THE DESIGN AND CONSTRUCTION OF AN AUTOMATIC SPECTROSCOPE A THESIS IN ELECTRICAL ENGINEERING
Zackie D, Reynolds
Approved
Texas Technological College May, 1951 Tp: DESIGN AND CQNSTEUCTION OP AN AUTOMATIC SPECTROSCOIE
A THESIS IK ELECTRICAL ENGINEERING
Submitted to the Faculty of the Division of Graduate Studlea of Texas Technological College in Partial Fulfillment of t^e Requirennts for the Degree of
MASTER OP SCIENCE
Zaokie D» Reynolda, B. S« It
Lubbock, Texaa
May, 1951 ACKNOTIXEDGSMENT
Ihe aulhor hereby expresaea hie appreciation to the m9sft)era of his Oliesls Committee, Professors C, £• Houston, chairman, H, A. Spuhler, and v . i:. Craig, ^o have given helpful criticism and suggestions in the preparation of t^is paper* La particular, appreciation la expreaaod to Dr« Craig, Professor of Chemistry, for providing the opportunity of working on this project, for his much needed moral support, and for hia assistance in coordinating the spectrographic science with the electronic*
in
U TABLE OF CONTENTS
Introduction •••••••••••••••• 1 1* Principls of Operation ••••• 9 2. The Spectroacope • • • • • • • • • 14 3, The Multiplier Phototube 21 4« The Power Supplies • 30 Theoretical Considerations Specific Construction Requiremonts Circuit resign The High Voltage Supply IhB / 250 Volt Supply 5* The Sequence and Timing Control •••••• 54 6. The Amplifiers • •••• 65 7* The Recordera • 76 8« Teated Operation and Suggested Improvementa 80 Bibliography • • 65
ill IHTRODUCTIOH 2 In 1666,^ Sir laaao Newton first separated white light into a band of colors with a prism* In 1814, v • H. V.oHas ten observed spectral lines, and In 1859, 0, l • Klrchhoff and R. Bunsen made the first practical spectroscope. Thus, it v/as that the visible electromagnetic radiation of ex* cited bodies was separated into regions of different wave lengths* It has since been determined that electromagnetic radiation exists not only in the visible region but extends also into the short wavelength region of the ultraviolet and the X-raya as well as into the relatively long wave length region of ttie Infrared, or heat waves and the longer radio waves* Radiation near and including the visible region is very useful for the Investigation of matter, v1thin the wavelength limits of approximately 1000 Angstrom units (lA: 10-^^ meter) to 80,000 A, lies the region of visible llpht, the transmission limits of both glass and quartz, and the spectral sensitivity regions of photocell cathodes. By en ergy shifts and vibrations in the outer electrons in atoms and molecules, excited matter produces radiation in this region* Thla excitation may easily be produced by electric arc, spark, or gas discharge. The specific frequency of radiation from a sample of inaterlal is determined by the
^0, R* Harrison, R. C« Lord, and J. R, Loofbourow, Practical Spectroscopy* (New York: Prentice-Hall, Inc*, 1943), pp. 2-4* state of excitation and by the elemental constituents of the Sample, each elemsnt of the periodic table radiating energy at specific frequencies in its characteristic spectra* The more restricted spectral region near to and including vis ible light, 1*0* from approxlnately SOOO A to 8000 A con tains spectral lines of most elements with sufficient avail able Intensity to observe and measure. A large portion of spectroscopic work is now done in this region. The spectroscope has been an important aid to the re search scientist in determining the kind and magnitude of the fundamental constituents of matter. The scope of its use in technological laboratories Is still increasing, and its usefulness is not restricted to any one field of science. As the information furnished by the spectrograph becomes more and more important in Industry and research, the im provement of the performance of the spectrograph likewise becomes necessary. Specific improvements would be greater accuracy, shorter time limits between sample injection and the determination of useful information, and Increased con venience* This paper presents a combination of electronic and optical arrangemsnts designed to facilitate the rapid and accurate determination of quantitative measuremsnts of known materials in a spectroscopic sample. Instruments performing these functions are commercially on the market; one is manu factured under the trade name of "^uantometer." The spect- roscope hez^ described was designed and constructed to off set the very high cost of the commercial instruments while providing an adequate instrument for college laboratories, small industries, etc. This work does not represent an in tention to compete with the quality or to knowingly infringe upon any of the circuit details used in ccmsnerclal instru ments of this type. Previously, nearly all quantitative analyses of test samples of material were accomplished by photographing the pattern of spectrum lines produced when the material was burned in an electric arc or spark. Since the intensities of the spectral lines are functions of the amount of each element present in the sample, the amount of exposure, and subsequent darkening of the film, may be used as indications of the percentage of each material present in the sample. In order to Interpret correctly the correlation between film exposure and percentage composition it becomes necessary to plot ''working grains'* of percentage composition versus den sity of the blackened film for each element. Ihls is ac complished b^ putting known samples of material in the spect roscope and using the film densities to plot the working cuz*vea* Since exposure time^ arc intensity, and densito meter measurements may vary between different runs, it be comes advantageous to refer all measurements to a standard, or known amount of a given material, in each sample* Thus, any variations in exposure from one run to another will af- foot the standard lines aa well as the "teat" lines* The relative denaitioa of the photo|praiphed lines may then bo uaod in calculating par cent age compoaition. Usually, the internal standard la a major conatituant of the sample, such aa the baao metal in an aHoy* In order to prevent large orrora, more than one teat is usually run on each sample, and the results are then averaged* 'Although the photographie method of quantitative an- alyaia la sexialtlve and pz
^A* M* Olevor, ""A Roviev of the Developmnt of Senai* tivo Phototubea,** Froc* j2£ !•£«£•# Vol* 29 (Auguat, 1941), pp* 4X5«4^* 6 ever, the aensitivity of the light receiver could be in- creaaed to one million times that of the conventional photo* tube, and this sensitivity could be controlled over a con siderably large range, to "handle" lines of different in tensity octaves, applied to logarithmic intensity scales* A few instruments have been developed which utilize multiplier phototubes for qualitative and quantitative an alyaia* Generally speaking, these units have been lar^e and expensive, notwithstanding the fact that the output of a multiplier phototube la generally sufficient to operate sensitive recorders with no intermediate stages of amplifi cation* The jnajor considerations are then the stability or constancy of amplification, short term reproductibility, long term stability, and general linearity*^ These are ea- aential for accuz*ate quantitative analysis, although not so important in qualitative measurements* An automatic apectroaonpe is herein described which uses multiplier phototuben aa detectors* Comblnationa of accepted electronic circuita and spectrographic procedure are applied to produce an inexpensive Instrummit designed to provide dependable and reproducible operation*
1E, A. Doettner and 0. F^ Brewing.ton, "The Application of Multiplier Photo-Tubea to Quantitative Spectroohenioal Analyaia," Jour* Opt* Soc* of Awer** Vol* 34 (January, 1944), "^Applied Haaearch Laboratoriea, SpectroRrapher*a Newa Letter, Vol* 11* No* 11 (San Pranciacoi April, 1949), pp, 1-5* Thla unit was cona true ted for Speotrographio Roaearoh in the Department of Chemiatry, where ita primary uae will be for the quantitative analyaia of aaxsplea of material with known oonatltuenta, but with unknown amounta of these mater ials* 1h9 time of analyaia for a aample is, after insertion in the arc, approximately one and one«half minutes for in- atrianent operation and fifteen seconds per element for in- torprotation and recording of percentage composition* Sineo oertain spectral linea of an instrument have move definite line-intensity funetlcna of concentration than others, the choice of the indioatlng line for any specific element will affect the accuracy of obaervation for that element* Since analyaia by thla method la quite rapid, several runs may bo made and the results averaged to give very good results in a short time* Conmeroial Instruments give results with a deviation of approximately 0«1I^ to a naxlBRin of 4^ over a large aver age of runa*^ There la no reaaon to expect greater error in the inatrunent dlacuaaed if operated under identleal con- ditiona, i*e« whan the factor a of traiperature and atray light are controlled to the aame extent*
<^mm It2iUl< ^tttr.
