V O L U M E 2 7 , NO. 9, S E P T E M B E R 1 9 5 5 back slowly to the red-colored reduced form. Although the indicator did not appear to be destroyed, i t was evident that the detection of end points by this visual indicator would be extremely laborious. No further study of other indicators was made. I t is obvious that the inherent difficulties involved in the stabilization and use of cobalt(II1) sulfate make it a much less practical titrant than cerium(1V) or permanganate. However, as it is almost certain that the accuracy of cobalt(II1) titrations can be improved by using ice-jacketed burets or refrigerated automatic pipets, this reagent should be very useful in simple and direct titrations of substances such as ce~ium(III), chromium(III), and manganese(I1) where very high oxidation potentials are needed. Furthermore, although all organic compounds studied were oxidized by cobalt(III), it is possible that this reagent may be selective and stoichiometric in its attack. Further work is being done along these lines. ACKNOWLEDGMENT
One of the authors (C. E. B.) gratefully acknowledges support from the Eugene Higgins Fund.
1423 LITERATURE CITED
(1) Bawn, C. E. G., and White, A. G., J . Chem. Sac., 1951, 331, 339, 343. (2) Bommer, V. H., 2. anorg. Chem., 246, 275 (1911). (3) Bricker. C. E., and Sweetser, P. B., ASAL. CHEM.,25, 764 (1953). (4) Brunner, E., Hela. Chim. Acta, 12, 208 (1929). ( 5 ) Fichter, H., and Wolfmann, H., Ibid.,9, 1093 (1926). (6) Jahn, S., Z . anorg. Chem., 60, 292 (1908). (7) Kitashima, S., Bull. Inst. Phys. Chem. Research (Tokyo). 70, 1035 (1928). (8) Noyes, A. -4., and Deahl, T. J., J . A m . Chenl. Soc., 59, 1337 (1937). (9) Schall, C., and hlarkgraf, H., Tians. Am. Electrochem. SOC.,45, 161 (1924). (10) Smith, G. F., “Cerate Oxidimetry,” G. F. Smith Publishing Co:, Columbus, Ohio, 1942. (11) . , Swann. Si.. and Xanthakoa, T. S.,J . Am. Chem. Soc., 53, 400 (1931). . 25, 253 (12) Sweetser, P. B., and Bricker, C. E., h r . 4 ~ CHEM., (1953). RECEIVED for review hlarch 12, 1955. Accepted M a y 4, 1955. Presented before t h e Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1955.
Constant Current Sources Based on Transistors N. HOWELL FURMAN, LOUIS 1. SAYEGH, and RALPH N. ADAMS Princeton University, Princeton,
N. 1.
This paper describes the characteristics of some electrical circuits based on the use of a germanium transistor of the p-n-p junction type. After a preliminary warmup of 15 minutes the circuits are used to give currents constant to within 0.0294 for half-hour intervals, at any setting in the range from 100 pa to 5 ma. The transistor is protected from ambient temperature changes by a simple thermostat. The current output varies only 0.01 to 0.1% when the resistance in this circuit is varied by 100 ohms.
