Internal voltage drop

Vqs could be viewed as composed of two parts, the externally applied bias and any internal voltage drop inside the device, according to Vas = Fo5. / + Fos / -... 

Examples of changes in vs Vgs characteristics for the situation of a gas-induced internal voltage drop are given in Fig. 4.2(c). 

As discussed in Chapter 9, the ionic conductivity of organic electrolytes is much lower than that of aqueous electrolytes. Therefore, the electrode areas should be as large as possible to reduce current density, internal resistance, and internal voltage drop. A single coating layer should not be thick, normally about 100 pm, and the total thickness of the coating should be about 200 pm, much thinner than that for Ni-Cd rechargeable batteries. To attain... 

Corresponding to the charge in the potential of single electrodes which is related to their different overpotentials, a shift in the overall cell voltage is observed. Moreover, an increasing cell temperature can be noticed. Besides Joule-effect heat losses Wj, caused by voltage drops due to the internal resistance Rt (electrolyte, contact to the electrodes, etc.) of the cell, thermal losses WK (related to overpotentials) are the reason for this phenomenon. 

FIGURE 2.28 Voltage drop due to sulfur poisoning versus CO concentration for an SOFC button cell operated on a HrCO fuel mixture containing 5 ppm H2S at 1000°C with current density of 200 mA/cm2. (From Sasaki, K. et al., Proceedings of the Ninth International Symposium on Solid Oxide Fuel Cells, 2005-07 1267-1275. Reproduced by permission of ECS-The Electrochemical Society.)... 

Tests with these inclined louvers [9] (Fig. 16.4) showed that the cell voltage dropped, this drop being more prominent (see Fig. 16.5) with an increasing current density, as was expected. This suggests that this innovation of the anolyte circulation acting upon the anode not only lowers the energy consumption but also lowers the material costs for the cell because no specific internals are required and manu-... 

In the polarization curve, three parts can be observed kinetic, ohmic, and mass transfer. In the kinetic part, the cell voltage drop is due to the charge-transfer kinetics, i.e., the 02 reduction and H2 oxidation rate at the electrode surface, which is dominated by the kinetic I-rj equation (Equation 1.37). In the ohmic part, the cell voltage drop is mainly due to the internal resistance of the fuel cell, including electrolyte membrane resistance, catalyst layer resistance, and contact resistance. In the mass transfer part, the voltage drop is due to the transfer speed of H2 and 02 to the electrode surface. 

At the mid-point of the polarization curve (Figure 1.20), the cell voltage drop is dominated by the drop caused by internal resistance. This internal resistance is also called ohmic resistance. The drop can be calculated as AEohmic = IceuRei-... 

Equation (11.28) resembles the theory of electrical circuits A max is the electromotive force, A is the voltage drop on a load, and XL is the voltage drop through internal resistance R,. External R and internal resistances Rt are defined by... 

Because of the high resistance of the glass membrane (10 to 100 MO), it is not practical to measure the emf directly. Instead, pH meters either use a direct-reading electronic voltmeter or electronically amplify the small current that flows through the cell and detect the voltage drop across a standard resistor potentiometrically. Both battery-operated and ac line-operated pH meters are available connnercially from such firms as Beckman Coulter, Thermo Orion, and Coming. Such pH meters are calibrated to read directly in pH units, have internal compensation for the temperature coefficient of emf, and have provision for scale adjustments. 

The second consideration is that even if, for simplicity, we assume zero forward voltage drops across both the switch and the diode, we still may not be able to deliver the required output voltage — because of maximum duty cycle limitations. So for example, in our case, what we need is a (theoretical) duty cycle of Vo/Vin = 14.5 V/15 V = 0.97, that is, 97%. However, most buck ICs in the market are not designed to guarantee such a high duty cycle. They usually come with an internally set maximum duty cycle limit ( Dmax X typically around 90 to 95%. And if that is so, D = 97% would be clearly out of their capability. 

Excitable systems as considered here are many particle systems far from eqnilibrium. Hence variables as voltage drop (neurons), light intensity (lasers) or densities (chemical reactions) are always subject to noise and fluctations. Their sources might be of quite different origin, first the thermal motion of the molecules, the discreteness of chemical events and the quantum uncertainness create some unavoidable internal fluctuations. Bnt in excitable systems, more importantly, the crucial role is played by external sources of fluctuations which act always in nonequilibrium and are not counterbalanced by dissipative forces. Hence their intensity and correlation times and lengths can be considered as independent variables and, subsequently, as new control parameters of the nonlinear dynamics. 

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