PRINCIPLE OF OPERATION 9 General circuit analyaia is best aided by reference to Fig. 1, the block diagram of liie apparatus. The sample to be analysed is first prepared and in serted in a small recess cut in the tip of the lower arc electrode. Whenever the arc is struck, this material la energlssed, or burned, and produces a characteristic light depending upon t^e elemental constituents of the electrodes and of the sample. The electrodes are usually made of some highly purified material such as carbon or copper, vlth a known spectral output, so that the spectral lines of the aample may be distinguished from t^oae of the electrodes. It is an obvious necessity that the electrodes be made of some material other than that to be analyzed. The electromagnetic energy of the sample is then op tically directed through a narrow slit to the surface of a apherically concave replica grating In the spectroscope, which diffracts and reflects the incident li('ht at different angles depending upon its constituent wavelengths. The shape of the grating is such that these different diffracted and reflected beams are focused as separate Images of the entering slit upon a curved surface. Upon this surface, known as the focal surface, the light lines are separated almost linearly with respect to their wavelengths. It is here that a photographic plate or film is usually placed when analyzing the aaaqple* In this particular application, no photographic plate 10 is necessary and the lines are optically directed to the in dividual cathodes of stationary photomuXtlpHer tubes. Only one selected line, which presents a ready IxKlication of the quantity of material present, is used for each element to be tested. As stated previously, these elements must be known beforehand *
As the light from each chosen line strikes the cathode of a phototube, electi*ons are given off which are electro statically directed through a series of "dynodes" in the multiplier section and collected by an anode. The output of the phototube is essentially a constant cunent whose magni tude is nearly a linear fimction of the amount of light striking the cathode* The above mentioned currents are used to charge separ ate capacitors for a short time so that the voltage across each capacitor is the time Integral of the current charging it* This is an "averaging" effect which diminishes the ef fects of an unstable arc intensity, wavering of the arc, mechanical vibrations, and erratic vaporization of the sam ple* The voltages across the oapacitora are Inputs to sepa rate DC asgpllfiers controlling plate relays. Aether these relays are energized or not depends upon tibe magnitude of the voltages on the Integrating capacitors. V^l en each capa citor has charged above a certain set minimum value, the output relays are closed. After the charging time, a se- 11
SPECTROSCOFE
SAIIPLE TO BE AWALTZED
UQLTIPLIER 60'VAC PHOTOTUBES PCWHR SOURCE AHC
SOURCE LINrE INTBC21ATING VOLTAGE RECJULATING CIRCaiT TRANSF0R1.IER I I TBONG AND Hu, REGULATED DC SBQOENCE PCWER COl^TEROL ^1/W\ SUPPLT I I DC AIIPLIFIHl
( AMPLITUDE CONTROLLED "ON-OFF"' CIRCUIT )
CALIBMTED
RECORDS:^
BLOCK DIAGR/L: OF APPARATUS
Fi.. 1. 12 quence control closes the circuits of tibe output relays to time recorders, disconnects the phototubes from the input capacitors, and shunts a known resistance across the capaci tors to allow them to discharge* Tho time recorders then operate until the capacitors voltages have dropped to the critical minimum voltage where the output relays open. Thus, spectral line intensity, maximum capacitor voltage, end capacitor discharge control-time are all functions of the percentage of eadi element in the sample to be analysed* As stated before, one spectral line is chosen for an internal standard element in the sample, this element being of a known percentage which is greater than the percentage of any test element* The time recorder for this standard element will Indicate a relative base time* The "normaliz ed" times for the test elements will then be a fraction of thla time* vrorking curves of intensity versus normalized time may then be used for the final analysis of information for quantitative data. By this means, any variations in operating conditions will not affect the normalized time, and accuracy will be retained. It is to be noted that if operating conditions can be precisely reproduced, only the time Indications of the test elements need be considered, and their recordera may then be calibrated directly in nor- nvillzed time, or in percentage composition* Aa further indicated on the block diagram, the DC am* plifier and the multiplier phototubes are supplied by a 13 highly regulated DC power supply, whldi is in turn supplied by a line voltage regulator. These regulators provide de pendable circuit operation under varying line supply and loading conditions* The "Timing and Sequence Control" controls the time of charge and discharge of the input capacitor, the connect^ Ing of a degenerative network within the amplifier to pre vent "drift" during the period when the aiit is on but not used for analysis, the starting of the timing indicators, and the striking of the arc, if this is desired* THE SFECTHOSCOEB 15 The spectroscope used la a modified form o. the Cen tral Scientific Company'a Orating Spectrot^raph No. C7102, of Fig. 2.
Fig. 2.
The arc pov/cr source used in this case Is 220 volt DC as preferred to 110 volt DC because of its greater sta bility* A high voltage AC arc (2000 to 2000 volts) and spark (10,000 to 40,000 volta) may alao be used for criti cal analysis,^ if special precautions are taken for safety and to secure proper firing* According to The Central Sol* entific Ccmipeny (CENCO) literature.