T
HERE is a need for circuits of simple nature that will deliver efficiently a small constant current. At present the majority of such circuits have been either the simple but rather inefficient ones that employ a battery of high voltage and a high resistance in series, or the vacuum tube circuits that make use of electrical or electromechanical feed-back loops. For the latter systems the efficiency is rather low, but they are capable of high current outputs. c (REO DOT) Recently fairly reproducible germanium transistors have been produced. Since transistor action may be obtained with bias batteries of a few volts and a t high efficiency, their use in certain types of instruments E B C offers much promise. Figure 1. Schematic diagram of
FFI
GENERAL DISCUSS103
transistor
I n Figure 1 a diagram of a tranE . Emitter C . Collector sistor and the connections, for the B . Base junction type is given. The familiar cathode, grid, and plate of the vacuum tube have their counterparts in the emitter, base, and collector of the transistor. When used in the grounded base assembly the characteristic curves indicated in Figure 2 are obtained with the junction-type transistor. The family of curves indicate the variation of collector current with collector voltage
a t the designated constant emitter currents. These curves show that the current output, I,, is essentially independent of the collector voltage, which strongly suggests its use as a constant current generator. Its vacuum tube counterpart would be the pentode. The point contact 3MA.1, transistor does not have the same characteristic curves as the junction unit, but hascurves that resemble those of a vacuum tube triode; therefore, only junction type and not point contact transistors must be used here. The fundamentals of transisIC tors, are covered in a standard Figure 2. Insensitivtreatise ( 2 ) . Two types of juncity of current output with collector voltage tion transistor exist-namely, at various operating the p-n-p (positive-negative-pospoints itive) and the n-p-n. It is thereVC. Collector voltage fore essential that the proper IC. Collector current polarity be observed-that is, I ? . Emitter current for a p-n-p, the emitter is biased positively with respect to the base and the collector negatively relative to the base. For the n-p-n type the polarity is reversed. A transistor may be permanently damaged by failure to observe the right polarity, or by overstepping the maximum ratings. Because transistors are semiconductors, they are sensitive to temperature changes, an important disadvantage. As there is no filsment to supply the carriers in a transistor, the problem of a stable filament supply does not exist. These temperature changes affect the transistor by essentially changing its operating point. Thus, it is fortunate that the grounded base network is less sensitive to temperature variations than the grounded emitter and gounded collector networks. If one plots the log of I,o (the output for zero-emitter current input) against the temperature, a straight line is obtained. I n order to make a transistor as insensitive percentagewise to temperature as possible, a transistor with low I,, should be used. This is apparent since I , = I,, a l e , where a is a constant.
+
1424
ANALYTICAL CHEMISTRY
Because of the changes due to temperature fluctuations the low current range of the transistor circuit is limited, as is the upper current. The output resistance falls at the higher current levels, because the increased power dissipation a t the base-collector junction increases the temperature of this junction and also I , Hence from the previous equation I , w-ill increase, which effectively decreases the output resistance. Preliminary experiments with a silicon-junction transistor gave inconclusive results, since the reproducibility was poor. As the silicon transistors have a much smaller I,, value than the germanium transistors, they qhould be much less sensitive to temperature variations percentagewise. This would tend to increase the upper range and decrease the lower range of operation. APPARATUS
The transistor was covered with a urethan plastic and maintained in a Dewar vessel filled with water. When changes in output current were measured us. time, it was found that the unit was feasible as a source of constant currents.
Constant Current Generator. The ret.istors and switches werr housed in an aluminum box, 4 X 5 X 6 inches. The insulated transistor was connected to the other components by a fourpronged jack. Switches S4 and S y (Figure 3) are reversing switches, so that the unit may be used with either p-n-p or n-p-n transistors. With Sl and Sp open when the transistor was put in circuit, the right polarity m-as set with S a and.&. I t wa8 extremely important that the right polarity be used when SIand Szwereclosed rISIwas closed with S 6in thc I I To J dummy position and R, ----- -> I in the circuit. PotentiI --,V ometer RZ was turned t o I the limit that makes Vt = 0, then S2 was closed. It is important to have J-< initially zero, since the full battery voltage might pass more than the mnximum current, 8 ma., and ruin the transistor. RePistor R1 was inserted in peries as an extra proterR tion against mistakes. It limited the input current to about 7 ma. for any setting of R2 assuming thr input resistance R, r a e about zero. Actually it was of the order of 100 Figure 4. Transistor and ohms, so that this would accessories protect it from anv m i s take in making the conJ. Four-prong jack nection. With S2 closed, V. Shield of two-wire cablr P. Plastic coatine RZwas adjusted to give the Socket rurrent dutput tGat w&q T. Transistor required. For this parE , B , C Emitter, base, and collector, respectively ticular circuit the miniR. Radiation fin mum current is of the order of 100 pa. and thc maximum about 5 ma The upper limit depends on the power dissipation, which in thr case of the R.C.A. 2N34 transistor is about 50 mw. It is obvious that the potential drop across the cell should never exceed 1:'. otherwise there will be no current regulation.
-
--
8 s .
Figure 3.
