Reference capacitor

The Sawyer Tower measurement circuit is based on a charge measurement method which relies on a reference capacitor in series with the ferroelectric capacitor [2], The voltage drop... 

IZrl is the complex impedance module of the equivalent circuit formed by the reference capacitor tilled with A1203... 

In the case of DS, the same type of circuit shown in Figure 4.41 is used, fed with a varying frequency alternating signal (Vm 0.1 V of amplitude and the frequency in the range 30 Hz standard capacitor (Cs), and the Fout was measured with an oscilloscope [15,109],... 

The powdered zeolite (grain size between 0.2 and 0.5 mm) to be tested is placed between the electrodes of the capacitor, which is also the sample holder. The dielectric sensor circuit, which is fed with an alternating voltage (amplitude 0.1 V 30Hz standard commercial capacitor of known capacitance (Cs = 1 lOpF) in place of the reference capacitor (Cr) (see Figure 4.49c) [15,16,111-113,120], For the calculation of the real part of the relative permittivity, at different frequencies and temperatures, the following relation was used [15,120]... 

Fig. 6.1.2 shows a simple electrical model in which the sense capacitor is placed in series with a reference capacitor Cref. Also shown are parasitic capacitors Cpj. 

In addition to the desired dependence on AC, it has a matching-dependent offset and gain that depends on parasitics. Any deviation of the reference capacitor Cref from the nominal value of the sense capacitance Cs appears as offset. Since in many applications AC is much smaller than C0, this offset often exceeds the signal. Offset cancellation should therefore occur early to minimize the dynamic range of the readout electronics. Care should also be taken for the trimming not to introduce a poor temperature coefficient. One solution fabricates the reference with the same process and in close proximity to the sense capacitor. The z axis accelerometer shown in Fig. 6.1.3 [7] utilizes two rnicromachined structures for the sense and reference. The suspension of the reference structure has been made intentionally stiff. 

Fig. 6.1.8 shows conceptually the two most common topologies for sensing small capacitance changes. For single-ended interfaces, a reference capacitor takes the place of one of the sense capacitors. For practical reasons, an ac waveform is used as excitation Vs, resulting in an amplitude-modulated output V0 that must be demodulated. Circuits for demodulation and biasing with the resistors Rt,la will be discussed in Section 6.1.3.3. To first order they have no impact on circuit operation and are ignored in the conceptual analysis below. 

Impedance interfaces often provide the facility for automatically switching between measurements of the sample to be measured and measurements of a low loss calibrated reference capacitor (Figure 3.2.10). An ideal reference capacitor would have a completely flat response (constant capacitance) across the entire frequency range. This ideal capacitor cannot be achieved in reality since there will always be some parallel resistance in the capacitor even though this can, in practice, be an extremely high value. However, the difference between an ideal and nonideal capacitor is sufficiently small for most purposes and the reference capacitor is a very useful tool that can be used to quantify errors due to cables and instrument measurement errors. The deviation of the capacitor from its ideal response due to cables... 

Usually external reference capacitors can be connected to the interface allowing an ultra-stable reference to be used if one is available. This also provides best matching of cable errors since the same type of connection cables can be used on the sample and on the reference. The reference technique is especially usefiil when using external amplifiers for high voltage tests, though the reference capacitor must be chosen with care to withstand the applied voltage level. 

Fig. 7.7 Gas response of the capacitive (RT) and chemoresistive (300 °C) gas sensors to nitrogen dioxide (0.1-1 ppm), ethanol (2-20 ppm), and relative humidity (0-70%). (a) Gas protocol (b) differential PEUT capacitive sensor response (ten times magnified), PEUT-sensing capacitor signal, and reference capacitor signal. The PEUT-sensing capacitor and reference capacitor signals (in fact in the 2 pF range) have been arbitrarily shifted in order to be displayed on the same panel with the differential capacitive sensor response (c) gas response of the SnO2 0.2% Pd and respectively WO, MOX sensors [Reprinted with permission from Oprea et al. (2012). Copyright 2012 Elsevier]...

Wlien an electrical coimection is made between two metal surfaces, a contact potential difference arises from the transfer of electrons from the metal of lower work function to the second metal until their Femii levels line up. The difference in contact potential between the two metals is just equal to the difference in their respective work fiinctions. In the absence of an applied emf, there is electric field between two parallel metal plates arranged as a capacitor. If a potential is applied, the field can be eliminated and at this point tire potential equals the contact potential difference of tlie two metal plates. If one plate of known work fiinction is used as a reference electrode, the work function of the second plate can be detennined by measuring tliis applied potential between the plates [ ]. One can detemiine the zero-electric-field condition between the two parallel plates by measuring directly the tendency for charge to flow through the external circuit. This is called the static capacitor method [59]. 

