Charles K. Mann and Verne1 c. Champeaux
Florida State University Tallahassee
A Constant Current Source for Student Use
The development of electrochemical methods of analysis which utilize accurately controlled or constant direct currents has been very rapid in recent years. Examples of such methods are coulometric titration a t constant current, potentiometric titration at constant current, chronopotentiometry, and current scanning polarography. These methods, particularly the first two, are very well suited for inclusion in the undergraduate instrumental analysis course.' I t is the purpose of this paper to describe a convenient aud inexpensive controlled current power supply which is well suited to use in a student laboratory. Controlled current sources presently in use can generally be considered in three categories: dropping resistor, vacuum tube, and transistor. One type involves the use of relatively high dc voltage, e.g., 100 volts, a resistor, and the load, all connected in series. The resistor is very much larger than the load, so that nearly all the voltage is dropped through the resistor and small changes in load resistance have essentially no effect on the total resistance, hence, no effect on current. This type of power supply is the simplest to construct. It has the disadvantage of requiring frequent calibration. In addition, heating of the resistor with use must be taken into account. A second type involves the use of vacuum tubes. Many different designs have been described, frequently utilizing the flat plate current-voltage characteristic of the pentode. Vacuum tube power supplies, designed specifically for constant current coulometry or constant current potentiometry, are commercially available. They have the advantage of convenience, high precision, and large load tolerance; they are, however, quite expensive. Use of transistors rather than vacuum tubes makes possible a power supply that is convenient and precise, but simple and inexpensive to build. To obtain an output current which does not vary with load changes, one makes use of the common base c~nfiguration.~In this configuration, collector current is directly proportional to emitter current within certain limits of load resistance. A transistorized instrument is simpler and less expensive than its vacuum tube counterpart because of its smaller power requirements. Small batteries with relatively long lifetimes and very high short term stahility takr the place of high voltage
WILLARD,H. H., MERRIZT,L. L., JR., AND DEAN,J. R., "Instrumental Methods of Andy&," 3rd ed., D. Van Nostrand Ca., Inc., New York, 1958, pp. 427,508. ~STROBEL, H. A,, "Chernicsl Instrumenhtion," AddisonWesley Publishing Co., Ine., Reading, Mass., 1960, p. 372.
power supplies, which require fairly complex filters and voltage regulators to achieve necessary stability. As usually built, the dropping resistor and the transistorized power supplies are battery-operated; accordingly, they provide extremely steady dc currents. By contrast, vacuum tube supplies are generally lineoperated and show more or less residual ripple from rectification of ac. Comparing the two batteryoperated types, the transistorized supply requires six volts; small, inexpensive batteries are sufficient. The dropping resistor supply generally uses about 100 volts, which requires larger and more expensive batteries. The Instrument
A schematic diagram of the power supply is shown in Figure 1. It consists of one P-N-P transistor with two small dry cell batteries furnishing power. Battery "B,," furnishing output current, is a 6v cell such as Eveready 5105. "Bzl' i n the emitter circuit is a smaller 1 . 5 ~cell, Eveready 950. "R? is a 2-watt wire wound potentiometer, the size of which is determined by the current output desired. '%," which is optional, serves to limit the wnge of output current, t,hereby simplifying close current adjustment by R,. "S" is an SPST toggle switch. These components, together with suitable binding posts, are mounted on an aluminum chassis, 4 X 6 X 3 in. (Premier ACH 432).
Figure 1.
Schematic diagram of the constant current power supply.
Two types of these power supplies are used in this laboratory. One has a maximum current rating of 50 ma. I t uses a Workman Power 6 transistor (Sylvania 2N68 is equally well suited, but more expensive). R, is 250 ohms; Rz,47 ohms. With this arrangement, the instrument has a current range of 4 to 20 ma. The load resistance tolerance of this supply is increased by substituting a Tung-Sol 2N242, or equivalent, for the Workman Power 6. The low level source is rated a t 10 ma. A G.E. 2N190 t,ransistor is used. With R1of 3000 ohms and R2of 500 ohms, this instrument has a wnge of 0.36 to 2.40 ma. With Volume 38, Number 10, October 1961
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R, of 10,000 and R? of 5000 ohms, the range is 87 to 250 pa. All of the components used were obtained locally a t a radio supply shop. Assembly is carried out with common shop tools and requires no special skill. The cost of all components is about $6.50 per unit for the low level or the high level supply with the Workman Power 6 transistor. Use of the 2N242 transistor adds about one dollar and larger batteries would increase the cost. Performance
The effect of change in load resistance for the high level source is shown in Figure 2. Curve 1 pertains to the Workman Power 6 and curve 2 to the Tung-Sol 2x242 transistor. Using the Workman transistor with an initial current of 10 ma, the error amounts to less than 3~0.5%a t loads up to about 300 ohms. The tolerable load is larger a t small currents and less a t higher currents. For example, a t 15 ma appreciable error is noted above 150 ohms. It may be noted that the I R drop in the load is approximately half the collector voltage.
