Square-wave current pulses

Fig. 12G Square-wave-current pulses and the resulting transients of potential. Both the residual solution resistance and the double-layer capacitance can. be obtained from such measurements.

To provide ultra-high reliability and independent operational control, two thermal battery types were developed and refined for optimum and reliable performance. One thermal battery was to be used for the EHP application, which requires a square wave current pulse load for its entire operating life. The second thermal battery was designed to provide power to the DC emergency bus bar and was required to meet a constant power output for its entire operating life. The operating life requirement was the same for both thermal batteries. Both of these thermal batteries have met the vibration, shock, and all other applicable military specifications. Specific structural and critical performance parameters will be described in Section 7.8 on thermal battery classification. Ordinance and nonordinance applications wiU be identified with an emphasis on performance capabihties and limitations. No other battery can outperform the LiAlFeSj thermal battery... 

Current pulse-. TBl provides square wave current pulse. 

Figure 8.6 Square-wave current pulses and the resulting transients of potential.

Figure 3.16. Waveforms for pulse and differential-pulse polarography. Curves A and D Excitation signal applied to the working electrode. Curves B and E Instantaneous current observed at a single drop as a function of time. Curves C and F The resulting current-versus-voltage curves. In pulse polarography, square-wave voltage pulses of 40-msec duration are applied to the mercury drop, of drop-life mechanically controlled at 2.5 sec (A) ta,ti,ti,... represent successive drops. The overall rate of increase of the amplitude of the voltage pulses is about...

In principle, this method is a combination of the DC and the NP mode. Square-wave DC pulses of small and constant amplitude (AU = 5-100 mV) during 40-60 msec ate superimposed on the continuously changing DC voltage. The application of the pulses and the measurements of the currents must be correctly synchronized. For every pulse the current is measured twice during precisely defined identical intervals (e.g., 20 msec). The first measurement ends just before the start of the pulse, the second with the end of the pulse. The current of the first measurement is subtracted from that of the second and the resulting derivative AilAU is plotted in function of the DC voltage ramp. The shape of the curve shows rather sharp peaks on a smooth baseline. 

The three-phase bridge circuit draws a square-wave current under ideal conditions from the mains (Figure 13.13). Harmonic oscillations of the ordinal v = p(k 1) are encountered (p = pulse number (p = 6) k=l, 2, 3,. ..). The amplitudes of the harmonic currents are inversely proportional to their ordinal number ... 

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

Potential-excitation signals and voltammograms for (a) normal pulse polarography, (b) differential pulse polarography, (c) staircase polarography, and (d) square-wave polarography. See text for an explanation of the symbols. Current is sampled at the time intervals indicated by the solid circles ( ). 

Ion chromatography (see Section 7.4). Conductivity cells can be coupled to ion chromatographic systems to provide a sensitive method for measuring ionic concentrations in the eluate. To achieve this end, special micro-conductivity cells have been developed of a flow-through pattern and placed in a thermostatted enclosure a typical cell may contain a volume of about 1.5 /iL and have a cell constant of approximately 15 cm-1. It is claimed15 that sensitivity is improved by use of a bipolar square-wave pulsed current which reduces polarisation and capacitance effects, and the changes in conductivity caused by the heating effect of the current (see Refs 16, 17). 

DC = direct current NP = normal pulse DP = differential pulse SW = square wave AC = alternating... 

In such a synthesis the lengths of the pulses are variable as well as the potentials of the square wave. Recently a potential-time profile has been used to maintain the activity of an electrode during the oxidation of organic compounds (Clark et al., 1972) at a steady potential the current for the oxidation process was observed to fall, but a periodic short pulse to cathodic potentials was sufficient to prevent this decrease in electrode activity. 

---