The capacitor in which the capacitance value is not fixed and can be changed mechanically or electronically is called variable capacitor.
The capacity of the device for storing electric charge is known as capacitance.
It is given by,
C: Capacitance of capacitor in farad.
A: Area of any one plate in m2.
D: Distance between two plates in m.
ε: Permittivity of material = ε0εr.
ε0: Absolute permittivity = 8.854X10-12 farad/meter.
εr: Relative permittivity.
From the equation, we can conclude that, the capacitance can be increased by
- Increasing the area of the plates,
- Decreasing their separation, or
- Changing the dielectric material.
Requirement of variable capacitor
The Requirements of variable capacitor are as given below.
- Variable capacitors are subjected to continuous adjustment hence the capacitor should have rigid structure so that the capacitor value is not changed.
- The spindles used should be very rigid and well mounted.
- The insulation must be proper to minimize the leakage loss.
- The design of the capacitor must be excellent reproduction of its capacitance.
- Spacing between the plates should be constant so that frequency is stable.
Now, lets discuss about various variable capacitors.
Air Gang Capacitor
- The most common form arranges a group of semicircular metal plates on a rotary axis (“rotor”) that are positioned in the gaps between a set of stationary plates (“stator”) so that the area of overlap can be changed by rotating the axis.
- Air is used as dielectric
- When the common shaft rotated, the common area of the movable and fixed plates changes accordingly. The greater the common area the larger the capacitance.
- Various forms of reduction gear mechanisms are often used to achieve finer tuning control, i.e. to spread the variation of capacity over a larger angle, often several turns.
- Approximate capacitance swing : 15 to 100pF and 350 to 550pF
- Power factor : 0.001
- Operating voltage : 750 V dc
- Temperature coefficient : 120 ppm/ C
Air gang capacitor is used as tuning capacitor in broadcast receivers. Also used in test equipments.
PVC Gang Capacitor
The variable PVC capacitor is used PVC as a dielectric material. There are alternate layers of fixed and variable plates. The variable plates are rotated by rotor. The whole structure is covered by plastic plates.
The rotor is made of two sections on common spindles. A screw shaft is attached to the plunger, when the shaft is turned the plunger moves in or out of the sleeve and the value of the capacitor changes.
- Approximate capacitance swing : 7 to 350pF and 5 to 250pF.
- Power factor : 0.1 to 0.5.
- Operating voltage : 750 V dc.
They are used in radio receivers.
Trimmer capacitors (trimmers) are miniature variable capacitors. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built.
A small screwdriver or similar tool is required to adjust trimmers.
- The movable parts of the trimmer capacitor are locked in place and remain in their fixed positions while the device is operating.
- It is generally a simplified parallel-plate variable capacitor with one stator plate and one rotor plate.
- Less frequently, it may be, for example, a system consisting of two or more concentric ceramic cylinders or metal plates with an air gap or a system consisting of a rod and a metal-plated ceramic tube.
- To change the value of trimmer capacitor, the screw is rotated. Here the distance between plates is changed.
- Depending on the rotation angle of movable electrode, capacitance value will change. The capacitance value changes continuously because almost they don’t have a stopper.
- Range of values : 3 to 150 pF, 1 to 10pF
- Working Voltage : 100Vdc
- Operating Temperature Range : -55°C to +85°C
Used in high frequency circuits where the value of capacitance is varied very rarely.