Iw. R* i^rode. Chemical £pQctrogcopy* (New Yorki John Wiley and Sons, 19£9), pp. 44-50* 16
The arc and the spark are both useful and should both be available. The spark has the advantage of not burning the electrodes and of seldom needing ad justment* The arc has the advantage of being quiet and requiring much lower voltage* The arc is hotter and more material is volatilized into the vapor state, while the spark provides much higher potential and is advantageous for those elements having high ionizat ion potentials. The material to be analyzed will us ually determine the choice**^ The electrodes used may either support the sample to be tested or they may be composed of the material Itself. Graphite, copx>er, and silver electrodes are often used to hold ^e material to be tested, graphite being the most common because it gives very few lines of its own,parti cularly from 2500A to 3500A. If the electrodes are com- posed of the material to be tested, they should have a high melting point or should be used vith a high voltage spark source. The light from the source is directed and condensed by a proper lena so that it completely fills the entrance alit, and impinges upon the concave grating. The grating disperses t^e 11^t proportionally to its wavelength and focuses Images of the silt upon a curved surface which co incides with what la known as the Rowland circle. To fur ther illustrate this, refer to Fig. 3 which shows the Pashen- 2 Runge mounting of the concave grating*
^antral scientific Company, C^co Grating Spectrp- graph. Cat. No* 87102, Tedanical Llterature (Chicago), p• 6* ^Harrison, Lord, and Loofbourow, ^* cit*, p. 79* tv
- Runge" Grating Mounting
mm Hio iUt S« and tlao gratii:^ a^ are mounted m the oir«» oiMrtroiiot of a oirolo with a diamaf^r equal to tho radiua of tho grating aurfaoon and if tho re ii no dloporsion^ the ijMiigt of tho iXit will bo foouiod on anothor portion of tho ^i»^miwmsi^om of tho Bow land oiro3ji« at point C, Point 0 ia oa tha optioal axia of tha grating; l^ere la a right angla ralatimithip batwaon tlie isipinging light and tha dia<» tanaa batwaan s and C aa ahovii# with diiperaioat the iinagao of tha ilito aill ha dlffraotad by an angla^fron tha angle 18 Of iBOldanaa 1# but will bo in foeua on the Rowland eirolo« Iba diaiwraiati ia related to the other conatanta of tha in- •tmnant by tha ralatioiitfhipi
(^',•/•• •••„•!• mA - V (-T/^ c± S//9 -&-) whara /7 ia tha order tX apaatru>i# ^ ia tha wave laagth# V is tha grating tpaaa* / ia tha angle of inoSdenoe, exid mmM '-^ia tha dlraotien of thm diffracted wave front having wave length ^ *^ For thla apaatrographt tha firat order apaatruai is ua- ad, tho grating haviiig ISldO.llnaa par inoh or
and i « 100 30** The diamatar of the Rowland oirolo ia 106 Otti and with tha givan grating, thara it a linear diaparaion of approdwitaly 1$ A/nn wit hin the wavalangth range of 2S50 A to 7000 A« Tha alit-width of thia apaotrograph may be varied i tha affeot ia ahown in Fig# 4* With a narrow alit^ the curve of llaa intanaity«varaiia«dl8tanoe along tha Rowland circle ii aa ahovn at (a}« Tha contour ia very narrow and many clooa Hnaa nay thui ba aaparatad« Tho lisit of reaolution, aa dafinad by the Raylalgh oritorion, ia the diatanea betwaen tha Maylawan and tha firat ninlaam of tho diffraction pat* tarn*
^antral Saiantifio Ooapany, 52J2* oit^* p* 11* 19 Aa the width of the slit is Increased, the contour re tains its shape for a time with the maximum Intensity in creasing. This effect may be analyzed by considering a wide slit as many narrow slits side by side. As the number of narrow slits Is asstmed to increase, their Images, which will overlap, will first cause the total Intensity-versus- distance contour to increase more in magnitude than in width. As the number of entrance silts Increases to the point where the maximiaa separation of individual Images approaches the elemental slit-width, the Intensity of the total slit-con tour tends to increase very slowly, while the width increas es z*apidly* This effect Is shown at (b) • For quantitative analysis, the maximtas intensity should be utilized as in (b) and the line separation should be accomplished, if pos sible, by selecting Indicator spectral lines which are not limited by the decrease in resolution* For either contour, a proper setting of the focused line on the phototube cathode is approximately that of points w-w' so that a large cathode area will be utilized, while at the same time asxple clearance Is left to prevent small line shifts, re-positioning of the phototubes, and mechanical vi bration from affecting the total quantity of light incident upon the cathode* It is not necessary that a slit image be exactly focused on the cathode, as a slightly de-focused line will provide more uniform cathode excitation* 20
LZ-IT 07 RESOLUTIOH 3T iUIL-ilJlI CRITSiilON
HALF-EfTENSITI BREADTH
Z2^ Z^s^ (a) (b) SPECTRUU - LINE COIWOURS ( INTETiSITr VS DISTANCE ) (a) FOR NARRa7 SLIT (b) FOR .TIDE SLIT
MJLTIFLIER PHOTOTUBE CATHODE SHOOLD BE COVERED BT SPECTRAL WIDIH (w^*«)
Fig. h. THE mWSXftlER PHOTOTTJBE 22 The principle of the multiplier phototube involves control of secondary emission to amplify very small quanti ties of electrons, from the cathode, to appreciable quanti ties useful for control and observational purposes. The manufacturer gives thia information: "Having small size, rugged construction, enormous sensitivity, low noise level, low dark current, freedom from distorlon, and a spectral re sponse covering the same range a a that of the eye, the IP22 is especially useful in colorlmetry and spectroscopy."*^
FOCUSING GRILL
> INCIDENT LIGHT
0 = PHOTOCATHOD MICA SHIELD 10 - ANODE 1-9= DYNODES 92CS-6549R2
Fig. 5» Schematic Arrangement of 1P22 Structure
technical Literature of Radio Corporation of America, RCA Victor Division, Photolithograph 1922-8-45, (Harrison, 25 The schematic diagram for the IP22, a photomultipller used in this instrunwnt, is shown in Flg» 5. Incident light i5)on the cathode £ cau«(es it to emit electrons which are directed by fixed electrostatic fields along curved paths to the first dynode, or secondary emitter* The electrons which impinge upon the dynode produce, or "knock loose", many other electrons, the number depending upon the energy of ttie im pinging electrons, ibis larger beam current is af cln direct ed to the second dynode and i:iven added velocity by an e- lectroatatlc field. Ihere other electrons are produced. This multiplying process continues until an appreciable cur rent leaves the last dynode (dynode No« 9) and is collected by the anode* This constitutes the current utilized in the output circuit. Dynode No. 9 is shaped to partially en close the anode» which is a fine grid struct\n?e. This shields the anode and prevents fluctuating anode potential from affecting the critical electron focusing in the Inter- dynode region* The electrons from dynode No. 8 pass throvigh the anode grid structure^ and cause secondary emission at dynode No* 9« Ihe resulting cloud of electrons is collected by the anode* The output current is thus substantially in* dependent of plate voltage, giving it very high output im- pedanoe# and allowing It to be coupled to almost any type of load* The incident light passes through a grill which ia connected to the photocathode* This grill serves as an 24 electrostatic field for the open side of the electrode structure* The projected area of the cathode to incident light la approximately 1" x i". Nearly all this area should be illuminated for maximum dependability of aispllfIcatlon, and the illumination should be as evenly distributed as possible since different portions of the cathode may have slightly different sensitivities and spectral response* The effective spectral response of the multiplier tubee depenSs upon the cathode coating, which is usually oxides of rare earths. For any particular application, there can usually be chosen a tube which will be sensitive within the region to be studied. Fig* 6 shows a comparison of spectral sensitivity of different cathodes relative to ihe human eye« Although the cathode response curves are extended into the ultraviolet region (wavelength shorter than 4000 A), the response of the actual phototube Is limit ed by the transmission of the glass walls of the tube* Un less special glass is used, the usual wavelength limit is approximately 3500 A or 4000 A* For some applications it may prove advantageous to insert a flourescent material be* tween the cathode and incident light to convert ultraviolet radiation into visible light* The successive stages of the 1P22 are normally operated at 75 to 100 volts per stage^ tha amplification increasing with stage voltage* The am plification increaaing with stage voltage* The amplificat- INTRA ESD
