Capacitance bridge

Parallel plate capacitor Capacitance Impedance bridge, capacitance meter Moderate to high Moderate Moderate to large... 

In some cases it is possible to form bridges of metal using air as the dielectric (150). However, if more than two levels of wiring are required then dielectric spacing is necessary. The ideal dielectric film has excellent adhesion and alow dielectric constant to minimize parasitic capacitances. The most common films include siUcon oxide, siUcon nitride, and a number of spin-on dielectrics (216). 

In the voltage method the normal voltage is applied across the capacitor terminals and line current, /, is measured. The value of capacitance, C, can be determined from the equations provided in Section 23.5.2. In the bridge method... 

The energy stored on a conductor can readily be calculated from the object s capacitance and voltage (C-1.2). The capacitance may be directly measured using a capacitance bridge or may be estimated from the object s size and shape. A-4-1.3 gives some capacitances of common objects. 

A key factor in the suitabihty of cokes for graphite production is their isotropy as determined by the coefficient of thermal expansion. After the calcined coke was manufactured into graphite, the axial CTE values of the graphite test bars were determined using a capacitance bridge method over a temperature range of 25 to 100°C. The results are summarized in Table 24. Also included in the table are bulk density measurement of calcined cokes and the resistivity values of their graphites. 

The ionic conductivity of a solvent is of critical importance in its selection for an electrochemical application. There are a variety of DC and AC methods available for the measurement of ionic conductivity. In the case of ionic liquids, however, the vast majority of data in the literature have been collected by one of two AC techniques the impedance bridge method or the complex impedance method [40]. Both of these methods employ simple two-electrode cells to measure the impedance of the ionic liquid (Z). This impedance arises from resistive (R) and capacitive contributions (C), and can be described by Equation (3.6-1) ... 

The specimen may be a sheet of any size convenient to test, but should have uniform thickness. The test may be run at standard room temperature and humidity, or in special sets of conditions as desired. In any case, the specimens should be preconditioned to the set of conditions used. Electrodes are applied to opposite faces of the test specimen. The capacitance and dielectric loss are then measured by comparison or substitution methods in an electric bridge circuit. From these measurements and the dimensions of the specimen, dielectric constant and loss factor are computed. 

Capacitive Sensors. This device usually consists of a capacitor which is formed either from two concentric cylinders or from a pair of parallel plates. The solid sample to be analyzed for moisture content is passed between these plates. Since w has a large dielectric constant, the w content of the sample causes a significant change in the dielectric constant of the solid, which is measured using bridge or frequency techniques. 

The differential capacity can be measured primarily with a capacity bridge, as originally proposed by W. Wien (see Section 5.5.3). The first precise experiments with this method were carried out by M. Proskurnin and A. N. Frumkin. D. C. Grahame perfected the apparatus, which employed a dropping mercury electrode located inside a spherical screen of platinized platinum. This platinum electrode has a high capacitance compared to a mercury drop and thus does not affect the meaurement, as the two capacitances are in series. The capacity component is measured for this system. As the flow rate of mercury is known, then the surface of the electrode A (square centimetres) is known at each instant ... 

Capacitance measurements are quite simple. A typical drawback is the need of coaxial cables that introduce a thermal load which is not negligible in low-power refrigerators. On the other hand, capacitance bridges null the cable capacitance. Multiplexing is more difficult than for resistance thermometers. In principle, capacitors have low loss due to Joule heating. This is not always true losses can be important, especially at very low temperatures. Dielectric constant thermometers have a high sensitivity capacitance differences of the order of 10-19F can be measured. 

In several capacitance bridges and a few resistance bridges, a computer is used to control the feedback, as shown in Fig. 10.6. An Anderson bridge is used in this diagram. The set range line controls several relays which select the value of R. The computer also controls the ratio transformer. 

The conductance measurements were made with an Automatic Capacitance Bridge Assembly (General Radio Company 1680) at a frequency of 1 kHZ. 

The resistor (R1) is used to control the capacitor. The first diode (D1) can be either a single diode (halfwave rectified) or a bridge rectifier. The second diode (D2) is to protect the capacitor from the HV spike from the coil and should be large enough to provide blocking resistance to this spike. The third diode (D3) similarly prevents the capacitance discharge from flowing back to the coil. 

The dilatometer stem is coaxially enclosed in an open-ended stainless-steel sheath which serves as one plate of a capacitor in a manner similar to the sheath used in the high-pressure porosimeter. When connected to the capacitance bridge of the porosimeter, the filling apparatus, using its own low-pressure transducer, measures the intruded volume and continuously plots the data on an X-Y recorder up to 24 psia. 

If the material exhibits slow polarization currents during switching, then the bridge technique fails, unless one uses an identical sample for the nuUing capacitance and employs a resistance ratio bridge. 

In the low-frequency direction, the capacitative impedance (1/icoC) dominates. It is remarkable how in recent years the capability of commercial ac bridges to measure very low frequencies (10—3 to 10-4 cycles s l) has increased. However, the limit here is not so much in the electronics, but more in the stability of the interlace. A frequency of 10-4 cycles s 1 means a cycle some 3 hr in duration, and it is difficult to maintain a clean and stable solid/solution interface for this length of time. 

The sample, a reverse-biased p-n or metal-semiconductor junction, is placed in a capacitance bridge and the quiescent capacitance signal nulled out. The diode is then repetitively pulsed, either to lower reverse bias or into forward bias, and the transient due to the emission of trapped carriers is analyzed. As discussed in the preceding section, for a single deep state with JVT Nd the transient is exponential with an initial amplitude that gives the trap concentration, and a time constant, its emission rate. The capacitance signal is processed by a rate window whose output peaks when the time constant of the input transient matches a preset value. The temperature of the sample is then scanned (usually from 77 to 450°K) and the output of the rate window plotted as a function of the temperature. This produces a trap spectrum that peaks when the emission rate of carriers equals the value determined by the window and is zero otherwise. If there are several traps present, the transient will be a sum of exponentials, each having a time... 

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