What is Air Capacitor : Circuit, Working & Its Applications

A variable capacitor is one type of capacitor that has a variable capacitance value. This capacitor includes two plates where the area in between these plates is simply adjusted for changing the capacitor’s capacitance. These capacitors are available in two types air capacitor & trimmer capacitor. Generally, these capacitors are used especially in LC circuits for frequency tuning within radios. So this article discusses an overview of one of the types of variable capacitors like an air capacitor – working & its applications.

What is Air Capacitor?

An Air capacitor definition is a capacitor that uses air as the dielectric medium. This capacitor can be designed in a fixed or variable capacitance form. Fixed capacitance type is not used frequently because there are different types of capacitors available with superior characteristics whereas variable capacitance type is used more frequently due to their simple construction.

Air capacitors are generally made with two sets of semicircular metal plates which are separated through an air dielectric material. In these metal plates, one set is permanent & the other set is connected to a shaft which allows the operator to turn the assembly to change the capacitance when required. When the overlap between two metal plates is larger, the capacitance is higher. So the highest capacitance condition is attained once the overlap between the two sets of metal plates is maximum whereas the lowest capacitance condition is attained once there is no overlap. For better capacitance control, finer tuning, and increased precision, reduction gear mechanisms are used.

Air capacitors have a small capacitance value that ranges from 100 pF – 1 nF whereas the operating voltage ranges from 10 to 1000V. The breakdown voltage of dielectric is less so electrical breakdown will change within capacitor so this can lead to the defective working of air capacitor.

Air Capacitor Construction & Its Working

An adjustable capacitor like an air capacitor includes a series of semi-circular, revolving aluminum plates on top of a central shaft arranged in between an equally spaced set of fixed aluminum plates. This capacitor has a drilled hole within its center to pass a control rod. To control this rod, alternate discs are connected to pass it freely throughout the others which means that the disc set is separated efficiently into two groups that jointly form the two plate regions of the capacitor.

Once the capacitor discs are in a semi-circular shape, then turning the moving set causes the amount in which the two groups overlap to change to the whole plate area. When the capacitance of this capacitor depends on its whole plate area, then the change within the area can cause an equivalent change within the capacitance of the component, so an operator is allowed to modify the value of the component at will.

When the moving aluminum plates are rotated, and the amount of overlap between the static & moving plates will be changed. The air in between these sets of plates works like an effective dielectric that insulates the sets from one another. When the capacitor’s capacitance depends on the plate’s mutual size, then this adjustment simply allows the air capacitor value to be adjusted.

Air Capacitor Circuit

The simple air capacitor circuit is shown below. This capacitor uses air as a dielectric and it is designed by using two metalized foil or metal plates connecting parallel with some distance to each other. Capacitors store the energy in an electrical charge form on the plates.

Once a voltage is applied to an air capacitor to measure the charge on the two plates, then the ratio of the ‘Q’ charge to the ‘V’ voltage will provide the value of capacitance for the capacitor thus, it is given like C = Q/V. This equation can also be written to provide the formula for measuring the charge quantity on the two plates like Q = C x V.

Once an electric current is supplied into the capacitor, then it charges up, thus the electrostatic field will become very stronger because it stores more energy in between the two plates.

Similarly, when the current flows out of the air capacitor then the potential difference in between these two plates will decrease & the electrostatic field reduces when the electric energy goes away from the plates. So capacitance is one of the properties of a capacitor which is used to store electric charge on its two plates in an electrostatic field form.

Permittivity of Air Capacitor

Permittivity can be defined as the property of each material otherwise the medium used to measure the resistance offered against an electric field formation. It is denoted with the Greek letter ‘ϵ’ (epsilon) & its unit is F/m or farad per meter.

If we consider a capacitor that includes two plates that are separated by distance ‘d’, the dielectric medium like air is used among these two plates. In between the two plates of a capacitor, molecules are present which forms electric dipole moments. The electric dipole means, a pair of opposite and equal charges. For instance, a single molecule includes a positive charge at one end and a negative charge at another end which is separated by some distance as shown in the following figure.

In the following diagram, the molecules are generally aligned randomly within the capacitor plates. Once we apply an electric field to these plates externally then the molecules within the capacitor bring into line themselves in a better manner which is known as polarizability. So, their dipole moment generates its own electric field. This electric field opposes the externally applied electric field thus, it becomes like the similar pole of two magnets that keep resisting each other.

When the molecules line up themselves or they polarize more, they oppose the external electric field which is what we called as permittivity. Here, permittivity measures the resistance offered by material or medium to an exterior electric field.

If the medium’s permittivity is higher, then that medium’s molecules polarize better & thus they offer more resistance to the external electric field. Likewise, if the medium’s permittivity is low, then molecules weakly polarize, so they offer less resistance to the external electric field.

Permittivity is not constant, so it varies with different factors like temperature, humidity, medium type, frequency of field, electric field strength, etc.

Permittivity plays a significant role in determining the capacitor’s capacitance. So, a parallel plate capacitor’s capacitance is calculated by

C = ϵ x A/d

Where,

‘A’ is the area of a single plate.

‘d’ is the distance between two capacitor plates.

‘ϵ’ is the permittivity of the medium in between the two plates of capacitors.

If you observe the following capacitors, the permittivity can affect clearly the capacitor’s capacitance.
In the following two capacitors, the dielectric used in the left side capacitor is air. So the relative permittivity of this air capacitor is little > 1 i.e, 1.0006.

Similarly, in the second capacitor, the dielectric used is glass. So the permittivity of this capacitor is approximately 4.9 to 7.5. So, as compared to an air capacitor, a capacitor with glass dielectric has high permittivity.

So, the material with less permittivity will provide less capacitance & material with higher permittivity will provide high capacitance. Thus, permittivity plays a major role in deciding the capacitance value.

Characteristics

The characteristics of an air capacitor include the following.

• Air capacitors are non-polar which means these capacitors can be securely used in AC applications until the highest voltage rating is not exceeded.
• These capacitors have a small capacitance that ranges between 100pF & 1nF.
• The maximum operating voltage mainly depends on the capacitor’s physical dimensions.
• A high working voltage needs that the space in between two plates is enough to avoid the electrical breakdown of air.
• The dielectric strength of air is lesser than a lot of other materials, which makes these capacitors inappropriate for high voltages.

Advantages

The advantages of air capacitors include the following.

• It has less leakage current which means that the operating losses within this capacitor are minimal, particularly if the humidity is not high.
• Insulation resistance is high.
• Good stability.
• They have less breakdown voltage.
• The dissipation factor is low.

The disadvantages of air capacitors include the following.

• Air capacitors are available in large sizes.
• These capacitors have less capacitance.
• These are expensive.
• It occupies more space as compared to other capacitors.

Applications

The applications of air capacitors include the following.

• This capacitor is normally used in resonant, LC circuits, which need changes within capacitance. These
• circuits comprise radio tuners, frequency mixers & impedance matching components for antenna tuners.
• These are used normally where adjustable capacitance is necessary like resonant circuits.
• This capacitor is used to tune radio circuits & also in circuits wherever fewer losses are necessary.

Thus, this is an overview of an air capacitor – working with applications. These capacitors are made with aluminum and they work well in very strong magnetic fields. Here is a question for you, what is dielectric in a capacitor?