/if. 6. Spectral Comparison Curves of Differe. t Cathodes 26 ion and aensitivity as a function of dynode voltage is pre sented in Pig* 7* The maximiaa amplification for the 1P22 ia generally 200,000 whldi is not as hin;h aj that of son© other multlplieraj but the stability of operation and accuracy of characteristics are more dependable* The amplification, as mentioned, pertains to that at very low light Intensities. For hi^er values of intensity, the tube may age, or become "fatigued", so that the sensitivity will decrease. If the tube is not used for a time, the sensitivity will increase to an appreciable percentage of its original value unless the cathode has been damaged by excessive currents. The makers of the conjnerclal ^imntometer consider it good prac tice to keep the tubes fatigued, or "tired", by providing an even illumination upon their cathodes then not in use. Al though this results in a considerable decrease in sensitivity, the atabillty and precision of readings are Improved. For different light intensities, the anode current varies with the voltage between anode and dynode No* 9 in a manner shown in Pig* 8* The voltage should be maintained at a value just sufficient to give anode saturation* This reduces the dark current due to leakage paths and also reduces the ion bom bardment of the dynodes, so that operating stability is in creased without a sacrifice in aensitivity* The sensitivity of the multiplier phototube may be
^ifeji' 27 easily controlled either by decreaaii^ the dynode voltages, or by changing only one dynode voltage so tiiat defocuslng of the electron stream reaulta*^ A major consideration In the use of the multiplier phototubes is dark current. This is spurious current above that caused by incident light and is the result of leakage paths, noise components, ion bombardment and leakage, and the cathode temperature* Although this current is usually negligible as compared with those produced by most light intensities, it may become appreciable at very low illumina tion* ?1iere maximum gain with unusually low dark current is required, the manufacturer recomrnends that at these low light levels the phototubes be kept refrigerated, preferably with liquid air* The multiplier tubes are. In this application, mounted on a stationary chassis diagonally in front of the film holder of the spectrograph* ^'Ith the film holder removed, thin front-surface mirrors will be used to separate the de sired spectrum linea and direct them to the required photo tubes* In this manner^ adjacent linea may be utilized if desired and the phototubes will be more mechanically secure tlmn if they were to be mounted on the spectroaeope face and moved to the indicating lines* Since the tubes are not
^R. H* Mullerf R* L. Germanf and U. S. rroSf« Experi- nunta•ntajLl Slectronioa* (New Yorki Prentice-Ha 11^ Inc .,T»2T, p* 46* 28
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- NOIiV3IJI14MV 1 29 directly on the focal surface of ths Instrumsnt, the lines will be indefinite images of the entrance slit upon the cathode, unless focused by lenses. This is an advantageous situation since the cathode is then more evenly illuminated* Pig* 9 shows the electrical connections of the photo tubea on the remote chassis* The cathode is operated at a high negative potential with respect to chassis, or ground, and the dynode potentials are obtained by Individual volt age dividers on each phototube socket. A rheostat in series with ea(^ voltage divider gives individual control of the sensitivity of each phototube» to partially compensate for the large range of relative Intensities of spectral lines* The anode ia operated at
•r-t
o o o o •p THE POWER SUPPLIES 31 Since the amplifier of the automatic spectroscope la DC in character, the absolute voltages at various points are Juat aa Important as the changes In voltages, the concept us- ed in the analysis of AC amplifiers. The currents In the tubes, the voltages at the grids and plates, and ths current through plate relays are all critical, and any change of sup< ply voltage will effect the stability and dependability of the circuit* As an example, a very small change of plate aupply voltage for the first amplifier of a series will mat erially cbange the bias of the succeeding tubes, and will change the quiescent output current in the plat circuit of the final tube. Therefore, it Is i^cesaary that the aupply voltage be very constant, both with respect to changes in load current, and with re aspect to changes in load current, and with respect to changes of the input AC supply* Photomultipller tubes themselves need very constant supply voltages* A tube with nine dynodea (which Is the us ual case) has a current «mpllflcation above that of a two- elerosnt phototube which is proportional to the ninth power of the dynode supply voltages* For example. If a 1P22 tube la used and all dynode voltages are equal, the current am plification changes from 100,000 to 70,000 when each dynode voltage la varied from 90 to 85 volts, as Indicated in Fig*
Two power supplies on one chassis have been developed for thia inatrumantft One auppliea ^900 volta with reapect 92 to ehaaaia for the phototube cathode and dynode voltageai the other auppliea |fe50 volta with reapect to ch&ssls for the DC amplifier* A seventy-five •volt gaaeoua regulator and ita aeriea reaiator are ahunted acroaa the /te50 volt output to aupply the anode voltage for the i^ototubea. Both najor I at^plies, i*e* the •900 volt and the /2bO volt supply, are regulated by a degenerative regulating circuit*^ The input line voltage la held at 115 volta constant within plus or minus one par cent by a magnetic-type cons tent-volt age trans* fomer, hence the regulating circuit haa extremely well- regulated output* THSORETICAI. COHSIDSRATIOHS The baaic diagram for a dagenaratlve regulation cir* ouit ia ahovn in Fig* 10* Tha dotted reotaagla repreaenta the equivalent cir* ouit of a vaeuuBKtube rectifier with output voltage /^' and ita aaaociatad internal iapadance ^y , whioh mey ba consid* erad to ba purely raaiativo* Tz la the aeriea control tuba whioh aniat ba capable of oarryix^ tha maxlanaa load ourrant without axoaaaive heat diaaipation* It la choaen to have a Harga mutual oonduotanca»^/77^ » and a low plata reaiatanca, ;:P^ . Mora than one tuba may be put in parallel to aooom.
^* B* Bareakin# "Voltage Ragulatad Power Suppliaa," Proo* itl^i£»S#f Vol, SI (February, 1945), p* 47* 33 pllsh the above requl-''erTcr.ts. Kith n ^vT^'r tvibes in -nrallel, the 3^9iTiisrlble current is n tinea that for one tube, the combined mutual conductance ia n tines that for one tube, end the effective dynamic plate resistance Ir l/n tinos that of cnc t:;.^-)0.
• REGUIATING SECTIOi
EFFECTIVE i.AVD
POIYER SOURCE
j'ig. 10. Basic Deg.ncrative .lerulation Circuit
// is an amplifier tube chosen to have a high amplifi cation factor,/-J , and a low d-c plate current. Several stages of amplification may be used If proper attention is {=;lven to polarity of the output relative to ttie input* The grid voltage of ^ is a fraction K of the total output volt- 34 aga« Tha value of K la determined by the relative valuea of /TI and ^ which are usually resiatancea of large values, to limit the current through them to a negligible value* 75 ia a gaa ecus-discharge tube whose voltage drop la very nearly constant over a large conduction-current range* It is used to bias t^e grid of 7/ to a negative potential with respect to the cathode to give maximum amplification with no grid current* The resistor ^ is used to provide minimum conduotion current for 7^ • ^ ia a resistor used to designate the effective load which constitutes a current drain of-Z^ amperes, Aa illuatratlon of the general action of the circuit followa* When the output voltage ^ teiads to drop by ^^o volta, a portion K of this change in voltage will be applied to tha grid of "^ and will be amplified to a magnitude 4^ Ji^ where >^/ia the voltage aisplification of 7J. This ohang« ia in the opposite direction to the original voltage drop* Tha grid of 7^ becomes less negative, decreasing the effect ive tiihe resistance of the series element* The voltage drop acroaa ;j ia than lasst and the output voltage ohange ia da« oreaaed by thia variation in aeriea-tuba voltage* v.hen tha output tanda to riae, the baaic circuit function is the same, however all voltage changes will ba reversed in direction or polarity# The aotio» ia effeetivaly inatantaneoua and pro- widaa a oontinuoua oontrol of the change in output voltage, kaapiag it within eartaln liaita whioh dapand, among othar 55 things, on tb^ anplific tlon of T, and tho trcn--nd-.ctance cf 7^.
