The capacitor is an electrical component used in many different applications to provide a static charge to the circuits. There are different types of capacitors available in the market, with different characteristics and uses.
Some of the most common ones are: variable capacitors, fixed capacitors, and film capacitors. All these types have different working mechanisms.
If you are getting into the world of electronics or want to buy variable capacitors and learn more about variable capacitors, you’re in the right place. In this article, we will concentrate on variable capacitors. You will know every essential detail about it.
What is a variable capacitor?
The term ‘variable’ should give a hint about the type of capacitor this is.
A variable capacitor is a type of capacitor that can be varied to change the capacitance. It is also known as an adjustable capacitor.
The capacitance of a capacitor is the ability to store charge. The greater the capacitance, the greater amount of charge it can store. This can be varied by changing the distance between the capacitor plates.
So, when you are using a variable capacitor, you can change its capacitance either by physically changing its size or by using an external electrical circuit.
What is the difference between variable capacitors vs fixed capacitors?
A fixed capacitor has a specified value of capacitance and is not dependent on the frequency or amplitude of the applied signal. A variable capacitor can change its capacitance in response to an applied voltage.
How does a variable capacitor work?
A variable capacitor consists of two metallic plates separated by an insulator. The two metallic plates are called electrodes, and they are connected through a conducting wire. They are mounted on a base that is made up of insulating material.
There is also a knob in between them that allows you to adjust their distance from each other. This allows you to vary the amount of space between them and thus vary the amount of charge stored in it, i.e., its capacitance value changes with it.
The two electrodes are connected with an external circuit through another wire. This wire is also connected to a knob which allows you to adjust the amount of charge that flows through it.
In other applications a rheostat is used and can be adjusted in such a way that the amount of charge flowing through.
The charge is directly proportional to the capacitance of the capacitor. When you adjust the knob, the capacitor charges up and discharges until it reaches a point when it holds a certain amount of charge. This point is called its final value.
As mentioned above, the capacitance of a capacitor is directly proportional to the amount of charge that it can hold. So as you adjust the knob, you adjust the amount of charge that flows through it, thus changing its capacitance value. The charging and discharging process continues until this capacitor reaches its final value, at which it can hold a certain amount of charge.
This process continues endlessly so that in some cases, this capacitor can be used as an energy source for other circuits where energy is needed to be stored temporarily for later use.
Types of variable capacitors
Variable capacitors are not all the same. Even though all of them tend to have the same working principle, they come in different designs and features.
Variable capacitors are divided into two broad classes. These are:
What is the difference between the two? Well, let’s have a brief discussion about them:
These are rated as the most popular types of variable capacitors. A typical tuning capacitor comprises a stator, a rotor and a supporting frame. The rotor is positioned inside the stator and the frame supports both of them.
The rotor is composed of many windings, and it acts as a variable resistor. It has a handle that is used to adjust its position to control the capacitance value across its terminals when switched on.
When more current flows through it, the rotor moves towards the top, increasing its capacitance value, increasing its charging ability. The stator has two terminals that are connected to two sets of connections in the circuit. One set is known as “input” and another set as “output”.
When a capacitor is switched on, it starts charging/discharging through these connections till it reaches its final value, at which point, it can hold a certain amount of charge for other circuits.
They are also called “capacitor trimmers”. You can adjust by turning the knob or wheel on their body, which changes their capacitance value.
For years these capacitors had screws, and you will adjust the capacitance using a screwdriver.
Trimmer capacitors are oftenly installed at strategic positions where you don’t need to change the capacitance value constantly once you have decided on a particular fixed value.
A typical trimmer capacitor comprises three leads; one is connected to the otary, another one to the stationary plate and the other lead is always regarded as the common lead.
The procedure of adjusting the trimmer capacitor is effortless. First, disconnect the power supply from the circuit.
Now, connect the capacitor to a variable resistor or variable voltage source. Switch on the power supply and adjust it till you get desired capacitance value.
Technical parameters to consider when buying variable capacitors
There are several parameters that you need to consider when you’re buying variable capacitors:
-Voltage rating: This parameter states the maximum voltage that a specific capacitor can withstand before breaking down or getting damaged.
-Temperature rating: This parameter states the maximum temperature at which a certain capacitor can work without getting damaged or breaking down.
-Capacitance: While buying variable capacitors, you will come across three types of capacitance values: low-value, medium-value, and high-value. The low-value capacitors have values from 0pF to 1nF; medium value capacitors have values from 1pF to 10nF; high-value capacitors have values from 10pF to 100nF.
-Rated frequency: The frequency rating of a capacitor is the maximum frequency at which it can work.
