Positive-feedback iR-compensation

The electrode potential was controlled with an EG G Princeton Applied Research (PAR) model 173 potentiostat/galvanostat and is referenced to a saturated calomel electrode (SCE). A PAR model 276 current-to-voltage converter allowed monitoring of current during the ORC and SERS experiments and it also provided for positive feedback iR compensation for accurate potential control. 

Positive feedback iR compensation in three-electrode measurements As described in Section 5.3, the influence of iR-drop is serious in two-electrode polarography or voltammetry. The influence is eliminated considerably with three-electrode instruments, if the tip of the reference electrode is placed near the surface of the indicator electrode. However, there still remains some iR-drop, which occurs by the residual resistance at... 

The anionic Ni(III) carborane, (Ni[C2B,H,(CH3)jJ2] , can be reversibly reduced at — 0.9 V in CHjCN to the dianion and cyclic voltammetry data at a hanging-Hg drop are given in Table 2. To minimize resistance losses, a luggin probe is used for the reference electrode and positive feedback iR compensation is employed. At each scan rate, the measured value of AEp is used to obtain >j/ from Table 1. Then, using a value of a calculated from the scan rate and Eq. (1), a value of kj/D is obtained at each V. The average of values at different scan rates is calculated D must be measured independently (from, e.g., the polarographic I value) and in the present example is 6.4 X 10 cm s . This yields ... 

Meyer JJ, Poupard D, Dubois JE (1982) Potentiostat with a positive feedback IR compensation and a high sensitivity current follower indicator circuit for direct determination of high second-order rate constants. Anal Chem 54 207-212. doi 10.1021/ac00239a014... 

Fig. 5.45 Circuit for a three-electrode polarograph. The bold lines are for positive feedback iR-drop compensation. Q is a capacitor to decrease the current fluctuation.

E. R. Brown, H. L. Hung, T.G. McCord, D. E. Smith, and G. L. Booman, A Study of Operational Amplifier Potentiostats Employing Positive Feedback for iR Compensation II. Application to AC Polarography, Anal. Chem. 40 1411 (1968). 

Four-electrode system — Figure. Electronic circuit of a four-electrode potentiostat (X, potential input Y, current output RE1 and RE2, reference electrodes CE1 and CE2, counter electrodes PF, positive feedback circuit for IR drop compensation)... 

Positive feedback circuit — Electronic circuit incorporated in a -> potentiostat, which is used for the - IR drop compensation. Through this circuit, a part of the voltage at the current output of a potentiostat is positively fed back to the potential input, so that the - IR drop can be automatically compensated for. However, note that the positive feedback makes the system unstable and occasionally leads to oscillation. See also - four-electrode system. 

In the positive feedback methods a voltage signal is produced which is proportional to IR and which is added to the control input voltage. The aim is to compensate for IR automatically. Depending on the principle of the potentiostatic circuit several solutions have been proposed [15] (for example Fig. 6.8). 

Fig. 6.8 Basic circuit for potentiostat with IR drop compensation by means of positive feedback in the control loop...

It should be realized that overcompensation does not always result in "instability" and so great care is needed to avoid it. Positive feedback in IR-compensated potentiostats has been discuss in detail by Britz [15] and by Me Kubre and MacDonald [13]. 

The CV curves were recorded by using a measuring system consisting of potentiostat, generator and x-y recorder. For rapid scan rates the curves were recorded on the screen of the monitor of an IBM PC-XT computer using a programme written in our laboratory. In kinetic experiments the positive feedback technique was used for cell resistance iR compensation. 

The most common online technique for compensation of iR-drop is positive feedback [33-47]. The corresponding circuits depend on the concept of the potentiostat and are presented for the adder potentiostat (Fig. 2) and for the hybrid type (separate adder. Fig. 3). The simple idea behind is to add the potential drop Uq across Rq to the desired voltage so that they compensate each other. The current is measured by a current-to-voltage converter (CV in Figs. 2 and 3). The output of CV is... 

Brown ER, Smith DE, Booman GL (1968) Operational amplifier potentiostats employing positive feedback for IR compensation I Theoretical analysis of stability and bandpass characteristics. Anal Chem 40 1411-1423. doi 10.1021/ac60266a024... 

Figure 6.1.4.2 Theoretical limitations on ultrafast cyclic voltammetry. The shaded area between the slanted lines represents the radius that a microdisk must have if the ohmic drop is to be less than 15 mV and distortions due to nonplanar diffusion account for less than 10% of the peak current, (a) Without iR drop compensation by positive feedback, and (b) with 90 and 99% ohmic drop compensation. The dotted area in (a) and (b) represent the regions where transport within the double layer affects the voltammetric response. Limits are indicative and correspond approximately to a 5-mM anthracene solution in acetonitrile, 0.3 M tetrafluoroborate as supporting electrolyte. [Reproduced by permission of Marcel Dekker from C. Amatore, Electrochemistry at Microelectrodes, I. Rubenstein, Ed., 1995, Chapter 4, p. 198.]...

Blitz D (1980) 100% IR compensation by damped positive feedback. Electrochim Acta... 

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