High-Stability Regulator for Light Sources. Marvin J. Johnson, Department of Biochemistry, Vniversity of Viscomin, lIadison, Wis. low-wattage light sources o f ciinstaiit liglit output :iw photoelectric instruments, storage bat,tery polver is usually employed. Conetarit-voltage t,ransformers, because of short-pericd fluctuations in the line frequency, are usually not satisfactory. LIost, devices not affected by line frequency are cumbersome and expensive. The regulator described below is simple and easily constructed, gives light regulation as good as that obtained by t8heuse o f a storage battery, and is independent ~ iline ’ frequency fluctuations and of line voltage fluctuations between 90 and 130 volts. Recovery from rapid changes in liiie voltage is i i o t inst;tntaneous. HERE
\1-needed, as in
PRINCIPLE
Regulatioii of a light .wurce Iiy :t nionitoi,ing piiotocell has l~eeii eriip:o)-ed by l’ondi,oni itnd 12ol~ertson[Rei,.Sci.Itistt~uttwnts,19, 561 (1948)] and is employed in the Beckmiin I112 infrared .qpec.trophotonieter. In such appliciLtions, the regulatory circuit usually employs a series transformer arrangement introduced by Ridenour and Lampson [Re!’.Sci. I / i s t r . u t ~ e n t s8, , 162 (1937)l. I n the present circuit, the series impedance is a cu1,rerit regulator tube, \rhich, because of its high dynamic resistance, lends itself well to a circuit containing it singlts transformer and a minimum of componentLi. The prinripie of the present devitle mny be explained :Is follo\vs:
Let, a transformer, the priniaq. of n.hicli is connected, in series ~ i t t ia resistor, wross the line, be provided with an output secondary and a control secondary. Let a control circuit be arranged such that, when the voltage delivered I>)- the output secondary rises, the control secondary is caused to draw much more current. The resulting additional current through the primary then causes more voltage drop in the series resistor, and less voltage drop in the transformer primary. This causes the output voltage to return to normal. If, in place of a convent,ional resistor, a current regulator tube is used, superior results are obtained, as the incremental resistunce of such a tube is very large. The control circuit includes a phototube illuniinated by a lamp connected to the output secondary. .I change in the light intensity changes the voltage across the load resistor of the phototube. This changes the grid voltage of a D.C. amplifier, the output voltage from ivhich controls the grid voltage of power tubes connected to the control secondary. The regulation obtained by this arrangement is excellent, as the light intensity a t u-ave lengths affecting the phototube used varies approximately as the 5.Sth power of the lamp voltage. Hence, this factor, the nonlinearity of the primary resistor, and the gain of the direct current amplifier, all operate to increase the degree of regulation obtained. The actual circuit employed in :I regulator designed to replace pon-er source for t,he lamp of a photoelectric colorimeter (6 volts, 0.2 ampere) is F ~ O W I iii Figure 1. The components may be obtained through any dealer in radio parte. The output secondary consists of the nomirial 6.3-volt and 5volt secondaries connected in series. The high-voltage secondary serves as the control secondary. The 929 phototube and 82 automobile-type lamp \T-hiEh illuminates it are contained in a light-tight housing, to prevent ambient illumination from affecting the regulator. The output voltage is adjusted to 6 volts by adjustment of the spacing between the lamp and the phototube. No electrical connection is made t,o the chassis. Several of these units have been in operat.ion up to 3 years. Some have never been turned off. After approximately a year of cont,inuous operation or a correspondingly longer period of inter-
niittent operation the 6V6GT tulirs I J I ~ J ’need replacement, or the output voltage m:iy have incrcwed c:onsiderably because of duzt entering t h r photocell coriip:rrtnic~rit: i i i ( l of decre:rsed reflectilit? o f the \vtiIls of this conipartrnt~nt. 0I’ER.iTION OF (:PRCI;IT
\\-hen t h e regulator is in operation a t a line voltage of, for example, 115 volts the voltage tlrop iii the 21130 tube is 50 volts, leaving 65 volts” drop in the transformer primary. The output voltage is 6 volts. The positive and nrg:itive voltages supplied by the 6H6 rectifier are about 82 volts each. Thus the grid of the 6SH7 ~ o u l dbe 82 volts negative to its cathode if it were not for the voltage drop through the ~4-1iicgoIiniresistor caused by the phototube ourrent. (The cwriti.ibutiori of grid current to this drop is small. the grid curreiit being iippiwiinately 10-9 ampere.) Tlir phototube currelit gives a voltage tlrop of almost 82 volts. T h e I,esuItiriggi,id bias is such that theplatc~to cathode voltageof the 6SH7 (identical with the neg:it,ivr grid Ihis of the 6Y6GT tubes) is approximately A volts. The alterti:iting c*urrentthrough the 6\’6GT tubes (onc. tube conducting 0 1 1 ~ a c l ihalf-cycle) results in :ui additional current through the 2H30 of ahout 70 ma. If the h i ( . voltage decreases, the decreased illumination of the pliotocell i i y the 82 lamp causes t,lie grid of the 6SB7 to become more negative, the resulting change in plate voltage d ~ c i ~ i s i n g the alternating cui,rent conduction t,hrough th(J potver tubes. This decreases thr current thmugh the 2H30, and tlie voltage drop across it, thus coi,recting the change in line voltage. The curi,rnt-voltage cliaracteristic~of the 2H30 ai’e .sur11 that a line voltage cliangt>froniI15 to 110, or froni I15 to 128. is cctrrevted by :i changsc, of 0.2 volt in the plate volkdge of the (iSH7, correspondiirg t0.a phototulx current change of 1 part, iii 60,000. Because thip cun.erit varies as the 5.8th powrr of the output voltage, the c l r g r c ~of regulation theoretically ol)t;iiiicd i* sufic,ientl>.gi,eat.