-Leakage current: This parameter is also known as leakage reactance and refers to the amount of current that escapes from the capacitor when there is no voltage applied.
-Dissipation factor: This parameter states the loss or dissipation factor of a capacitor, calculated by dividing the change in voltage by the change in current with a certain frequency in between them. A higher dissipation factor means that more energy is lost and vice versa.
-Package type: Variable capacitors come with different package types such as axial, radial, DIP, SMD, etc., so choose one according to your needs and requirements.
-Breakdown voltage: This parameter is the maximum voltage at which the capacitor will break down.
-Tolerance: The tolerance of a capacitor is the range in which its value can vary. A capacitor with a tolerance of 10% means that it can be within 10% of its nominal value.
-Temperature coefficient: This parameter is also known as the temperature coefficient of capacitance (TC). It is the change in capacitance that occurs due to a temperature change. High-value capacitors have lower TC than low-value capacitors.
-Leaded or leadless: Leaded capacitors have leads as connections and are usually used for high-frequency applications such as RF circuits. Leadless capacitors are used for low to medium frequency applications such as filters and bypassing in power supplies.
How to control a variable capacitor?
We have seen that variable capacitors allow you to adjust the capacitance value. But how do you execute these adjustments? There are two main methods for controlling; Mechanically controlled capacitance and electronically controlled capacitance.
Let’s look at how they work.
Mechanically controlled capacitance
As the name suggests, you can manually adjust the capacitance of these variable capacitors. This is achieved by adjusting the position of the metal plate that separates the capacitors. The capacitance value is adjusted by moving the metal plate.
Alternatively, you can do the adjustment using a screw or a knob.
Mechanically controlled capacitors are primarily used in applications where very high-frequency changes are required. These are typically used in RF circuits and power supplies.
These capacitors come in different designs, including:
-Multiple section capacitors: These capacitors have different rotor sections. Each section has a different capacitance value.
-Butterfly capacitor: This type of capacitor is made up of two metal plates connected together. The capacitance of this capacitor is adjusted by sliding the metal plates.
-Star-shaped capacitors: These capacitors are made up of a star-shaped plate with a gap between each plate. The gap acts as an insulator and increases the capacitance value.
Electronically controlled capacitance
Electronic variable capacitors are also called “digital” variable capacitors because they can be programmed with a digital controller to change their value.
Electronic variable capacitors do not require any physical adjustments to change the value of the capacitor and can be adjusted at any time without affecting its performance.
Electronic variable capacitors can be used for many applications, including filters, bypassing, precision reference, tuning, etc.
An electronic variable capacitor is controlled by a microcontroller or digital controller, which provides a digital voltage signal to adjust the variable capacitor’s position on a linear scale from 0 VDC to 5VAC at 2 kHz.
Application of variable capacitors
Where are variable capacitors used? To answer this question, we will outline the most common real-world applications of variable capacitors.
Variable capacitors are widely used in filters because they can be adjusted to provide any desired frequency response. When a filter is created using a fixed capacitor, its cutoff frequency depends on the size of the capacitor, which limits its tuning range. As a result, it is impossible to create perfectly tuned filters with fixed capacitors.
However, when using an electronic variable capacitor as a filter’s input or output, it is possible to make any frequency response from 0 Hz to 2 MHz with no loss in performance or efficiency. This is a common application in radios.
Variable capacitors are widely used for bypassing because they can be adjusted in values. This can be used to reduce the number of components needed in a circuit.
Variable capacitors are widely used in audio circuits for volume control, and are sometimes called “volume pots”. The most common type is a potentiometer with a fixed value capacitor, but there are also some other types.
Variable capacitors are widely used in sensors because they can be adjusted. They are used to detect the presence of a magnetic field, or to detect the amount of voltage across a capacitor.
Variable capacitors are widely used in controls because they can be adjusted. They are used to control things such as motor speed and light intensity in appliances.
What is the accuracy of a variable capacitor?
The accuracy of a variable capacitor is the difference between its actual value and the value it should have according to a specified table. The physics of capacitors limits the accuracy of a variable capacitor. In general, a variable capacitor has an error that is proportional to the ratio between its physical size and its rated capacitance. This can be reduced by increasing the physical size or decreasing the rated capacitance.
Another way to reduce error is by using an electronic variable capacitor as an input or output filter in a circuit with an exact voltage divider or transmission line.
Where to buy variable capacitors
The surest way to buy variable capacitors is from reliable suppliers and manufacturers. They have the experience, knowledge, and resources to deal with these electronic components.
For this reason, we at ICRFQ are committed to helping you get reliable variable capacitors in China. We have an expansive network of the best electronic component manufacturers in China hence can help you find the best products.
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