Circuit for constant current
1.5-volt dry cell Vq. 6-volt d r v rpll S i , SZ. S.P:S;?. %oggle switches Sa, SI. D.P.D.T. toggle switches SS. S.P.D.T. toggle switch 100-ohm wire wound resistor, I / , watt Ri. Rz. 50.000-ohm wire wound Dotentiometer. watt. 1 t o 3 turns Ra. Dummy resistor t o match resistance of t h e electrolytic cell R.C.A. 2N34 p-n-p junction transistor, or equivalent T. .4. .4mmeter, 0 t o 50 pa.. with suitable shunts to extend range t o 5 ma. VI.
The circuit and its components are shown diagrammatically in Figure 3. The transistor socket was connected to a two-wire shielded cable, and the transistor connections were made to the socket (Figure 4). This overcame the danger of overheating and of ruining the transistor by soldering. The unit with the transistor in the socket was dipped inta carbon tetrachloride, allowed to dry, and then coated with a plaPtic of the following composition: Castor oil Grade D.B. (Baker Co.), 29.8 grams. Dioctyl sebacate (Rohm & Haas Co., Resinous Productb Division) 5.76 grams. 3. Dipropylene glycol (Carbide and Carbon Chemicals Co.), 5.96 grams. 4. m-Tolylene 2,4-diisocyanate (Monsanto Chemical Co.), 16.10 I.
2.
grams.
The components were mixed as follows: Nos. 1, 2, and 3 wertb mixed together and then No. 4 was added. The mixture was poured into a 22 X 135 mm. test tube, to which the transistor unit was lowered and properly positioned so that it was completely covered. After the mixture had gelled a day or so a t room temperature, drying &'as completed a t about 35' C. in an oven for 4 days. If an oven is not available, drying may be completed a t room temperature over a 6-day period. The glass tube was then cracked, and the fragments were removed. As shown in Figure 4 the plastic was cut half-way down the transistor with a razor blade. Next a radiation fin, k, was attached as indicated in Figure 4. This fin is a sheet of copper foil, 4 X l l / z inches, and is held in place by an alligator clip. The plastic coating, which has extremely low water absorption, was ideal for insulating the unit from the water. When immersed in water in a Dewar vessel of 1-quart capacity the transistor was &able over halfhour periods.
EXPERIMENT4L AND RESULTS
With the unit set a t various operating points, readings a e r r taken with a Type K potentiometer, Leeds & Northrup Co measuring the voltage drop across a standard resistor. A Ghl Laboratories Go., Chicago, Ill., galvanometer of 0.02 pa. per mni sensitivity was used for balancing. The data are based on thc procedure of allowing a &minute stabilization period for each operating current (Table I). The results are summarized in Table I. The effect of a change of 100 ohms in resistance iincluded in the table. The need for a 15-minute stabilization period can be seen from Figure 5, which shows the rapid initial rise in current as the transistor, immersed in water a t room temperature, was warmed by the current. The steady state was reached in about 5 minutes if the transistor unit was placed in ice water. If the initial reading Tyas taken after 15 minutes and thc final one a t the end of 45 miniitec, the reading a t 30 minute- was the average of the two.
.
Table I. Operation of Water-cooled Transistor, Radiation Fin, and Dewar Vessel Used Average Mean Current. Current, 1st and 3rd 3 Readings Readings 104.48 pa. 104.48 pa. 521.30 pa. 521.29 pa. 1.0493 ma. 1.0492 ma. 3.0880 ma. 3.0879 ma. 5 1661 ma. 5.1662 ma.
Average Deviation.
Maximum Deviation,
Effect of 100 Ohm Change (Output).
0.02 0.02 0.01 0.01 0 01
0.03 0.03 0.01 0.01 0 01
0.01 0.01 0.015 0.053 0 12
70
%
70
Water was used because of its high heat capacity, and because it served to thermostat the transistor against rapid changes in the temperature of the air of the room. Ice water was the best simple stabilizing medium. When run in air the current was unsteady and not reproducible from one experiment to another, because of ambient temperature changes.
