What is a variable capacitor?

  • May 20, 2022
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Ⅰ Introduction

One type of primary capacitor is the variable capacitor. Capacitors are classified into two types based on their capacitance. These are known as 'Fixed Capacitors' and 'Variable Capacitors.' Capacitors with a fixed capacitance value are referred to as 'Fixed Capacitors.' Similarly, capacitors with varying capacitance are referred to as Variable Capacitors.

This type of capacitor has the competence to change the capacitance values "Electrically" or "Mechanically." Variable Capacitors, unlike Fixed Capacitors, provide specific ranges of values rather than deciding the values during manufacturing. These capacitors are chosen based on the required values. This type of capacitor is preferred in the majority of Tuning Circuits.


Ⅰ Introduction

Ⅱ What is a Variable Capacitor?

Ⅲ Construction of Variable Capacitor

3.1 Summery

Ⅳ Types  of Variable Capacitors

4.1 Tuning Capacitors

4.2 Common problems in turning circuits

4.3 Trimmer Capacitors

4.4 Mechanical Capacitors

4.5 Electronic Capacitors

Ⅴ Working principle of Variable Capacitors

5.1 Turning types

5.2 Ceramic trimmers

5.3 Plastic foil types

Ⅵ Variable Capacitor Applications


Ⅱ What is a Variable Capacitor?

  • A variable Capacitor is defined as a capacitor whose capacitance can be varied to a specific range of values based on the requirement.

  • Metal plates are used in the construction of these capacitors.

  • One of these plates will be settled, while the other will be movable.

  • The capacitance range provided by these capacitors is from 10 picofarads to 500 picofarads.

  • It is known as a 'Ganged Capacitor' that these capacitors can connect with one and another.

  • This combination worked in such a way that only one shaft can be able to rotate the variable capacitor ends.

  • Variable capacitors can be defined in this manner. The symbol for this type of capacitor is straightforward. It has an arrow on it to indicate that it is a variable one.


Figure1:Variable Capacitor Symbol

Ⅲ Construction of Variable Capacitor

The variable capacitor's construction is shown below. As their simple construction, these capacitors are commonly used in a variety of applications. These capacitors are typically constructed with two sets of hemispherical metal plates separated by air gaps. One set of metal plates is fixed, while the other is connected to a shaft, allowing the user to turn the assembly and change the capacitance as needed. As a result, the construction of each type of capacitor differs.


Figure2: construction-of-variable-capacitor

The working principle of a basic capacitor can be applied to design this capacitor. The conductive plates of this capacitor are arranged in parallel and separated by dielectric coatings made of various materials such as reinforced paper, mica, or certain types of ceramics. These capacitors, unlike traditional fixed capacitors, are designed to change capacitance levels. In most cases, we can gain variable capacitance by varying the distance between parallel plates within a capacitor.

3.1 Summery

  • This capacitor is built by inserting metal plates into it.

  • Some of them are 'Fixed Plates,' while the rest are 'Movable Plates.'

  • The area between the Fixed and the movable plates changes as a consequence of the rotation of the movable plates.

  • Each type of capacitor in this category is built uniquely.

Ⅳ Types  of Variable Capacitors

In the market, there are two types of variable capacitors, which are as follows. The capacitance of the following capacitors can be changed manually with screwdrivers; otherwise, any device can be used.

  • Tuning Capacitors

  • Trimmer Capacitors

  • Mechanical Capacitors

  • Electronic Capacitors

4.1 Tuning Capacitors

Tuning capacitors can be designed with the help of a frame. This frame has both a stator and a rotor. The frame of the capacitor can support both the mica material and the stator. When the stator is turned off, the rotors begin to rotate with the assistance of a shaft.

When the movable rotor plates enter the immobile stator, the capacitance value is maximum; otherwise, it is minimum. These capacitors can provide capacitance values ranging from picofarads to tens of picofarads.


Figure3:Tuning Capacitors

These capacitors are used in radio receivers with LC circuits. Tuning condensers is another name for these capacitors.

4.2 Common problems in turning circuits

Variable capacitors used in tuning circuits can cause several issues. The main issue with the older types is a bend in the plates, which causes the component to short and thus become inoperative. As shown in Figure 1, this short can be detected.

Figure 4 - Testing a variable capacitor with a multimeter.

There must be no indication of low resistances around the entire turn of the variable capacitor.

In good condition, the meter's needle should remain on infinite to a variable. If a short is discovered between the plates, the first step should be to attempt to correct the alignment of these plates using the screw that exists in the mobile variable axis.

It is possible to adjust the plates if they are bent by pressing and releasing this screw. If the plates are bent, the technician can attempt to correct the defect very carefully, always performing a continuity test to detect the moment when the component is recovered. The insulating sheets inside them may have a small variable short problem.

By carefully dismantling the component, you can obtain the defective sheets and attempt to correct the defect, or with insulating use of other variables abandoned by the same problem, or even with the improvisation of insulating sheets. Figure 5 depicts how the metal plates are arranged, with an insulating foil sandwiched between them to prevent contact between the mobile and fixed assemblies.

