Variable capacitors / trimmers are used for trimming and tuning purposes. They represent a small but important part of the capacitor assortment.
By means of an electrode system consisting of one fixed and one movable part – stator and rotor – the capacitance can be varied between a minimum and a maximum value, the so called capacitance swing.
The temperature coefficient (TC) for the different occurring dielectrics usually differs considerably from the corresponding values for fixed capacitors. The variations are, except for what applies for the best precision components, considerably larger which has to do with the mechanical conditions that the whole construction is based upon.
Trimmer capacitors are mainly designed for mounting on printed circuit boards (PCB) but surface mount designs are getting more and more common. Trimmers often have a friction that increases the turning moment and thus locks the capacitor in its adjusted position.
The classic variable capacitor consists of semicircular electrodes that can be turned into each other as shown in Figure 1. The styles are intended either for PCB or panel mounting. They are used preferably for tuning of resonance circuits.
Figure 1. Schematic of an air-insulated variable capacitor and an example of Tronser’s make.
The air-insulated electrodes put great demands for mechanical precision. The plate distance usually is 0.2 to 1 mm. The price is relatively high.
If we reduce the plates in Figure 1 to only one silver plated ceramic rotor that is turned in over the stator electrode we have got a ceramic trimmer capacitor. An example of this design is shown in Figure 2. Multilayer designs also exist. The capacitor styles are designed for either hole mount or surface mount.
Plastic foil types
If we exchange the air insulation in Figure 1. for some plastic foil the electrode distance can be decreased contemporaneously as the εr – and thus the capacitance, – increases, unfortunately to the trade off of a somewhat poorer Q value. Low loss plastics dielectrics like teflon (PTFE), polypropylene (PP) and polycarbonate (PC) are common but also polyester (PETP) exists.
In the concentric type air usually serves as dielectric. Rotor and stator are made from, for example, gold-plated brass. The concentric type consists of concentrically-orientated metal tubes on both the movable rotor and the stator.
They are inserted into each other from the rotor side and are separated from each other by an air gap. It is a matter of precision design that permits air gaps down to 0.1 mm (4 mils). The Q value will be high and the dimensions comparatively large. The temperature coefficient is very small.
In order to get a setting stability one manufacturer uses one or several slits in the rotor that with spring force increase the friction and lock the trimmer in the adjusted position.
The piston design is used with air, teflon or sapphire as dielectric. With an εr of approximately 8 the sapphire dielectric achieves a certain capacitance per volume gain. Both hole and surface mount exist. A cross section through an SMD design with sapphire dielectric is shown in Figure 4. As for the concentric design the TC will be small.
Trimmers for commercial use sometimes lack satisfactory encapsulation. Then they must not be washed or exposed to solder fluxes or cleaning solvents. Encapsulated types with plastic housings should not be exposed to trichloroethylene.
For sensitive applications like medical magnetic resonance imaging (MRI), these components help to optimize performance where any instability in time or temperature could impact the image output.
In essence, when a patient is positioned inside an MRI scanner tube and a magnetic field is applied, the proton spins inside their body’s molecules line up, facing the same direction. When a short, computer-generated RF signal is applied to a proton in the uniform field, it’s “nudged” to break formation. After the interruption, protons return to their original state of alignment. In the process of realigning, energy is emitted; the emitted energy can be measured and used to identify different types of molecules and their locations in the body. Trimmer capacitors are used to tune the TX and RX coils to Lamor Frequency, the frequency at which this energy is emitted. By extension, trimmer capacitor tuning is pivotal in MRI imaging accuracy.
In order to meet the high demand of MRI, important considerations for trimmer capacitor selection include:
|Q Factor||Influences power handling; higher Q lessens self-heating under RF conditions Important in filter circuits; impacts insertion loss|
|K Value (dielectric constant)||Determines the capacitance density in conjunction with dielectric withstand voltage; higher the K Value, smaller the component can be|
|Dielectric Withstand Voltage||The maximum DC voltage the part can withstand without failure|
|Non-magnetic Properties||Essential for MRI components, especially body coil and surface coils Close control of raw materials and processes required to ensure MRI accuracy and performance|
|Half-turn vs. Multi-turn Trimmers||Half turn trimmers have a lower Q, DWV, and precision; typically used in lower voltage and power environments where some tuning is required Multi turn trimmers boast higher Q, DWV, and performance; provides precision where exact tuning is imperative|
Just as with potentiometers, variable capacitors are electromechanical creations with a correspondingly high failure rate. The contact resistance to the rotor can vary and produce contact disturbances.
Contamination – not the least from flux cleaning – may cause similar problems. Contamination and corrosion of mechanical parts may occur. Short circuits due to severe mechanical misalignment of air insulated parts are reported.