Scott Newman
Voltronics Corporation, 100 Ford Road, Denville, NJ 07834; (973) 586-8585, FAX: (973) 586-3405, Internet: www.voltronicscorp.com.

Trimmer capacitors add a great deal of flexibility to production-line circuits as well as tuning capability to prototype designs. For many circuits, the capability of selecting a capacitance value from a range, rather than settling for a fixed value, brings with it the ability to set the frequency of an oscillator or adjust the center frequency of a bandpass filter. The applications for trimmer capacitors are many, usually at only slightly more cost than that of fixed-value components. Once a trimmer capacitor makes sense for a circuit, however, comes the task of specifying the right trimmer for the application.

Specifying a trimmer capacitor is a simple matter of making a few decisions regarding the size and performance of the component. First of all, how much capacitance adjustment range is needed? Trimmer capacitors come in many forms, based on different dielectric materials and package styles. The capacitance range should be at least large enough to accommodate the tuning task, whether it involves correcting the frequency of an oscillator due to drift over time or because of material tolerances, or to move the center frequency of a filter based on the needs of a communications system. When deciding upon a capacitance range, it should be noted that too-large a range will nonetheless support whatever frequency adjustments need to be made, whereas too narrow a capacitance will not provide the frequency correction needed.

In addition, different manufacturers specify capacitance ranges differently, depending upon how their components were measured and how conservative the manufacturer chooses to be. Voltronics Corp. measures the capacitance range of trimmer capacitors with proven, precision test equipment from Boonton Electronics (www.boonton.com) and publishes conservative specifications for its parts. As a result, all components provide tuning ranges that exceed the minimum and maximum values provided in data sheets, ensuring designers that the range they specify will be at least the range they add to their circuits.

Trimmer capacitors provide adjustment ranges that can be controlled by a half-turn, single-turn, or multiple turns of a screw. Although the capacitance range of half-turn components tends to be narrower than the ranges typically handled by single-turn or multi-turn trimmers, the number of turns translates into higher tuning resolution. Engineers needing tight control over the capacitance adjustment range are better served by a multi-turn trimmer, whereas a half-turn trimmer is fine for applications requiring quick tuning over a narrow capacitance adjustment range. In addition, the manner in which the screw turns can be an issue. Trimmer capacitors from Voltronics, for example, employ non-rotating pistons so that the position and height of the tuning screw remain fixed on the trimmer capacitor body. For trimmers using a rotating piston, the height of the tuning screw will change as a function of the capacitance selected, which may be a limiting factor in dense circuit designs. In addition to the use of non-rotating pistons, Voltronics’ trimmers employ an O-ring seal that protects against contamination. No external sealing mechanism is needed and tuning can be accomplished even as the seal provides environmental protection, compared to rotating designs which when sealed do not allow further tuning.

How high in frequency must the trimmer operate? This question can quickly be answered by referring to the data sheet for a trimmer capacitor and noting the quality factor (Q) and self-resonant frequency (SRF). The higher the Q value, the higher in frequency the trimmer will operate. Since Q is a function of frequency (decreasing with increasing frequency), any comparison between trimmer capacitors from different suppliers should be referenced to a common frequency, such as 1 MHz or 100 MHz. When comparing the SRF values for different trimmer capacitors, be aware that this frequency is referenced to a specific capacitance value. Conservative suppliers will measure the SRF at the highest capacitance value.

What kind of voltage rating is required? The voltage capabilities of trimmer capacitors from Voltronics, for example, are delineated by two parameters: DC working voltage and DC withstanding voltage. The first refers to a typical maximum value of continuous voltage for a circuit while the second is a short-term peak value above which will generally result in damage to the capacitor. The voltage rating is a function of the amount of isolation between conductors in a trimmer capacitor, with isolation provided by the dielectric material. Dielectrics used in trimmer capacitors range from air, to glass, to sapphire, to PTFE, with different voltage ratings depending upon the dielectric material and the physical size of the trimmer capacitor. The P series of sapphire trimmer capacitors from Voltronics, for example, have a DC working voltage rating of 500 V and DC withstanding voltage that is twice as high, at 1000 V. The company’s smaller glass-dielectric S series trimmers have DC working voltage rating of 250 V.

For applications requiring high voltages, Voltronics manufactures the NT series of PTFE-dielectric trimmer capacitors with DC working voltage ratings as high as 7500 V and DC withstanding voltages as high as 15,000 V. (Voltronics typically rates its trimmer capacitors for DC withstanding voltages that are twice the value of a component’s DC working voltage.) The NT series trimmers cover capacitance-tuning ranges as wide as 2 to 100 pF. The tradeoff for high voltage is size: the NT trimmers with the widest capacitance tuning ranges are about 5 inches in length.

The different dielectric materials bring different performance characteristics to trimmer capacitors. Sapphire dielectrics feature a very low loss tangent for good performance at high frequencies (past 12 GHz, depending upon the capacitance range). The material does not exhibit a change in dielectric constant with frequency, making these trimmers ideal for applications requiring high stability. Sapphire is mechanically strong and moisture resistant, and sapphire trimmers are well suited for precision capacitance adjustments at voltages to about 500 V. In contrast, trimmers with PTFE dielectric material are capable of higher voltages but at lower frequencies, although less subject to the ionization problems suffered by low-loss air-dielectric trimmers in space applications.

Another issue to be addressed is one of operating temperature range, which usually depends on whether the final application is for commercial or military use. Glass-dielectric trimmers from Voltronics, for example, are rated for operating temperatures from -55 to +125ºC, while the firm’s quartz-dielectric trimmers are rated for operating temperatures from -55 to +150ºC. Trimmer capacitors intended for military use should be specified according to MIL-PRF-14409 specifications which call out screening of parts according to specific levels of mechanical shock and vibration.

Along with temperature range, trimmer capacitors are specified by their capability to maintain their nominal capacitance values over temperature, using a temperature coefficient. The temperature coefficient is stated in terms of capacitance deviation in parts per million (ppm) per degree of temperature change. For example, the temperature coefficient for a Voltronics A1 series PTFE-dielectric trimmer with tuning range of 0.5 to 8 pF is 0 ± 100 ppm/ºC, while the temperature coefficient for a trimmer with narrower tuning range of 0.45 to 4 pF is 0 ± 50 ppm/ºC.

Some final considerations in selecting a trimmer capacitor are tuning torque and size. Tuning torque is simply the amount of force needed to turn the adjustment screw on a trimmer. A certain amount of torque is desirable to allow for precision adjustments without overshoot when tuning. Tuning torque can be as little as 0.2 inch-ounces to more than 5 inch-ounces, depending on the size and type of trimmer.

Traditionally, trimmer capacitors have been supplied in tubular shaped packages with leaded connections to a printed-circuit board (PCB). In recent years, chip-sized trimmer capacitors such as the J Series ceramic chip capacitors from Voltronics have become available in sizes comparable to fixed-value capacitors. For example, the firm’s JN Series trimmers measure only 0.067 x 0.059 x 0.035 in. (1.7 x 1.5 x 0.9 mm) but provide adjustable capacitance from 1.5 to 4.0 pF.

In addition to deciding upon a package style and size, specifiers may ask one final question regarding the application: Does it require a non-magnetic trimmer capacitor. In some systems, notably magnetic-resonance-imaging (MRI) systems used in hospitals and research facilities in which a large magnetic field is generated, trimmer capacitors must be absolutely non-magnetic in order to not distort the magnetic field used for imaging. Voltronics offers non-magnetic versions of its different trimmer capacitor types for these demanding medical and research applications.