Like all capacitors, variable capacitors are made by placing two sets of metal plates parallel to each other (Fig. A) separated by a dielectric of air, mica, ceramic, or a vacuum. The difference between variable and fixed capacitors is that, in variable capacitors, the plates are constructed in such a way that the capacitance can be changed. There are two principal ways to vary the capacitance: either the spacing between the plates is varied or the cross-sectional area of the plates that face each other is varied.
(A) Schematic view
Figure B shows the construction of a typical variable capacitor used for the main tuning control in radio receivers. The capacitor consists of two sets of parallel plates. The stator plates are fixed in their position and are attached to the frame of the capacitor. The rotor plates are attached to the shaft that is used to adjust the capacitance.
(B) construction
Another form of variable capacitor used in radio receivers is the compression capacitor shown in Fig. C. It consists of metal plates separated by sheets of mica dielectric. In order to increase the capacitance, the manufacturer may increase the area of the plates and mica or the number of layers (alternating mica/metal) in the assembly. The entire capacitor will be mounted on a ceramic or other form of holder. If mounting screws or holes are provided then they will be part of the holder assembly.
(C) mica compression variable capacitor
Still another form of variable capacitor is the piston capacitor shown in Fig. D. This type of capacitor consists of an inner cylinder of metal coaxial to, and inside of, an outer cylinder of metal. An air, vacuum, or (as shown) ceramic dielectric separates the two cylinders. The capacitance is increased by inserting the inner cylinder further into the outer cylinder.
(D) piston variable capacitor.
The small-compression or piston-style variable capacitors are sometimes combined with air variable capacitors. Although not exactly correct word usage, the smaller capacitor used in conjunction with the larger air variable is called a trimmer capacitor. These capacitors are often mounted directly on the air variable frame or very close by in the circuit. In many cases, the “trimmer” is actually part of the air variable capacitor.
Trimmer capacitor
There are actually two uses for small variable capacitors in conjunction with the main tuning capacitor in radios. First, there is the true “trimmer,” i.e., a small-valued variable capacitor in parallel with the main capacitor (Fig. 2A). These trimmer capacitors (C2) are used to trim the exact value of the main capacitor (C1). The other form of small capacitor is the padder capacitor (Fig. 2B), which is connected in series with the main capacitor. This error in terminology is calling both series and parallel capacitors “trimmers,” when only the parallel connected capacitor is properly so-called.
(2A) Trimmer capacitor connected in parallel with the main tuning capacitor
(2B) padder capacitor is connected in series with the main tuning capacitor.
The capacitance of an air variable capacitor at any given setting is a function of how much of the rotor plate set is shaded by the stator plates. In Fig. 3A, the rotor plates are completely outside of the stator plate area. Because the shading is zero, the capacitance is minimum. In Fig. 3B, however, the rotor plate set has been slightly meshed with the stator plate, so some of its area is shaded by the stator. The capacitance in this position is at an intermediate-value. Finally, in Fig. 3C, the rotor is completely meshed with the stator so the cross-sectional area of the rotor that is shaded by the stator is maximum. Therefore, the capacitance is also maximum. Remember these two rules:
(3A) capacitance is minimum
(3B) intermediate capacitance
(3C) maximum capacitance.
1. Minimum capacitance is found when the rotor plates are completely unmeshed with the stator plates; and
2. Maximum capacitance is found when the rotor plates are completely meshed with the stator plates.
Figure 4 single-section variable capacitor
Figure 4 shows a typical single-section variable capacitor. The stator plates are attached to the frame of the capacitor, which in most radio circuits is grounded. Front and rear mounts have bearing surfaces to ease the rotor’s action. The ganged variable capacitor
(Fig. 5) Dual section air variable capacitor.
(Fig. 5) was invented to provide tracking between two related LC-tuned circuits, as in a radio receiver. Such capacitors are basically two (in the case of Fig. 5) or more variable capacitors mechanically ganged on the same rotor shaft.
In Fig. 5, both sections of the variable capacitor have the same capacitance, so they are identical to each other. If this capacitor is used in a superheterodyne radio, the section used for the local oscillator (LO) tuning must be padded with a series capacitance in order to reduce the overall capacitance. This trick is done to permit the higher-frequency LO to track with the RF amplifiers on the dial.
In many superheterodyne radios, you will find variable tuning capacitors in which one section (usually the front section) has fewer plates than the other section. One section tunes the RF amplifier of the radio, and the other tunes the local oscillator. These capacitors are sometimes called cut-plate capacitors because the LO section plates are cut to permit tracking of the LO with the RF.