Figure 1.
Circuit Diagram
1-uliinresistor 1 w a t t . others 0.25 watt. 0.0002rf. condenser should h a r e high leakage rrsiatance. Windings of 40-ina. power transforIrler are: .I, 11.5-volts priinary; B , center-tapped C,.3-\-ult secondary; C, 3-vult secondary; D , ,100- or 5.50-volt center-tajrped sectindary
For operation of light sources requiring other voltages and currents, circuit modifications must, be made. By the use of two 2H30 tubes in parallel, the current out,put of the regulator, without other circuit changes, is increased to about 2 amperes. For output voltages differing greatly from 6 volts, a change must be made in the transformer used. Higher wattages may be handled by t’he use of 6L6-G or 6Y6-G power tubes, with appropriate circuit modification. Care must be taken to use, on the power tubes! a heater voltage suffici2ntly Ion to prevent undue grid emission. PERFORM4NCE
Data on drift and warm-up characteristics were obtained by einploying a n Evelyn photoelectric colorimeter, equipped v i t h a
ANALYTICAL CHEMISTRY
610
510 mp filter, for measuring changes in voltage output from the regulator by measurement of the changes in galvanometer reading. As it was found experimentally that the galvanometer reading varied as the fifth power of the lamp supply voltage, changes of approximately 0.02% in output voltage could be detected. Errors due to drift of the measuring instrument were minimized by substituting a resistance box for the series resistors of the colorimeter, and by allowing an overnight warm-up period, during which an auxiliary poxer source was used, for elimination of drift due to fatigue of the colorimeter lamp and the barrier layer photocell. The external load on the regulator was 0.18 ampere. The output voltage of the regulator decreases by 0.2% when the line voltage is raised from 90 to 130 volts. The change with line voltage is approximately uniform throughout this range This overcorrection is apparently caused by the small amount of wave-form distortion produced by the regulator with the attendant change in the ratio of the RMS voltage (which determines the light output of the 82 lamp, and hence the voltage drop through the phototube load resistor) to the peak voltage (which to a largp extent determines the D.C. voltage to which this drop must be equal). Drift of the regulator output voltage after narm up was found to be just detectable (0.1% change in light output, corresponding to 0.02% change in output voltage) over a 4-hour period. A 150 ampere-hour storage battery was capable of equally good performance after overnight operation. Forty minutes are required after the regulator is turned on to reach complete stability (less than 0.02% voltage drift per hour). Duiing the first 5 minutes, downward voltage drift is a t the rate of 0.04% per minute. Comparison with battery warm-up characteristics is difficult because these characteristics vary greatly with the type of battery, its state of charge, and the length of time of disuse before the trial. Thirty minutes were required for a 150 ampere-hour storage battery, charged, connected to the colorimeter for 20 hours, then allowed to stand idle 13 hours before the test, to reach the same degree of stability. The response of the regulator to line voltage changes is not instantaneous. If the line voltage is changed from 90 to 130 volts in less than 0.1 second, when the regulator is serving as a power supply for an Evelyn photoelectric colorimeter, an upward excursion of the galvanometer spot, amounting to about 10% of the total deflection, occurs. Approximately 2 seconds are required for return to normal. As the galvanometer time constant is longer than that of the regulator, only large sudden changes in voltage are apparent as temporary excursions of the galvanometer from its equilibrium reading If a galvanometer of very short period were used, flutter would probably be apparent. The effect of supply-voltage wave-form variations on output voltage has not been studied, In the author’s laboratory no variations ascribable to this source have been detected. However, in a few laboratories, with devices drawing heavy currents and having nonlinear current-voltage characteristics-e.g., thyratron-controlled devices-the waveform distortion encountered is sufficient to make the voltage regulation poor. h modified version of the voltage regulator