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.

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.

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.

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.

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