Historically, the first and most important capacitance method is the vibrating capacitor approach implemented by Lord Kelvin in 1897. In this technique (now called the Kelvin probe), the reference plate moves relative to the sample surface at some constant frequency and tlie capacitance changes as tlie interelectrode separation changes. An AC current thus flows in the external circuit. Upon reduction of the electric field to zero, the AC current is also reduced to zero. Originally, Kelvin detected the zero point manually using his quadrant electrometer. Nowadays, there are many elegant and sensitive versions of this technique. A piezoceramic foil can be used to vibrate the reference plate. To minimize noise and maximize sensitivity, a phase-locked... 

When the process medium is electrically conductive (dielectric values > 10), the capacitor developed above does not work the iasulatiag material needed between the two conductive plates is lost. The conductive Hquid surrounding the probe acts as a short circuit to the tank wall (second plate of the capacitor). To reestabUsh the dielectric (iasulatiag material), the probe can be iasulated with a nonconductive material such as tetrafluoroethylene (TFE), poly(vinyhdene fluoride) (PVDF), poly(vinyl chloride) (PVC), etc. The capacitor exists between the probe rod, through the thickness of the iasulation (dielectric), to the conductive Hquid which is now acting as the second plate of the capacitor, or ground reference (Fig. 9). 

In nonmetaUic vessels, the second plate of the capacitor is missing and must be suppHed. A stiUweU probe, one with a concentric metal tube, is utilized. The concentric tube suppHes the second plate. StiUweU probes have numerous other uses. In appHcations of nonconductive media, a stiUweU probe is more sensitive and suppHes a greater amount of capacitance because the ground reference is so close to the probe. Further, if a tank waU offers a ground reference that is a varyiag distance to the probe, eg, a horizontal cylinder, the stiUweU offers a much more consistent (linear) ground reference. 

Other. Insoluble alkaline-earth metal and heavy metal stannates are prepared by the metathetic reaction of a soluble salt of the metal with a soluble alkah—metal stannate. They are used as additives to ceramic dielectric bodies (32). The use of bismuth stannate [12777-45-6] Bi2(Sn02)3 5H20, with barium titanate produces a ceramic capacitor body of uniform dielectric constant over a substantial temperature range (33). Ceramic and dielectric properties of individual stannates are given in Reference 34. Other typical commercially available stannates are barium stannate [12009-18-6] BaSnO calcium stannate [12013 6-6] CaSnO magnesium stannate [12032-29-0], MgSnO and strontium stannate [12143-34-9], SrSnO. ... 

In maldug electrochemical impedance measurements, one vec tor is examined, using the others as the frame of reference. The voltage vector is divided by the current vec tor, as in Ohm s law. Electrochemical impedance measures the impedance of an electrochemical system and then mathematically models the response using simple circuit elements such as resistors, capacitors, and inductors. In some cases, the circuit elements are used to yield information about the kinetics of the corrosion process. 

Additional tests on a capacitor VT The tests discussed above refer generally to the electromagnetic unit only. To test the whole VT, the following tests are recommended. For the test procedure and results refer to lEC 60186. 

These capacitors differ from standard p.f, improvement capacitors, as they are designed to withstand higher test voltages and have a low internal inductance. They should preferably be non-inflammable, synthetic liquid impregnated and provided with a built-in discharge resistance. For specifications refer to VDE 0675 and VDE 0560 III,... 

With this particular arrester, the motor has to be specially designed for the higher level of FOW impulse voltage withstand. But the manufacturer can always modify the protective characteristics of the arrester, depending upon the system s requirements. The matter may therefore be referred to the manufacturer for recommendations. The arrester may be fitted with a 0.25, uF surge capacitor in parallel, to reduce the steepness of the FOW (for the arrester of example 17.5, it is 14/0.2 kV/jUS) to a safer value. 

Referring to the data available from experiments, as shi)wn in Table 23.1, it hits been estimated that a Vp, of I. Hj should be sufficient to account for the harmonic effects. For this dielectric strength is designed a capacitor unit and selected a switching or protective device. 

Coupling capacitor or CVT is used when it is required lor measurement and protection. For details on CVT refer to Section 15.4.1.3,... 

Refer to Table 23.1, which shows the average cumulative effect of all the harmonics that may be present in a power system. If we can provide a series reactor of 6% of the total kVAr of the capacitor banks connected on the system, most of the harmonics present in the. system can be suppressed. With this reactance, the system would be tuned to below the fifth harmonic (at 204 Hz) for a 50 Hz system as derived below. 

Use of a reactor will enhance the voltage across the capacitor banks and must be considered in the design of the capacitor units. Refer to Figures 23.16(a) and (b) illustrating this. If... 

Refer to a typical distribution network shown in Figure 2.5..5 I. The capacitor is of maximum use when located as near to the load-point as possible, especially in induction motors, because ... 

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