•AAr- /•• r \ ^4 ^
r- 1 / i ^h h ^f' f MV. 1 I I 1
. "i /l Fig. 11.- - Equivalent Giro it of the Degenerative Peculator /•I
If Klrchhoff's Laws are applied to the equivalent AC circuit , or the equivalent circuit to changes of voltages and currents, relations may be derived showing the regulat- \ ion of the arrangement.-^ These relations are derived rith \ \ , 1 n y A. Abnte, Sasic Theory and Design of Electronically Regulated Power Supplies," rroc. ci I.n.^., vol, c: (July. 1946), p. 478. ^*
I I reference to the equivalent. 'olj.'cult Tor tiie degenorctive i-- oUl£.tci-, ri(^. 11, Tl.'j pi'actical assumption Ic made that t;ie currezit •a:.rouo- tlic voDtagf) dlvicler nupplylri^ the gric po tential of 7^ eiiu llio curi'cnt Vavo^a^h the L.:.. ^h\inG t^''- ^^* negligible coi^parcd v.ILL -.he loaL. cui'
.-his la tho I'atio oi' the change in output voltage to a cor- ro3pcnuin«3 chaiii^o in Liput voltage. If tho in>-ut \rcltage Is consldercL constant, tho out put voltage variation doipendo on th3 chan^/j of load c:i>uit- ions only. ^liosQ equatloiij hold j 57 (V
A/ • 00 /(^/S^z y
The last expression, C^^J , gives the output imi)edance, ^d , of the regulated power supply. The rec,ulatlon of such a power supply Is the ratio of the change of output voltage with a definite load current to the no-load output voltage. The percentage regulation is then:
SPECIFIC CONSTRUCTION RE^UIREIriENTS The regulated negative nine-hundred-volt supply was designed to supply a maximum current of 20 ma. which is now adequate for four phototubes. As other multiplier tubes are added to the spectroscope, either an additional series oontrol tube will be put in parallel with the original, or 58 the voltage-divider resistors for the phototabes will be changed from 22,000 ohms each to 50,000 ohms. The latter procedure appears more desirable, as the present regulating circuit would be ample for nine phototubes, and the sllcht loss of amplification due to degeneration in the dropping resistors would be negligible, particularly at low light levels • The poaitlve two-hundred-fifty-volt source which sup plies the DC amplifier Is designed to give excellent recu- latlon for load currents up to 50 ma* This "capacity" is adequate for the 9 ma* shimt current of the 75-volt gaseous ragulating tube and for three stages of amplification* To Increase the current rating for nine sections, another ser ies-control tube will be added in parallel with the one al ready in the circuit* Since t^e current of each phototube la nearly inde pendent of anode voltage, the regulation of the 75-volt anode supply need not be as good as that for the other sup plies, thus the gaseous type of regulating tube is suffici ent to aupply the necessary voltage In that its supply vol tage, the /260 volt aupply, will not vary* The total cur rant drain will be considerably less than 9 ma*,which la sufficient for nine phototubes* m designing this particular supply combination, sever* al factors had to be considered* Since the spectroscope will probably be used by untrained personnel, it was neces- 59 sary that all parts be exceptionally well insulated and that all precautions possible be taken to prevent accidents vhen replacing tubes, making adjuatmsnts, etc. A further re quirement was t^at a minlm\an of space be used for the sup plies, since most of the mounting space would be used for the mmpliflBvn and recorders* This necessitated placing all voltage supplies and regulators on one chaasls, using minia ture components where possible, an3 arranging components in such a way as to provide adequate wiring space and to allow for the new components as the current capacity is increased. The other requirements were that all parts should be stand ard and durable as possible, which is consistent with the provision for rapid repair in case of fault* The outpixt of the chassis is a standard high-voltage 8ix<»pin female socket* This socket Is the terminus for all voltages supplied by the chassis* Thus, chances of accident al contact with supply voltages are lessened. An effort was made to keep all high-voltage connections well Insulated and shielded or below the surface of the chasaia. Whenever pos sible, each component was so placed in the circuit as to minimize insulation strain cmd to have as low a maximim volt age with respect to the chassis as possible* This chaasls, the amplifiers, the indicating volt meter, and the recorder panels are mounted in a standard re lay rack, with coasters, to allow it to be placed in the most conventlent location* 40 CIRCUIT DESIGN The circuit design for the complete power supply chas sis as constructed is shown in Fig. 12. Specific design factors will be discussed with reference to this diagram* The input AC supply shown at the left of the diagram is Intended to be the output of an external constant-voltage transformer which will hold the primary voltage of the in put transformers at 115 volts within one per cent* The 115 volt line is also connected to the output socket on the right of the figure. This voltage will be used to operate timing relays and recording meters in other parts of the In- s trument *
Sas. High Voltage Si^pply The upper power supply of Fig, 12 supplies a negative nine-hundred volts to ground. High-vacuum rectifier tubes, type 2X2-A, were used as recosmended by the phototube manu- factiirer. The allo??able DC current through each tube is only 7.5 ma*; therefore, it was necessary to use four of them in a full-wave rectifying circuit to secure ample cur rent reserve for nine photoelectric tubes and to provide reasonable regulation* A 500-ohm resistor was put in series with the rectifiers to act as a peak-current llmlter*^ The
%. J* Reich, Theory and Applications of Electron Tubes* (2d ed*j New Yorkt MoOraw-Hlll Book ^mpany, Inc*, TSST, pp. 576-581* 41 output from this section was then filtered through a "Pi" section filter composed of two 2-mfd oil-filled capacitors and a 12-henry swinging-choke coll* The choke was connect ed in the loF-potential side of the circuit to minimize in sulation strain* It was not necessary to have exception ally good filtering action at this point since the regulat ing section which follows has a filtering effect also. Small values of ripple voltage would also have a negligible effect on the Integrated output of the phototubes* The output of the filter section was connected to a degenerative voltage regulator of the type already discus sed* The series tube chosen was a miniature 6J6 which has a mutual conductance,^/>7^, of 5500 mlcro-nlios per tube sect ion, and an amplification factor, ^ , of 38* Since the tube is a dual trlode, the total current rating Tlth simi lar elemsnta connected in parallel is twice that of one side alone, or 17 ma. For the low current drain of three original phototubes, this tube will provide very good re gulation* The tube was Inserted between ground and the poy- itivQ lead from the filter section, to prevent the indirect ly heated cathode from exceeding a voltage difference of plua-or-mlnua 90 volts with reapect to the heater supply. Low voltages on the tube also provide a margin of safety for any person who replaces the tube* The 6AK5 voltage-amplifying tube was also a miniature* It had a mutual conductance of 5000 micro-mhos and a high U2
> Oi