Figure 5 - Construction of a variable with plastic insulation.

After reassembly, an isolation test should be carried out to ensure that the short between the plates was indeed removed. If the problem is moisture or dirt, the variable can be disassembled for cleaning, but be careful not to bend plates and remember to reassemble any insulating sheet.

4.3 Trimmer Capacitors

Trimmer capacitors are used to provide basic calibration of equipment during manufacturing or servicing. These capacitors are frequently arranged on the printed circuit board in such a way that the user does not have access to change them. As a result, these capacitors are inexpensive.

These capacitors are used in circuits to set the oscillator frequency, rise, latencies, and fall times. These capacitors enable servicemen to adjust devices as needed. These capacitors are classified into two types: air trimmer and ceramic trimmer.

There are three leads in this capacitor. The first lead is connected to the immobile part, the second to the rotary, and the third to the common. The purpose of a semi-circle-shaped movable disc is to observe the movement of this capacitor. This capacitor has two plates that are separated by a dielectric material and are arranged in parallel to each other.


Figure6:Trimmer Capacitors

These capacitors can be classified based on the dielectric material used, such as air trimmer or ceramic trimmer.

4.4 Mechanical Capacitors

These capacitors are made up of a series of curved plates that are connected to a knob. The main advantage of this is that the capacitance of the capacitor can be easily changed if necessary. Mechanically, these are dependable because they are not overly complicated.

4.5 Electronic Capacitors

By applying a DC voltage to these capacitors, you can change their capacitance. These capacitors' main applications are multimeters, resistance, and amperage. The DC (direct current) here refers to the type of current supplied by a battery.

Ⅴ Working principle of Variable Capacitors

The capacitance can be varied between a minimum and a maximum value using an electrode system composed of one fixed and one movable part – stator and rotor.

The temperature coefficient (TC) for the various dielectrics has a great difference from the corresponding values for fixed capacitors. Except for the best precision components, the variations are significantly larger, which has to do with the mechanical conditions as well as the entire construction.

Trimmer capacitors are primarily used on printed circuit boards (PCBs), but surface mount designs are becoming increasingly popular. Trimmers frequently have friction, which increases the turning moment and, as a result, locks the capacitor in its adjusted position.

5.1 Turning types

Air dielectric

As shown in Figure 7, the classic variable capacitor is be composed of semicircular electrodes that can be turned into each other. The styles are designed for either PCB or panel mounting. They are mainly used for tuning resonance circuits.

Figure7: A variable air-insulated capacitor schematic and an example of Tronser's design.

Mechanical precision is required due to the air-insulated electrodes. The plate distance is usually between 0.2 and 1 mm. The cost is fairly high.

5.2 Ceramic trimmers

We can make a ceramic trimmer capacitor by reducing the plates in Figure 7 to just one silver-plated ceramic rotor that is turned in over the stator electrode. Figure C5-2 depicts an example of this design. There are also multilayer designs. The capacitors are available in both hole mount and surface mount configurations.Because Type 1 ceramics are used, losses will be minimal.

Figure 8: Explanatory sketch of a cross-cut using a ceramic trimmer.

5.3 Plastic foil types

If we replace the air insulation in Figure 7 with some plastic foil, the electrode distance can be reduced while the r – and thus the capacitance – increases, albeit at the expense of a slightly lower Q value. Low loss plastics such as Teflon (PTFE), polypropylene (PP), and polycarbonate (PC) are common, but polyester (PETP) is also available.

Ⅵ Variable Capacitor Applications

  • The applications of the variable capacitor include the following:

  • Trimmer capacitors are used in the manufacturing process when a capacitance value needs to be matched to a specific circuit.

  • The main reason for using this capacitor is that the components in the circuit have different tolerances. As a result, the tolerance values in a circuit can be changed by 20% from what the designer expected to notice. As a result, these capacitors are used to adjust these tolerances.

  • These are frequently used in a variety of microwave circuits.

  • These capacitors are used in medical instruments such as NMR scanners and MRI machines to generate extremely high magnetic fields.

  • Tuners, oscillators, filters, and crystal oscillators are examples of common applications.

  • These capacitors can be found in communication devices such as mobile radios, aerospace transmitters and receivers, CATV amplifiers, and signal splitters.


(1). What is the main function of the variable capacitor?

It is used to fix the resonant frequency in the LC circuit.

(2). How these capacitors are made?

These are made with two sets of curved metal plates and they are divided by air gaps

(3). What is a ganged capacitor?

The combination of two capacitors that are connected together is known as a ganged capacitor.

(4). What are the two types of variable capacitors?

They are tuning capacitors and trimming capacitors.

(5). What is the capacitance values of a variable capacitor?

Typically ranges from 100pF to 500pF

Thus, this is all about variable capacitors and the characteristics of the variable capacitor mainly include accuracy, tolerance, polarity, voltage rating, and capacitance range. Here is a question for you, what are the advantages of a variable capacitor?

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