100
200 300 400 Load Resistonce (ohms)
500
With the 2N242 transistor, load tolerance is approximately doubled. With a 10-ma current, breakdown occurs sharply a t 600 ohms. Power dissipation by this transistor is only a small fraction of the supply. Thus Ohm's law can be used to estimate load tolerance for a given collector voltage. This remains true a t least up to 12 v furnishing 20 ma for which the tolerable load is 600 ohms. Since this transistor is rated to break down a t 45 v and 2 amp, it should be possible to increase the load beyond 600 ohms a t 20 ma by using larger collector voltages. For reference, cell resistance under several conditions typically encountered in constant current coulometry are given to show order of magnitude. With two metallic electrodes (wire helix, about 2 X 3 cm) immersed in 0.2 M KCI, as would be the case for acidbase or argentimetric titrations, a resistance of 5 ohms was measured. When the electrodes were separated by a short salt bridge containing saturated KC1 and one frit with 0.2 M KC1 in the external solution, the resistance was about 50 ohms. The same arrangement with a salt bridge having two frits showed a resistance of about 80 ohms. With 1.0 M, rather than saturated, KC1 in the double salt bridge the resistance was 150 ohms. Changing to 0.05 M KC1 in the external solution wit,h 1.0 M KC1 in the double salt bridge gave a resistance of 220 ohms. These data indicate t,hat it will generally be possible, with aqueous solutions, to use the high level current source with the
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600
Figure 2. Chonge in Ovtpvt wilh Load. Curve 1, Workman Power Six Ironsirtor; curve 2, Tung-Sol 2N242 Transistor.
520
Workman Power 6 transistor in the 10 to 15 ma range. The low level current source showed measurable deviation only above 5000 to 10,000 ohms load. Since this is a far larger value than is likely to be encountered in aqueous solutions, no further investigation was carried out. The low level source should be useful with nonaqueous solutions as well. To determine the effects of time, two different tests have been carried out. One involved allowing the high level source to discharge continuously through a load with periodic checking of the current. The result of this test is shown in Figure 3. After an initial drop, the current remains reasonably stable for about a week and then begins to drop off. The current was steady and with recalibration would have been usable a t any time during the test period. At the end of the test the unit was still capable of providing 12 ma on readjustment of R1. It would appear that for maximum stability, the instrument should be allowed to discharge for a while before calibration. The low level source would of course have a much longer life.
Figure 3.
2
4
6 .
8
10
12
14
16
18
20
Time (Days) Vmrialion of current with time. High level current source with
100-ohm load.
Under actual conditions of operation, current is delivered for only relatively short periods of time so that the batteries should have essentially shelf lie. I t was established that the current can be turned off and on repeatedly without change in calibration. At the current levels described here it is unnecessary to use a heat sink to prevent transistor heating. The instruments are calibrated periodically. Table 1 shows results of calibration of the low level source. The dial of the high level source does not permit highly reproducible settings of R,. Table 1.
Calibration of Low Level Current Source
Time of ealibrstion
Bt assembly
After 23 days
After 42 days
* All ourrents measured in milliamperee
After 58 days
The transistor power supply is very well suited to student experiments in constant current coulometry and potent,iometry a t constant current. With coulometric generation of titrant, any suitable end point can be used. The end point can be approached as slowly as necessary by repeatedly breaking the circuit. Time is most simply measured with a stopwatch. For best results, a precision electric timer is desirable. For potentiometry a t constant current, the power supply is connected to the electrodes in parallel with a vacuum tube voltmeter. This arrangement is useful
either when a pH meter with constant current feature is unavailable or when the single current value provided by a meter is unsatisfactory. In addition, these supplies provide a source of small do current and voltage for general use in the laboratory. For example, with a known resistance it makes a convenient calibrator for variable span potentiometer recorders. Acknowledgment
The writers wish to thank Warren Cloer, Ronald Dapo, and John Motes for some of the measurements.
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