Transient current generation

Figure 7 displays computer generated plots of Je(t)/8(1 + K ) vs. t for comparison with the transient current responses reported... 

N olecules in their excited states and molecules of transient existence generated by photochemical stimulation or by other processes, such as electrochemistry, are rapidly drawing considerable interest and gaining importance. The excited state of a molecule is, in many ways, a new species different chemically from the ground state molecule and endowed with additional energy it is often capable of chemical processes that are not possible in the ground state. The ability to do test tube experiments with such short-lived species is currently under intensive development. 

Acetylcholineesterase Bilayer lipid membranes were prepared by adding a solution of egg phosphatidylcholine and dipalmi-toyl phosphatidic acid dropwise into the surface of aqueous 0.1 M KC1/10 mM HEPES, near the Saran Wrap partition of a two compartment plexiglass cell. A portion of AChE solution in 10 mM Tris hydrochloride buffer solution of pH 7.4 was applied. The electrolyte level was momentarily dropped below the orifice and raised to form a membrane. The membranes were used as transducers for the reaction of AChE with ACh. An external voltage (25 mV) was applied across the membrane between two Ag/AgCl reference electrodes. Enzymatically generated hydronium ion causes transient current due to alteration of the electrostatic field by the ionization of dipalmitoyl phosphatidic acid. The response delay time was directly related to the substrate concentration where acetylcholine can be determined from 1 pM upto mM level. [113]... 

Figure 94 (a) The SCL transient currents for various normalized trapping times (R = Ttrap/t0) as calculated from theory (see Ref. 26) R = oo denotes the trap-free case is the steady-state current without trapping, (b) t trap-free SCL transient current injected from ITO under a positive step voltage applied to an IT0/PPV/TPD PC/A1 device jScl corresponds to in part (a). Bottom TOF photocurrent transient for holes generated by a light pulse at the A1/(TPD PC) interface (the negative polarity applied to ITO). (From Ref. 428). 

Hole Transport in PMPS. In the experiments with layered structures (20) and visible excitation (to which PMPS is transparent), transient currents were observed only when the top electrode was negatively biased with respect to the substrate. The substrate was composed of a visible photoconductor (charge generation layer) overcoated aluminum ground plane. When the polymer top surface was directly (intrinsically) photoexcited with pulsed 337-nm excitation, current transit pulses were observed only when the top electrode was positively biased. Therefore, under the experimental conditions described, only hole transient transport could be directly observed. Transit pulses were nondispersive over a wide range of temperature. Figure 14 illustrates the relative increase in dispersion with decreasing temperature. In addition, no evidence for anomalous thickness dependence at the transit time was obtained, even at the lowest temperature. 

The net photocurrent and the quantum yield are a function of a number of competing processes " as shown in Fig. 1.22. For an n-type semiconductor, the externally measurable current i is the difference between the photocurrent and the forward current of electrons. The electron current is decreased to zero under certain anodic bias. While the flux of holes to the surface is exclusively controlled by the solid-state properties, all the other reaction steps depend on the surface properties of the semiconductor. The holes arriving at the surface can either (i) transfer to an electron donor in the solution, (ii) be trapped at the surface states, or (iii) recombine with electrons in the conduction band in the depletion region or at the surface. Process (iii) does not generate current in the external circuit, whereas process (ii) produces only transient current charging up the surface states. Only process (i) produces steady photocurrent. The measured photocurrent /ph can therefore be different from the flux of holes to the surface due to these processes. 

Constraint i) will need the sub-transient reactances of the generators to be higher than for a standard design. It may also require the starting impedance of the motors to be higher than normal in order to reduce their sub-transient currents. 

If generators are physically remote from the switchboard, e.g. interconnected by long cables or overhead lines, then the impedance between the generators and the switchboard may be large enough to swamp the sub-transient and transient current contributions, as well as reducing the DC component effects. 

Modem switchboard circuit breakers are often able to clear a major fault current within 120 milliseconds, which is typically five or six cycles of the fundamental current. When these circuit breakers are used with generators, and switchboards that are fed by generators located only a short distance away, the decay of the sub-transient current merges with the decay of the transient current. Even at 120 milliseconds the current may have a substantial value. There are several ways of assessing the breaking duty current. 

Oil industry power systems often have generators and large motors connected to the same high-voltage switchboards. Hence there is the possibility of, more than may be expected, contributions of sub-transient current from the generators and motors. This will unduly stress the switchgear. 

In addition to its low threshold for activation, LVA Ca " channels [33] are characterized by a similar permeability for Ca " and Ba [34,35], This channel was termed T (for Transient or Tiny ), with its fast inactivation, which generates a transient current, and their inactivation when the holding potential is fixed between -60 and -50 mV as the main characteristics of this channel. The single-channel conductance has been estimated to be around 8 pS. 

---