The capacitor that can change the capacity is caIIed a variable capacitor.
Like variable resistors, variable capacitors are found in several types. AII of them are manufactured starting from the same principles: a moving plate can slide in front of a fixed plate with an insulator between them, or a moving plate can change the distance from a fixed plate also with a dielectric between them.
Two basic variable capacitor types are found in electronic applications the trimmer, one that can be adjusted by a screwdriver, and the variable capacitor, or varicon, that is adjusted by a button placed in its axis.
Symbols and Views
Figure 1 gives the symbols used to represent variable capacitors and views of the most common types.
The variable capacitor shown in (a) is found in antique devices such as radios and tuners. The dielectric is the air gap between the moving plates and the fixed plates. In (b) are variable capacitors like those found in modern small transistor radios. In this type the dielectric is a polyester foil between the moving plates and the fixed plates.
The trimmer capacitors can be found with plastic, ceramic, and mica as the dielectric. Adjustment is made by a screw that moves one of the plates as shown in Figure 1 (c).
Specifications
Two main specifications are important when choosing variable or trimmer capacitors.
1. Capacitance
For variable capacitors, normally maximum capacitance is shown as nominal capacitance. Thus, a 120 pF capacitor is a capacitor that can change capacitance between a smaller value and 120 pF.
However, even when totally open the capacitance isn’t zero, which is normal, to indicate the minimum capacitance. For trimmer capacitors the capacitance range is given. For example, a 2-20 pF trimmer capacitor can be adjusted to present any value between 2 and 20 pF.
2. Type
Type indicates how many sections the capacitor has and of what material the dielectric is made. A two-section, air-dielectric capacitor with 365 pF sections is a type commonly found in old radios.
Where they are found
Variable capacitors are found in equipment operating with RF or radio signals like radios, remote control receivers and transmitters, wireless telephones, wireless communicators, and many others. They are used for adjustment or tuning circuits, placing them in a determined frequency.
Testing
The main problem of variable capacitors is shorts between plates. To test this, measure the resistance between the fixed and moving plates (the component must be disconnected from the circuit for this test). The resistance must be infinite in any position of the adjustment to indicate a good device.
