Trimmer Capacitor

 

What are trimmer capacitors?

Trimmer capacitors are variable capacitors which serve the purpose of initial calibration of equipment during manufacturing or servicing. They are not intended for end-user interaction. Trimmer capacitors are almost always mounted directly on the PCB (Printed Circuit Board), so the user does not have access to them, and set during manufacturing using a small screwdriver. Due to their nature, trimmer capacitors are cheaper than full sized variable capacitors and rated for many fewer adjustments.

Trimmer capacitors are used to initially set oscillator frequency values, latencies, rise and fall times and other variables in a circuit. Should the values drift over time, these trimmer capacitors allow repairmen to re-calibrate equipment when needed. There are two types of trimmer capacitors: air trimmer capacitor and ceramic trimmer capacitor.

Trimmer capacitor definition

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.

Applications for trimmer capacitors

The potential applications for trimmer capacitors are numerous. They are used whenever there is a capacitance value that needs to be matched to a certain circuit during the manufacturing process. The reason for their use (instead of using precise fixed-value capacitors) is that other elements in the circuit have their own tolerances and their values could differ by as much as 20% from what the engineer expected to see in a circuit. In order to adapt to those tolerances, trimmer capacitors are used.

They are commonly used in various RF circuits, VHF through microwave. Special non-magnetic types are used in medical devices such as MRI and NMR scanners, which produce very high magnetic fields that would otherwise destroy capacitors containing ferromagnetic materials such as steel. Other common applications include oscillators, tuners, crystal oscillators and filters. Trimmer capacitors can be found in communication equipment such as mobile radios and aerospace transmitters and receivers, signal splitters and CATV amplifiers.