Variable Capacitors

A variable capacitor is constructed to allow a user to adjust its capacitance mechanically. For example, in a radio application, using a variable capacitor coupled with an inductor allows the capacitance to be manipulated to resonate 

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In another type of measurement, the parallel between mechanical and electrical networks can be exploited by using variable capacitors and resistors to balance the impedance of the transducer circuit. These electrical measurements readily lend themselves to computer interfacing for data acquisition and analysis. 

Fig. 14.4 Apparatus for electrodeless photochemical irradiation. A. antenna, B. transmitter, C-j. capacitor, C2. variable capacitor,...

Figure 8.2.2 (a) Schematic of the four-coil probehead introduced in Reference [12]. The four individual solenoidal coils are represented by an inductance (L), series resistance (R), and inter-turn capacitance (Q. The entire circuit was impedance-matched to 50 it at a frequency of 300 MHz by using the variable capacitors Ct and Cm. (b) Photograph of the four-coil assembly. Reprinted from MacNamara, E., Hou. T., Fisher, G., Wilhams, S. and Raftery, D., Multiplex Sample NMR an approach to high-throughput NMR using a parallel coil probe , Anal. Chem. Acta, 397, 9-16, copyright (1999), with permission of Elsevier Science... 

The probe tuning rods are long extensions of the variable capacitors located at the top of the probe, near the probe coil. The capacitors are delicate and there are two ends of the travel of the knob If any force at all is applied at the end of the travel, the capacitor will break. This will usually require that the probe be sent back to the manufacturer for repair, a process requiring a week or two and costing many thousands of dollars. For this reason many NMR labs do not allow users to tune the probe ... 

A capacitance cell suitable for work with liquids or solutions is shown in Fig. 1 it is made with a small variable-air capacitor of the type formerly in common use in radios and electronic circuits. It should have a maximum capacitance of 50 to 200 pF. This device is more convenient than a fixed-plate capacitor, since with the latter device it is necessary to measure separately the stray capacitance due to electrical leads, etc. In the cell shown, the variable capacitor is used in two positions fully closed (maximum capacitance) and fully open (minimum capacitance) these positions are defined by mechanical stops for the pointer on the knob that rotates the capacitor shaft.f The difference ACbetween the closed (b) and open (a) positions is independent of the stray capacitance. Thus the dielectric constant of the liquid or solution is given by... 

Rinse the cell beaker and variable capacitor with the pure nonpolar solvent, fill the cell beaker to a level that will completely immerse the capacitor plates, and reassemble the cell. Make measurements in the open and closed positions as described above. Repeat this procedure with all the solutions of the polar solute in the nonpolar solvent. Note the temperature of the bath. 

Variable capacitors are available with values up to a few hundred picofarads. These are commonly formed from sets of interleaved plates, one fixed and the other attached to a shaft. Rotation changes the effective area and thereby the capacitance. Arrangements with sliding cylinders are also used and dielectrics include air, mica, and ceramic. Varacter diodes, junction diodes in which the capacitance is determined by the reverse bias voltage, are now finding increasing use in circuits, because their capacitance can be actively controlled by other electronic circuit elements. 

Equal resistive ratio arms are used and a small variable capacitor is applied across resistance 1 to balance the conductance of the unknown in arm 4. A substitution procedure is generally used. First, the bridge is balanced with the sample cell out (switch S open). Then the sample cell is connected in (switch S closed) and balance regained by compensating for the cell capacitance Cx by decreasing C4 and for the cell conductance Gx by increasing Cj. From the two sets of balance conditions we obtain, for tan Sx < 10-2,... 

In 1961 Winefordner et al. [19] described a GC detector that functioned on the change in dielectric constant of the carrier gas when a vapor or another gas was present. The detector responded to most gases and vapors providing a suitable carrier gas was chosen. The sensor consisted of a variable capacitor mounted in a special cell which... 

Where clock-radios (figure 30) are employed, a variety of significant debris may be discovered. The bulk of all radio enclosures are manufactured cither from wood or plastics and these would, therefore, disintegrate in a fire of any magnitude. Most of these radios arc built on a rather heavy steel or aluminum chassis, and various components are often riveted to this metal base. A normal superheterodyne receiver will probably include a double-section variable capacitor having a radier heavy mounting. Carbon resistors or certain metal-encased capacitors may come through the fire reasonably intact. Parts of the clock... 

To measure spike intensity versus steepness of voltage pulse edges we introduced small variable capacitor connected in parallel with the sample. Increase in capacitive load led to a commensurate increase of the voltage pulse rise time. As we varied pulse rise-time from 2 ns to 90 ns, we observed the attenuation and eventual disappearance of transient EL peak. Therefore transient EL spikes can... 