•r-i 43 plate reaiatanea of 0«54 Magoha* Thia tube alao has the de sirable character is tie of a low average plate current for normal oparating conditions. Thla decreases the load on tha power supply* Since its cathode was only approximately 85 volta above ground, the filamsnt was supplied by the same transformer as the 6J6* Instead of a single clo^»-<3 is charge VR tube aa a voltage reference as shown in rig. 12, tha lar ger voltage drop to the cathode of the amplifier tube neces sitated uaing 15 type NH«51 midget neon glow-bulba in seriea, providing the necesaary voltage drop with low current drain, approxiiiately 5 ma* The char aotar 1st Ics of each glow-bulb ia stable under operating conditions, but all tube a do not have idantioal voltage dropa, the average drop par bulb ba* ing aboufe fifl^ volts* The grSd of the 6AK5, although daalgned to be negative with reapect 1:0 the cathode, was connected through a pro tective current«»limitiiae resiator to a potentiometer whioh ia part of a voltage divider across the output of the supply* By adjusting tha potentiometer from the front nanel, the cir cuit can be adjusted for any variations in replaced NE-51 bulbs and for output operating voltage* Tha limit of con trol of no-»load voltage is over a range of approximately 850 volta to 950 voXta* The output of tha raotifler la protect*
H. ©• Reynolda, "KS-51 Glow Tubes," Unpublished Sini. nar Report, Texaa Taohnolc^ical Collage, Spring, 1949* 44 «d by a fuse, mounted on the front panel. A neon indicating bulb shows the condition of the fuse. Graphs of the operation of this supply are shown on pages 7^ and V7. Fig. 13 shows the variation of output voltage with input RKS supply-voltac.e variations with dif ferent loads. By using the pre-roctifIcation llne-voltage- regulatlng transformer, the operating range is kept within the limits shown ty the dashed lines, vithin this region, the change of output DC voltage from the regulator with in put voltage change is effectively zero. Ihe variation of output voltage with load current, vVfh a rated line voltage, is shown in Fig. 14. \'lth the instruments available, there was no detectable voltage change for load currents of 0 ma. to 20 ma*, l»31cating very good regulation* The regulation of this supply may be calculated from the equations ^iven In the theoretical discussion. The vari ation of the output voltage vith Input voltage with rated tube constants is:
when 1 T-^rn riL^U...-'^^^^^^^^^^^^-;y. • and the output impedance is:
^=-^:^5^ V^.^;r/if^r/'?'^>r//^^ ^''•''^^•''^^^ ^^^ 45 nhero:
For a current drain of 20 ma., the regulation isi
The i^50 Volt Supply Ohe positive 250-volt supply (See lower portion of Pig* aa) uses a high-vacuum full-wave rectifier, 5Y3 or 5Z4, so that the current drain allowable, as connected, is 125 ma* A 47-ohm resistor was added in seriea with the recti fied voltage source to limit the peak current, and the out put was filtered through a "Pl"-sectlon filter* ^e output from this filter was then regulated by a degenerative regu lator* The series tube for this regulator is a miniature beam- power 6AQ5 with a design^maximum current rating of 100 ma*, if grid current la not allowed to flow. Its transconduct- ance la 3700 micro-mhos and its plate resistance is 58,000
OhXBS* The ampllfyli^ tube is a 6SJ7 with a trans conductance of 1650 micro-mhos and a plate resistance greater than 1 Megohm* The grid voltage was supplied by a potentiometer tap on the voltage divider across the output of the regulat or* Thla potentiometer was also mounted on the front panel of the chassis to provide a measvu*e of control of the load U6
-9/0-1
k
t k 0 o
B5 95 f05 IIS 125
Fig. 13. Out.^iit-Volta e Versus Inputs-
Voltage for the 900-Volt Ai -ily. — • -920-
• QAA- t ) » « ' y * «
\ til QAA A I - • 1 t
, ! • ' 1 '
^ A4A-
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1 <) i n7 /i5 ^ D Z 5 30 LOAD c VRR£N. r-MA
-. - \ Fig. lU. Load-Voltage Versus Load-Current
for the 900-Volt Supply. 46 voltage. The cathode was kept at a reference potential by a miniature gaseous•discharge tube, an 0B2/VR108* The screen voltage was supplied by a tap on a series of voltage-divid ing resistors used to provide the sustaining current for the VR tvibe* The center-tap of the filament transformer for the 6AC;^ and the 6SJ7 was connected to another tap to establish its potential at a median between the 6AQ5 and 6SJ7 cathode potentials, decreasing the voltage gradient between cathodes and heaters for these tubes* An &-mfd electrolytic capacitor across the output of the regulator further Improves the filt ering and regulation for very sudden dianges in load current. Both of the outputs of the 250-volt supply and the 75- volt supply were protected by cartridge fuses azid indicated by neon lamps mounted on the front panel of the chassis* Incandescent lamps across the secondaries of the fila ment transformers Indicate when the power and regulating sections are on* Since the maximum current change for the 75-volt sup ply will be 9 ma*, there will be no appreciable change in output voltage* The actual amount of regulation with this type tvibe la not prediotable, and its absolute voltage may vary slightly from tube to tube because of manufacturers' tolerance of gaa mixture and cathode coatlnga in the glow tubeai however, its stability Is less Important than the others since the anode voltage on the phototubes is not crl- tical* 49 The operating curves of the 250-volt supply are shown on pages 5*0 and 5 ' • ^i?« 15 shows the variation of out put voltage v/lth input volta-e for different lead resistan ces* Within the range limited by the voltage-regulating transformer, any small input voltage change will cause neg ligible output-voltage change. Fig. 16 diowa load voltage as a function of load current for the correct line voltage* For this curve, the VR-75 waa removed. Vflth the VR-75 in the circuit, current available for amplifiers was dininlshcd by a fairly constant 9 ma. l.ith available Instruments, the regulation was insignificant for a total current drain of approximately 52 aa. Calculations for this supply follow. The change of output voltage for a change of Input voltage is calculated to be J A£. I _ i ,c ^ ,
where 4 ^gm^J^^r^O.^SX/o^X/^oy/^^)^ UB
Glia output impedance is t ^' >w;^ V^.s^X'tsjcj-vA/^-^j-- ^'^ °^^> ^'^-^ an:3 the regulation la: 50
-tz^Ch
SO lOo Ifo 115 /iO LINi VOLTAGE - RMS,eO^
^C« 15. OuV^it-Volta; :e Versus Line- Voltage for the 250-7olt lupply. 51
i-aso^
2so~:^K
f • + t *" * ^• 2^0 ^ • • - Kr * * * ^ ^ ^
• ^
• Ci i3o % ^
220
-tzto 20 ^o : eo bO LOAD CUnRENTrMA I . . . , Fig. 16. Load-Voltage Versus Load-
Current for the 250-VDlt Supply. 52 for a 50 ma* load* Thla also is very good regulation in the operating range* Pig. 17 (a), (b), and (c) are photographs of the com pleted supply and best show the arrangement of parts and the final appearance of the supply chassis with front panel. 53
(a) Front View
(b) Bottom View
(c) Rear View-
Fig. 17- DC Povrer Supply Chassis THE SBQTJBirCE AHD TBilUS CONTROL
^ • • • • 55 Tha three DC amplifiars and tha sequence-and-timing control cireulta for this instrument are located on another ohaaaia* A general diagram of these two circuits is shown in Pig* 18, page 57 * A very brief discussion of the ampli fier section (aee the lower part of the figure^ is necessary to properly daacribe the sequence control action. At the left of the amplifier diagram is the input from the phototubes. The relay contacts encountered from the in** put are tha •'charge-discharge" contacts which connect or dis connect \he input integrating capacitor C)^ from the photo- tuba output cir