described here, involving the use of voltage regulator tubes in the direct current voltage supply for elimination of wave-form difficulties where they exist, is being marketed by the Rubicon Co. Applying Spectrochemical Analysis to 2 M Perchloric Acid Solutions. John G. Conway and Milton F. Moore, University of California Radiation Laboratory, Berkeley 4, Calif. HE spectrographic analysis of perchloric acid solutions presents T m a n y difficult problems not usually encountered in the analysis of solutions, The main difficulty is the chemical reaction between the electrodes and perchloric acid. Platinum or gold electrodes in sufficient quantities for such work are prohibitive in cost. The copper spark method modified by the use of acidresistant paint and evaporation in vacuum offers a solution to
this problem (1, 8). The copper electrodes are not expensive and are available in sufficient quantities. Electroplating a thin film of gold on the end of the machined copper electrode was tried, but even the best plated electrodes had small pinholes through which the acid would attack the copper. This etching became greater upon concentration of the acid by evaporation in the drying process. Salts were formed in such quantities that mechanical dislodgment occurred when the spark first &truck. The salts also increased the background to such an extent that it vas possible to densitometer the lines. Several acid-resista n t p a i n t s were T O V A C PUMP f o u n d satisfactory, the best of which is a vehicle from a paint known as Prufcoat (Prufcoat Laboratories, Inc, 50 East 42nd St., New York, ,, c12 N. Y.). For use in this work the Prufcoat is diluted with 6 parts of benzene.
A freshly machined copper electrode is dipped into the diluted paint to a depth of 0.5 inch, withdrawn, and placed upright in a holder to dry. Drying may be accelerated by heating. The dried electrodes are then placed in a copper block and the samples and standards are discharged onto the protective coat. If the sample or standard is too large to be placed on the electrode a t one time, it may be reduced by partial evaporation in air. The solution should not be permitted to go to dryness in air, because the action of the perchloric acid is enough to decompose the protective coat. The partially dried samples and standards are transferred to a copper electrode holder and this is placed in a vacuum desiccator which is then evacuated to betn-een 0.2 and 0.5 mm. of mercury and held there until the solution dries. This drying in vacuum may be hastened by irradiation of the copper block and electrodes with an infrared lamp. Perchloric acid will distill undecomposed a t reduced, but not a t atmospheric, pressures (3). Calcium chloride is used as a drying agent; it is considered adequate because under under the conditions of sample preparation-heating vacuum-the desiccant is maintained close to its maximum efficiency. The dried electrodes are kept in the desiccator until ready to use because of the hygroscopic nature of the sample. Once dry they are sparked by the copper spark method. The protective coat increases the background to some extent. This method of handling samples was originally worked out for the analysis of hafnium and zirconium in perchloric acid and haB been very satisfactory. LITERATURE CITED
Bachelder, M. C., Conway, J. G.. Nachtrieb, N. H., and Wildi, B. S Atomic Enernv Commission Declassified Document. MDDC 511 (Nov. 6, “945). (2) Fred, M., Nachtrieb, N. H., and Tomkins, F. S., J. Optical SOC. Am., 37,279 (1947). (3) Prescott and Johnson, “Qualitative Chemical Analysis,” p . 536, Xew York, D. Van Nostrand Co.. 1932. WORKperformed under auspices of U. 5. Atomic Energy Commission. (1)
Increased Life for Sodium Arc Lamp. Norman G. Adams, Bureau of hfines, Petroleum Experiment Station, Bartlesville, Okla. useful life of a General Electric laboratory sodium arc lamp In most cases the useful life of the lamp is determined when an excessive time is required before the discharge begins. It is possible, with a lamp of this age, t o induce the discharge by illuminating it for a few seconds with an intense tungsten lamp such as a Westinghouse 150-watt reflector spot lamp. This laboratory is a t present using a sodium arc lamp which would otherwise have been discarded 6 months ago. HE
Tcan be greatly increased by a very simple procedure.