When properly shielded and connected to the appropriate apparatus, ion-selective microelectrodes should show the response and selectivity similar to conventional size electrodes. They should be calibrated in a solution of similar composition to that of the sample solution to be analyzed. The response time of the electrode itself should be comparable to that of conventional electrodes. However, with these electrodes the diffusion of the ions through the stagnant layer near the ion-selective membrane has a smaller effect on the transient response than the time constant of the measuring circuitry. Measuring instruments often contain a variable capacitor to adjust the delay caused by the capacitance of the input stage. 

In Fig. 6.1.8a the variable capacitors are configured as a capacitive voltage divider. Conceptually, the center electrode samples the electrostatic field set up by the sense... 

In both cases the output is proportional to AC/Cg and the sense voltage Vs. Increasing the sense voltage results in higher sensitivity but is limited by supply voltage, electrostatic force exerted on the variable capacitor, and pull-in. 

Microelectromechanical systems (MEMS) combine the electronics of microchips with micromechanical features and microfluidics to create unique devices. The multitude of MEMS applications continues to grow including many types of accelerometers, radio frequency (RF) devices, variable capacitors, strain and pressure sensors, deformable micromirrors for image projection systems, vibrating micro-membranes for acoustic devices, ultrasound probes, micro-optical electromechanical systems (MOEMS) and MEMS gyroscopes, to name a few. 

There are many techniques used to measure the dielectric properties of solids. One of the more popular ones is known as ac impedance spectroscopy. described below. Another technique compares the response of the dielectric to that of a calibrated variable capacitor. In this method, the capacitance of a... 

The high impedance of the parallel resonance circuit is transformed to 50 Q nominally by the second variable capacitor. Both variable capacitors are Johan son 5341 which are piston air capacitors with Be-Cu stators and Ag rotors. They work very nicely except for the problem of high voltage breakdown. The dc breakdown under test conditions is listed as 1200 volts at sea level but that number has to be severely derated for radio frequency and altitude. (The latter problem is serious at Los Alamos where the atmospheric pressure is 590 mm Hg and the altitude correction to the breakdown is about 10%.)... 

This metallic layer acts as the fixed electrode of a variable capacitor, the variable electrode being the mercury inside the glass capillary tube. A wide tube which is metallised except for a double slit is electrically connected to the mercury and acts as shield. Details are shown in Figure 3.7. 

Electrical Measurements. Both ac and dc conductivity measurements were made on elemental sulfur and P4S3. The ac measurements were made with an Electro Scientific Industries model 290A impedance bridge in conjunction with a model 860A generator/detector at frequencies of 0.1, 1.0, and 10.0 kHz. A compensating variable capacitor was used either in series or in parallel with the variable resistance arm of the bridge. The... 

A, antenna B, transmitter Q, capacitor C2, variable capacitor D, jacketed flask E, EDL E, reaction mixture G, circulating coolant. Adapted from Ref [91]. 

Note that in Figure 5.3 a variable capacitor shunts Ri. This is to balance out any phase shifts in the alternating signal caused by the capacity eifects present at the electrode surfaces. It is adjusted to give the sharpest minimum in the null signal. For conductometric titrations it is generally not needed. 

Figure 4-100. OAUGDP experimental setup schematie A, water electrodes Hy high-voltage probes PMT, photomultiplier probe CT, eurrent transformer BIAS, parasitie current elimination tool Cy, variable capacitor OSC, oscilloscope PC, computer SIGNAL, harmonic signal generator RF AMP, radiofrequency power amplifier CCD, digital camera.

The equipment is basically a Wheatstone bridge and conductivity cell, as illustrated in Fig. 15.21. Resistance A is made up of the cell containing the sample B is a variable resistance resistances D and E are fixed. Resistor B and variable capacitor C may be adjusted so that the balance point can be reached. At this point... 

Dielectric elastomer (DE) actuators are essentially compliant variable capacitors. They consist of a thin elastomeric film coated on both sides by compliant electrodes. When an electric field is applied across the electrodes, the electrostatic attraction between the opposite charges on opposing electrode and the repulsion of the like charges on each electrode generate stress on the film causing it to contract in thickness and expand in area (Fig. 1.9). Most elastomers used are essentially incompressible, so any decrease in thickness results in a concomitant increase in the planar area. 

The extension of their previously developed low temperature method to an 18.8 T system has been described by Lipton et al. A new probe was used with a cross coil and variable capacitors that are operational at cryogenic temperatures. The limitations to sensitivity are discussed, including a new diode network, the utilisation of a cryogenic band pass filter, and the consequences of the RF profiles of the coil. Further, details of the spectroscopy of quadrupolar nuclei in a protein are discussed, such as the observation of the outer transitions and how to distinguish them from the desired 1/2 transition. 

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