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NOfHONOO „ 3 7JJ J„ ' llr'tVi/OA/ /ON 3Nt1 l¥dlD3t^S lOdlNOO 3JN3nt>3S JLS31 yoj y3/3/idi^y 58 the output contacta of the standard spectral line, f^^ * Under the normel quiescent or "cycle" condition, the timing re3ay,-^^ , and the "degenerative control" relay, H^ are energlEcd, and the "charge-discharge" relay, J^^, , and "ahortii^-control" relcy, ^tf^j , are de-en*»rgized, vith the atandard line contacta open * To atart the sequence, the "start" button on the front panel Is momentarily depressed, opening the circuit to the timing relay Tvc^m This relay de^enargizes, first discon necting the degenerative circuit, JC^^^ , and then energizing the coil A^ , allowing the input capacitors to charge* Be cause of the time delay action of the "Timing Relay," this condition remains for about twenty seconds during which the integrated voltage across the input capacitors increases, and the contacts of the standard spectrum-line output (which is the strongest line and the first output contact to close) closes, opening the aborting contacts, oc*^ , of the Input capacitors and connecting tl^ output relay contacts of the test spectral linea to an open contact of the "Timing Relay". After tha time delay in /?c^t and with the "Start" button circuit cloaed, tha "Timing Relay" energlees, discharging the input capacitor ^roug^ a fixed resistor A^, and closing the series circuit to the output re3ay contacts and time recorders* ^is condition remains for a short time during which the two teat elemsnt output contacta open. The stand ard-element output contacts open last, ahcrting the input 59 oirouit, closing the degenerative circuit of AV^, and open ing the power source to the output relay contacts. During thla last Interval of time, all recorders should indicate a time output which is some function of the concentration of each element* The timer f cr the "Timing Relay" is thermostatic and Adjustable from f^bout five seconds to one minute. After one analysis, the cycle should not be repeated for at least one minute to aHow the bl-natallic element to cool off and to insure accurate timing* All sequence-control relays are 115 V-AC except the output coil of the standard element* The different condit- ^lona of operation, in order, are denoted as "Cycle" for tha quiescent condition, "Charge" for the condition of input voltage build up and open shorting and degenerative controla, and "Dladmrge" for the tljie«recorder operation* These con ditions are indicated on ^e front panel by green, red, and amber lights respectively, being connected as shewn in the diagram* If the arc source or some other part of the spectro graph fails, the standard-lina-control relay will not close and the sequence-control will Jump from "Charge" to "Cycle" condition immadiately after the time lag of ^^ * This may ba readily observed from the front panel lights, and the oauaa siay then ba found end remedied* Alao, the output re cordera will not accurately indicate any apectrum line of 60 greater intanalty than the standard line, since the standard has a direct effect on the sequence control which provides a completa circuit for the output contacts* The output of the final spectrum-line relays is 115 V-»AC connected to female two-pole sockets mounted on the front panel* Contacts to close the arc circuit during "Charge" are also provided from the sequence control and are c<»mected to a 115V-AC female panel socket to be used. If desired, for automatic arc operation. The actual circuit diagram for the sequence control is ahown in Fig. 19, and an explanation chart with approximate time limits ia shown on page CZ • Four-pole-double-throw ralaya were used to save space and to insure simultaneous action of control for all amplifiers* Vhen other asipliflera, and chasses are constructed, aimilar relays on these chasses may be connected across nc^ , f?^^ , and ocs • 61
iSlJ. JO SAVTlb indlftO Oi- ®- 30MVH3Sia
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nOMlNO? — -dte-didA-dte^V^f^-itlAiivnaNaoia; •i-i M t 0_| P"! U (NaiMO) 0 0 UJ < h-®— «3 I 3-IOAO h < Oh" Ofc." hO .^ -lOHlNO? % ONUUOHS Is—-^^-d^-d^-d Q t^U
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RZL\TTJE TUX OF 3S-JUa;CE COirTROL V/1TH RSLAI CIRCUIT COIDITIOIIS .1':D
THE O.IDER OF RELAI CHANGE
TBE COiroiTION Re^ Re^ Re2 Re3 Res RaiARIiS (SBC.)
(l)NOHHAL COimiTION , I "CrCLE" - aiEEK CLOSED OPEN OPEN CLOSED OPQi 9 \ LIGHT (2)I1IHJT CAPACITOR IS SHORTED (3)ACTIVE DBGEKERATION (U)VISUAL "SET" IS [•(•oej (1) (3) (2) ADJUSTED IF NEEDED (5)THIS CONDITION SHOOID BE UAmTAm- ED AT LEAST 1 ICDT. AFTER THE PRECEED- ING CICI£ '' 1 ' ^' OPEN (l)INHIT CAPACITOR, i k 1 "CHARGE" - RED OFQI CLOSED 0 ?W OPEN t*V LIOTP Ci, CHARGING ^("START" BOTTON (2)ARC CONTROL IKHEWTARHI (2) (1) CONTACTS CLOSE PRESSED ) (3)PASSIVE DEGENER 1 ATION T Y LM. OPEN CLOSED CLOSED OPEN CLOSED (1)INPUT CAPACITOR 1 1 SHORTING COHTACTS (1) (2) OPEH 20 1 (2)PASSIVE DEGENER 1 ATION '' } ' t CLOSED (l)TILIING RECORDERS 11 "DISCHARGE" - ALBSR CLOSED OPEN CLOSED 0. Liorr ^m OPERATE (2)PASSIVE DEGENER (3) (2) c .) ATION 5-»«0 (3)INHJT CAPACITOR SHORTING CONTACTS OPEH '' \ > \ ' T •ICICLE" - GREEN CLOSED OPEN O'FW CUDHKD OPEN (1)NORMAL CONDITION LIGHT FOR SETTING ALL (SEE ABOVE) CONTROLS
Pig. 20. i ^-
THE AMFLXFIiRS 6A Tha circuit diagmn for one of the amplifying sectiona ia ahown in Pig. 2l, page 65, The amplifiers for all spect rum Unas are identical. The diagram also shows connon ter- minala used for all three amplifiers. Socket connections for the input phototubes (socket on the front panel) aw3 for the coupling aockata (for any chassis to ba added) are shown in the upper left corner. The power for the amplifier (Cassia la obtained from a flexible 6-conductor cable connected to the powar*aupply chassis* The input circuit for the amplifier la on the left aide of the diagram* The output of the phototube for test line No* 1 ia connected to tbe *'^arge«dis^arge'' relay contacts, as discussed previotuily. Shunted from these con* tacts, to the anode supply for the phototubes, is a resistor K^ , used to determine the maximum allowable voltage at tha input to tha amplifier* Since the maximum allowable photo tube currant ia 1 ma», the maximum possible input voltage at oharge is thia currant times the reslatance of /C5 and ^ in parallel* Practically, tha current does not approach thia level and 7^5 nay be varied to materially change ths senaitivity for any atage. V^lth a small value of ^5- , the capaoitor Cx will rapidly charge to a constant voltage, de pending upon tha spectral»line intensity during the charge period* For a large value cf l?^ , the capacitor potential will mora nearly be tha time-integral of phototube currant, and aaiy not reaoh the maximum poaalbla voltage. *" r* ^S 6C ^ ji' the "DisciiaroQ" con ition, the capacitor d *-r ch.ir'~9S slowly through 7? • Choice of this reals ta:v;o d3torrlnes tho lor:^;th of timo the output circuit is ci:. ed .ro:-' any given spectral-line intensity. If *]? la very IBV^Q, t'r dinr;.arge will be almost line: r with t v e, an3 the out:-)ut recorderc will indicate time proportionally to line Inti in'ty. -tlio auo.ljoic conaIdorations of the capacitor c'-ar^'e and c, is charge 1*? t
by nodal analysis.
