Stephen W. Tobeyl West Virginia Wesleyan College Buckhannon
Constant Current Supply for Electrokinetic Experiments
The electronically controlled power supply described herein is designed to deliver the constant current required in electrokinetic experiments in physical chemistry. It is used in this laboratory in determining cation transference numbers by the movinghoundary method.2. In this particular application it performs the same function as a unit described earlier by Bender and L e w i ~ . ~ However, in THIS JOURNAL the unit described below is based on quite a different operating principle and is consequently capable of delivering an appreciably more constant current over a considerably wider current range. During an electrokinetic experiment, such as the determination of the transference number of a cation by the moving-boundary method, the electrochemical changes occurring within the transference cell may cause as much as a tenfold increase in the cell resistance as has been pointed In the absence of any compensating action by the power supply being used, a tenfold decrease in the cell current would result. Since t h ~ cell current must ordinarily be maintained constant, some type of automatically compensating power supply is a practical necessity. The circuit shown in Figure I mill deliver any current from 1.0 to 10.0 milliampsto a load varying in resistance hetweeu 1 and 50,000 ohms, or up to 5.0 ma to a load varying between 1 and 100,000 ohms, with a total change in current delivered during a run of 1% or less without any adjustment. With only very occasional readjustment during an experiment, the current regulotion can be held to within 0.2%. To place the unit in operation, the cell or other load (RL)is connected as shown and the unit is turned on. Negligible warmup time is required. Rs-Rs is adjusted until the desired load current (IL) is read on MI. Final adjustment in ILand minor readjustments during an experiment are made with R4. Ordinary caution should be exercised in using the apparatus, since relatively high voltages are present a t the output terminals. Table 1 summarizes the performance of the unit under typical operating conditions. I t also provides information on the calibration accuracy of the particular current meter used. The data mere obtained
' Present address: University of Wisconsin, Madison. ' LONGSWORTH, L. G., J. CHEM.EDUC.,11, 420 (1934). ET AL., "Experimental Physical DANIELS, FARRINGTON, Chemistry," 5th ed., MeGraw-Hill Book Co., Ine., New York, 1956, p. 150. 'BENDER,P., AND LEWIS,D. R., J. CHEM.EDUC.,24, 454 (1947). R*WLINGB,R. J., "Current and Voltage Regulators" in lladiotrn Desirmr's Handbook, Lilogford-Smith, F., Editor, 4th ed., Radio Corporrttion of America, Harrison, N . J., 1953, pp. 1213-1222 and references therein.
by measuring with a potentiometer the voltage drop across a 100 ohm precision resistor connected in series with the indicated RL. In obtaining the data, MI was set to the desired IL with RL equal to 10,000 ohms. The actual current delivered was measured, then R L was increased and the measurement repeated. R4 wa.s not adjusted, except as noted. Table 1. MI reading milliamps
Performance of current reaulato~
Measured IL milliamp~
" Mt reading obviously lower. M, reset to original reading with R,. ED required to maintain I L constant reaches EA (vide infra).
In further test,s, a load of 25,000 ohms was placed on the unit and MI was set to 5.00 ma. The drift in IL over a one-hour period was random ~ t 0 . 0 2ma. When the unit was plugged into a Variac and the line voltage varied between 120 and 100 volts ac, the resulting variation in I,. was *0.03 ma. The mechanism by which the circuit in Figure 1 maintains IL constant despite changes in RL is similar to that employed in constant-voltage power ~upplies.~ and depends upon the fact that Ti, acts as a uariable resistor which is automatically adjusted by the circuit to just offset any change in RL. A detailed discussion of the mechanism by which the circuit operates folloms. The functions and critical values of all components are given. Assume that the unit is in operation, that Rs-Rs has been adjusted to give the desired IL,and that RL is increasing with time. The necessary high dc voltage is provided by TI, V1, L,, C1,and Cz. TI should deliver approximately 750 rms volts. The current drain on the power supply is very low, so that by running V , as a half-wave rectifier the voltage a t point A (E*) is maintained at about +900 volts, close to the peak voltage output of TI. V1 must be able to withstand a peak inverse voltage of a t least 2500 volts. R1is a protective resistor which discharges C, and Cs when the unit is turned off. V 2and V3are voltage reference tubes which in conjunction with R1 provide constant voltages of approximately f75 and +225 volts a t points B and C. The 110 volt ac input is run through the jumpers Volume 38, Number 10, October 1961
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517
Parts Lid CX, C, 4 mfd. CJ 0.1 mfd.
RI 500 K R? 50 K 10won R, 5 K R4 2.5K potentiometer Rr-Rr 100 K dual-section potentiometer. Ohmitetvoe CCU 1041
R,
1 Meg0hm
15 Hy. 75 ma TI Stancor type P-8171 MI 0-1 0 ma. dc milliommeter. Triple* model 420 S, SPST toggle switch 1,
PI F,
6.3 volt pilot light 1 om0 f w e
Figure
1.
Circuit diagram of Current Regulator
in V , and V 3to prevent injury to V 5if these tubes are removed. As may be seen from Figure 1, EA appears between point A and ground, and I L (electron flow) passes serially through R3, Rl, Rs-Rs, R7, RL,MI, and V4. Rs and R, are protective resistors which prevent damage to MI if I