V O L U M E 2 7 , NO. 9, S E P T E M B E R 1 9 5 5 I n other experiments it was found that a change of 1000 ohms in the output circuit caused only a 0.03% change in output in the 100-pa. range and a 0.07% change in the 500-wa. range. Use of Transistor for Variable Controlled Currents. Since the output of transistor circuit (Figure 3 ) may be varied continuously, the unit can be used to take polarograms in which the current is the controlled variable (1). For this application it is unnecessary to coat the transistor with plastic and to thermostat it ns the transistor need not be stable over a 30-minute period. The current is set, its value is read, and the corresponding voltage i+ noted. The transistor unit offers decided advantages because the rurrent held constant regardless of changes in resistance such .is those a t the dropping mercury electrode. The ideal type of transistor would again be one with low I,, since this would irquire a smaller B battery in the circuit that is used to buck out this Ice. T h r circuit for this application IS shown in Figure 6 The operation is the same as current-scan polarography. After connecting the dropping mercury electrode and reference half cell, Rz (Figure 6 ) was set so that the microammeter read about 5 M.; a t this point I , v a s approximately zero. Then SS was rload and R1 was adjusted to give zero current. From then on
1425 resistance characteristic of the junction transistor in the groundedbase configuration. I n this paper two possible applications havr, been explored-namely, the use of the transistor in a constant current circuit for coulometry or other applications, and the drvelopment of a convenient source of steady and continuous11 variable current for current scanning polarography. I n the case of the germanium transistor of the junction type (R.C.A. p-n-p), the stability of the unit after a 15-minute warm-up period is from 0.02% to better than 0.01 % for the range from 100 pa. t,o Ra
T
Figure 6.
Circuit for current scan applications
vi.
45-yolt d r y cell v2. &volt dry cell Va . 90-volt d r y cell, or two 45-volt cells in series Ri. 270 000-ohm carbon resistor watt R?. ii0.600-ohm wire-wound potkntiometer, 111 w a t t , 1 to 3 turns R3. 10- to 15-megohm carbon resistor, ‘/4 w a t t R4. %megohm carbon potentiometer, watt A. brqmeter 0 t o 50 microamp., with shunts t o extend rang‘, Si, S I , Sb. S.P.S.T. ioggle switches Sa, S4,S s . D.P.D.T. toggle swjtche? T. R.C.A. 2N34 p-n-p junction-type transistor os equivalent
5 ma., respectively. Changes in load resistance or cell resistance. from 0 to 100 ohms produced a change of ’0.01 to 0.1% in current output. The chief advantage of the transistor is its long lift. and stability toward shock. The power requirements are much IeFs than for varuum tube units, and the cirruits are simplified. ACKNOWLEDGMENT
Appreciation is expressed to scientist8 of the Radio Carp Research Laboratories, Princeton, N. J., to Arthur Rossoff of thtx Radio Receptor Co. for help on transistor problems, and to D. S Trifan and Ralph Christensen of the Plastics Laborator), Princeton University, on the problem of a suitahle coating Eo1 the transiptor and its leads. 0
15
30
45
LITERATURE CITEL)
T (MINI
Figure 5.
Change of current w i t h time
it W:IY only necessary t,o adjust K? t o give currents up to approsimately 100 pa., which is ample for the dropping mercury electrode. K 1 was made large to reduce I,, and thus require less biasing current. RBwas large in order to obtain a biasing current of the order of I,bwithout effectively decreasing the output resistance of the unit. This unit has an exceptionally high output resistance; therefore the current will fluctuate little, if any, with drop size. The constancy of current during drop formation w w demonstrated for a mixture of 25 ml. of 0.lM cadmium chloride plus 15 ml. of saturated pot:tssium chloride. Only a t the lowest scale of the recorder (2 pa.) n--crethere visible fluctuations on the record of the order of 0.02 microamp. or less. Fluctuations of thc same size were found with a fixed resist,or in place of the dropping electrode assembly.
Adams, R. N., Reilley, C. N., and Furman, 9.H.. ANAL.CHEM., 25, 1160 (1953). (2) Shea, R . F., “Principles of Transistor Circuits,” Wiley. New York. (1)
1953, RF:CEIVED for review Derenilwr 27. 19%.
.Accepted M a y 1 9 . 1 9 5 3
CONCLUSION
With proper attention to temperature control, there art’ numerous possibilities for taking advantage of the high output-
S K I N N E R & SHERMAN, I N C
