bridges. A feature of this type will be that it has two rows of stator plates and one row of rotary ones. The arrangement of the groups of plates is as follows: when one group leaves the gap, the second immediately takes their place. At this point, the capacitance of the differential type AC capacitor will decrease between the plates of the first stator group and the rotor group. But between the second group of stator plates and the rotor group, this figure will increase. Thus, the total valuewill remain unchanged all the time.

Trimming KPIs

Another type of KPI is trimmer capacitors. They are used to set the initial capacitance of the oscillatory circuit, which will determine the maximum frequency of its tuning. The capacitance of a capacitor in this type of AC circuit can be changed from a few picoFarads to several tens of picoFarads. Larger capacities can be achieved in some cases.

The main requirement for such types of KPI is the ability to smoothly change the capacitance indicator. Also, this capacitor must provide reliable fixation of the rotor in a given position.

PDA design

The most common type of trimmer capacitor is ceramic. The design of this device is as follows. The base of the part is a ceramic stator, as well as a movable base fixed on it in the form of a disk - a rotor. The plates in this element are thin layers of silver. They are applied by burning. Burning is carried out on the stator, as well as on the outer wall of the rotor.

In order to change or determine the capacitance of this type of variable capacitor, it is necessary to rotate the rotor. If we talk about the simplest equipment, then it most often uses a wire trimmer capacitor. This part consists of a piece of copper wire with a diameter of 1-2 mm. The length of this element is 15-20 mm. The wire is very tight, coil to coil, woundinsulated wire with a diameter of 0.2-0.3 mm. In order to change the capacitance in this device, it is necessary to unwind the wire. So that at this time the winding does not slip from it, it is necessary to impregnate it with any insulating compound.

Resistance Capacitor in AC Circuit

It is important to note here that the current in a circuit in which there is a capacitor can only flow if the applied voltage changes. You also need to understand that the strength of the current that will circulate in the circuit during the discharge and charge of this element will be the greater, the greater the capacitance of the capacitor itself, and will also depend on the speed at which changes in the electromotive force (EMF) occur.

One more property. A capacitor with a variable capacitance, which is included in a circuit with an alternating current, will be resistance for this circuit. In other words, the value of the capacitive resistance will be the smaller, the greater the value of the capacitance itself and the higher the frequency of the operating current. However, this statement is true only for a circuit in which the current is alternating. The capacitance of the capacitor is equal to infinity, that is, its resistance will be infinite if such an element is placed in a circuit with direct current.

Key parameters for KPI

There are several basic parameters for this kind of capacitors.

One of the main ones is the law of capacitance change. This law determines the nature of the capacitance change. This setting will changedepending on the angle of rotation or on the linear movement of the movable part of the capacitor plates in relation to their fixed parts.

Another property is temperature stability. This indicator directly depends on the design of the capacitor itself. Most often, this indicator is positive, and for capacitors with air as a dielectric, the indicator does not exceed (200:300) 10-61 / deg. If we talk about capacitors with a solid dielectric, then they have this value exceeding this indicator.