Capacitors are basic circuit elements that store and provide charges on demand, and are the basic components of passive circuits and auxiliary components of active circuits. When designing a circuit, engineers often have to weigh many factors, such as choosing a fixed capacitor or an adjustable capacitor. Although most circuits choose to use fixed capacitors, in some cases, the use of adjustable capacitors is more ideal or necessary. Next, I will take you to learn more about adjustable capacitors and help you make your ideal choice.
What Is An Adjustable Capacitor?
The tunable capacitor is a variable capacitor, which allows a small range of variable tuning, such as the frequency value of the oscillator or the number of rises and falls. In addition, if the value shifts during the life of the device, the adjustable capacitor can be recalibrated as needed. For sensitive applications such as magnetic resonance imaging (MRI), any instability due to time or temperature may affect the image output. Choosing an adjustable capacitor can help optimize device performance.
What is the difference between adjustable capacitors and fixed capacitors?
Generally speaking, although the cost of adjustable capacitors is slightly higher than that of fixed capacitors, it also provides greater flexibility. But when the problem of capacitance accuracy is encountered, the cost of using a fixed capacitor with high accuracy will be higher, so adjustable capacitors may be more cost-effective. Although the volume of fixed capacitors was significantly smaller than that of adjustable capacitors, the emergence of chip adjustable capacitors has made up for this gap.
Unlike fixed capacitors, which have a fixed capacitance value, adjustable capacitors can adjust the performance of active and passive circuits. The value of the adjustable capacitor is usually between 1 pF and 2 pF, but it can also be expanded to 200 pF and above. Fixed capacitors store charges through two fixed metal layers, but in adjustable capacitors, it is necessary to adjust the distance between these metal layers or adjust the area of the effective area to change the capacitance.
Basic Structure Of Adjustable Capacitor
As with fixed capacitors, in adjustable capacitors, a variety of dielectric materials are used as electrical insulating materials between metal layers or other metalized surfaces. The accuracy and consistency of the dielectric have a great influence on the accuracy and stability of the adjustable capacitor capacitance.
Adjustable capacitors can be designed into tubular and plate-like structures. By moving the piston inside the outer metalized dielectric tube, the capacitance will change, and as the piston overlaps with more fixed metal inner walls, the capacitance will increase. You can also use a piston and a set of movable concentric metal rings to achieve this change. These rings are installed in a set of fixed parallel rings. As the rings are engaged, the capacitance increases. In tubular adjustable capacitors, a piston connected to an adjustment screw can be used to adjust the capacitance.
How To Choose The Dielectric In A Tunable Capacitor?
In tunable capacitors, a variety of dielectrics can be used to fill the space between the metal layers. These materials include air, ceramics, glass, PTFE, and sapphire. Among them, air provides the least insulation and is very limited in terms of voltage handling capability and capacitance value. Adjustable capacitors that use glass, quartz and polytetrafluoroethylene as dielectric materials can provide sufficient insulation at higher rated voltages and can obtain higher capacitance values.
In high-frequency applications with higher requirements for quality (Q) factor and series resonance frequency, multi-turn adjustable capacitors using air, sapphire or PTFE as dielectric materials can provide the lowest loss and the best overall performance. The amount of insulation provided by the dielectric material affects the rated voltage of the capacitor, which is usually expressed in terms of its withstand voltage (DWV). For example, polytetrafluoroethylene has a higher dielectric constant than air (the dielectric constant of air is equal to 1), which can provide a higher withstand voltage (DWV) for adjustable capacitors-about 15,000V or higher.
Adjustable capacitors using ceramic dielectrics are small in size, inexpensive, and can be packaged on tape and are suitable for automated manufacturing machines. The capacitance range of this type of capacitor can be set to around 40 pF, which is very suitable for applications that need to meet the needs of small size and low cost. However, ceramic adjustable capacitors are often limited by temperature stability, and the performance will decrease as the capacitance increases. These components have a quality factor of approximately 1,500 at 1 MHz and a nominal temperature coefficient of 0 to 750 ppm/°C. The capacitance drift range is about ±1% to ±5%, and the maximum withstand voltage (DWV) does not exceed 220 VDC.
The adjustable capacitor with sapphire as the dielectric is very durable. Its dielectric constant value does not change with frequency, has high mechanical strength and moisture resistance, and its loss is very low, even above 10 GHz. The Giga-Trim adjustable capacitor of Lou's Capacitor uses sapphire as the dielectric and is a high-strength miniature adjustable capacitor with excellent electrical characteristics. This type of capacitor can withstand high-temperature welding, over-tuning and rough operation. Sapphire has excellent dielectric and insulation properties, so it can withstand high breakdown voltage.
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