Device construction & Distinguishing traits
Trimmer and variable capacitors are devices that provide a capacitance which is variable within some range, the difference between the two terms being mostly one of design intent; a “trimmer” capacitor is usually intended to be adjusted only a handful of times over its service life, while a “variable” capacitor anticipates routine adjustment. Numerous different construction types are used, but with few to zero exceptions, they are of the electrostatic variety and achieve their adjustability by changing the effective surface area between the electrodes, the distance between them, or perhaps both.
One common design approach resembles two small wheels on a common axle, with a semicircle (or similar shape) of electrode material plated on each. By changing the angle of rotation of the two “wheels” relative to each other, the effective capacitance between them can be varied. Beyond this, changing the shape of the electrodes on each “wheel” can produce varying relationships between rotational adjustment angle and device capacitance as needed for a given application. A variation on the approach may involve using a worm gear or similar mechanical arrangement to vary the relative rotation of the two “wheels” in order to provide a higher adjustment resolution within the device’s range of variation. Other designs include variable piston capacitors, which operate by varying the degree of overlap between concentric cylinders, and vacuum capacitors that use a screw or other mechanism to vary the mechanical relation between electrode plates in a vacuum which is maintained through the use of a flexible membrane.
Common usages & applications
Trimmer and variable capacitors are generally used for tuning & matching applications in RF circuits. Radio receivers that indicate the selected tuning frequency by sweeping a mechanical indicator past a scale (or vice-versa) typically have a mechanical linkage between the indicator and the variable capacitor(s) used in the tuning circuit. Most such receivers are of older vintage or lower cost/quality design, though modern applications may still include trimmer capacitors for fine-tuning or calibration purposes. Variable capacitors (the sort designed for frequent adjustment) on the other hand are something of an endangered species; the use of alternative design techniques enabled by better manufacturing tolerances and newer technologies renders the characteristically bulky, drifty, mechanically cumbersome, and expensive variable capacitor a less desirable design element than the alternatives.
Common failure mechanisms/critical design considerations
The wide variations in device construction that can be found within trimmer and variable capacitors preclude extended discussion on their specific merits and drawbacks herein. If one considers the basic principle behind an electrostatic capacitor however, a given device’s merits can often be discerned though observation; anything that affects the dielectric, electrode geometry, or electrode positioning will affect device capacitance. For example, an air-dielectric device will exhibit changes in capacitance at a given setting with changes in barometric pressure, temperature, and humidity because all of these factors affect the dielectric constant of air to a small degree. Similarly, a vacuum-dielectric capacitor will be affected by leakage or loss of vacuum.
From a mechanical perspective, the rigidity of the final assembly will affect capacitance stability with respect to mechanical shock or vibration, and the design of the adjustment mechanism will also influence the tendency for drift over time.