A trimmer capacitor is a variable capacitor used for initial calibration and recalibration of equipment. It is commonly mounted directly on a PCB and accessed only by professional repairmen, not the end-user.
Characteristics
Voltage rating, capacitance range, polarity.
Trimmer capacitors can be rated for voltages up to 300 volts, although voltage ratings of up to 100 volts are much more common. Since trim caps are variable capacitors, they come in a capacitance range rather than a
single capacitance value. The minimum capacitance is usually between 0.5 pF and 10 pF, while the maximum capacitance is usually between 1 pF and 120 pF. The actual capacitance value can be varied between the
minimum and maximum capacitance values for a given trimmer capacitor, but it can never be set to zero. It is worth noting that trimmer capacitors are not polarized.
Tolerances and accuracy
Trimmer capacitors do not boast a good capacitance value tolerance. Sometimes, the tolerances can be as high as -0 to +100%. This means that a trimmer capacitor can have a maximum capacitance two times larger
than nominal. However, bad tolerances do not pose a great problem to engineers because trimmer capacitors are variable. Even if the maximum value is different between individual capacitors, they can still be set by turning the screwdriver a certain angle. Accuracy depends mostly on the operator, as he can choose to spend more time in order to set the capacitor to a desired value. Often, trimmer capacitors are set by robots instead of human operators, and they can achieve much better precision. In order to achieve a better accuracy, it is advised to use a non-metallic tool, since metal screwdrivers will introduce a source of capacitance that will vary the capacitance value when the tool is moved away from the capacitor.
Construction and properties of trimmer capacitors
There are two types of trimmer capacitors: air trimmer capacitor and ceramic trimmer capacitor. These two types use different materials as the dielectric. Both types use rotating action to change the capacitance value.
The construction of trimmer capacitors is similar to the construction of their larger variant, the variable capacitor. Trimmer capacitors can be made of semi-circular metal plates. One is fixed, while the other can be rotated using a screwdriver. The user changes the capacitance by rotating the shaft and increasing or reducing the amount of overlap between the two plates. Gear mechanisms may be used to improve the precision by allowing multiple turns between the lowest and highest settings.
Another way to make a trimmer capacitor is to place a metallic screw in a non-conductive threaded cylinder. The screw represents one electrode, while the other is located at the base of the cylinder. By rotating the screw, the distance between the two plates is varied which results in a change of capacitance. This construction is used in RF and microwave applications.