7hei7y £ = <2z)
(23)
' ^iis Is he approximation which considers the capacitor as an Integrating device. More accuratelyi 67 a general solution of
ia
and an exact solution for the charge condition is
-^
ainoa
The exact solution for the discharge condition is
since
When "3 ia small compared v;ith 'r^ ^ 9 the change in capacitor voltage is nearly linear with respect to time. Fig. 22 shows the curves of magnitude-versus-normal- lEed-time for the capacitor charge and discharge. It will be noted that when the discharge time is SITBII compared with the time constant of the circuit (time constant: l/KC), 68 the change of magnitude with respect to time is effectively linear.
1.0
-
0.8 - t^RC UE SCALE)
/-•*/": 0.6
o.U
\ -t/RC
0.2
^ 0 1 2 3 U 5 RELATIVE TBIE ( t/RC ) Fig. 22. Capacitpr Charge-Discharge Curves
All three input capacitors, C, , for these amplifiers were in a common metal case which was used as one terminal. This case was therefore connected to the chassis because of electrical Insulating and mounting considerations. The charging and discharging of the capacitors will follcsr the general analysis with reapect to changes in capacitor volt age. V/hen the capacitors are shorted during the "Cyde" condition, thsir voltage with respect to chaasls la / 75 69 volta, but with reapect to the cathodes of the input ampli fiers, their potential is zero volts* A 1,2-volt bias cell is connected in the grid circuit of 7, , following the input capacitor, to provide a nega tive bias when the capacitor is shorted, and to provide a constant potential at the input for accurate balancing of the circuit by the degenerative-control potentiometer on the front of ths chassis. The resistor ^ , in series with this bias cell is an added precaution to limit any possible grid current* The first amplifier is a 6SJ7 with a trans conductance of 1650 micro-mhos and a plat^reslstance greater than 1 Megohm when operated at rated conditions. The actual trane- conductanca is determined by the screen voltage of the tube, which in turn la controlled by the degenerative amplifier. A diange of screen voltage of 25 volts will change the trans conductance by 600 micronnhos, Kajar considerations in the design of this first stage were a minimum of tube drift, low screen-grid currents, and low control-grid cur rents • The first stage of an amplifier Is usually the only stage herein the tube drift is of much Importance, since drift in following stages is less effective by a factor e- qual to the voltage gain of the first. The minimum drift in most receiving tubes, for example the 6SJ7, occurs when 70 the plate current is between 0.1 aix3 1 ma., and at plate and screen voltages as low as permissible from the stand point of control-grid current,^ In pentodes, the potential distribution between cathode and plate and the distribution of space current between various electrodes are not altered when all electrode voltages are changed in the same pro- portion. Iherefore, It appears advantageous to decrease the tube currant and the plate and screen voltages. It al so la desirable to keep the screen current in this tube at a minimum since appreciable current causes a loss of gain and a decrease in the effect of the degenerative-balance amplifier, whose plate circuit uses a common resistor with the screen cf TJ • Grid ciarrent will have the greatest effect in the discharge of the small bias batteries which are not designed for an appreciable current drain, A very good method of reducing the screen grid and control grid current is to decrease the filament supply voltage about twenty par cent, as was done in this case. An change In heater voltage is equivalsnt to changing the cathode potential with respect to its terminal pin. At low current levels, as previously established to be desirable, tiie grid of a tube with oxide-coated unipotential cathode
%. K. Valley, Jr* and H, Vallman, Vacuum Tube Aaoli- fiars, (1st ed.i New York: McGraw-Hill Book Company, inc.. 15151,2? • 750. Reich, 0£* cit*, p. 160, 71 nuat !)• changed apFroxiaately 0,2 volt for a twenty par cent ohanga of heater voltage above the normal value, to keep tha tube curreiA constant* Ohis equivalent change of grid voltage by heater voltage ohange ia not affected by tha plate voltage at any given current, nor by the current if it ia laas than approximately 1 ma * For higher ourranta, the effacta of heater voltage variation ia greater and mora erratic,^ Oiam of tha more important reaaons for lowerix« tha filament voltage on this tube is that the useful life of a tuba variaa approximBttely inversely as the square of the heater voltage* low voltage inaurea greater dependability and longor Ufa* The plate potential of 7^ ia directly impreaaed on the grid of TJ , which la the trlode power amplifier* Un» dar tha natvml or "Cyde" condition, tbis tiibe ia very near out«»off, and ita plate voltage la high* When the grid ia made a lightly mere poaitlve, tha plate relay, whoae aenai* tivity may ba controlled by ^^ , will cloae at approximata- 3y 5 BA* Tha re31aiy opena when ^e plate currant decreaaea alightXy below tiie anarglElng currant, or about 2 ma* Tha tuba choaen for thia use waa a mediu»4BU twin- trio with an octal base, a 6692 RCA K^ tuba with a trana- conductanoa of 8200 mioro-niioa, a plata resistance of 1900
^Vallay and Wallaan, 0£* cit*, pp* 421*422* 72 ohoa, and an aaplification factor of 20. This ia an in- duatrial tuba deaignad for loi^ life (10,000 bra. ain*), rigid conatruotion, uniformity, and stability, it was re- coxanended by tha manufacturer for DC operation,^ Because of its axeaptional long life and stability characteristlca, the tube did not need to be operated at reduced heater volt age* In order to bias the grid to a atable negative potent ial with z^aapaat to the cathode, the cathode was connected to the plata of an 0B2/VR108 gaaeoua regulator* Another reaiator, J^i^ * was put in parallel with T-^ to provide the sustaining current for T^ * A voltage-dividii^ network and potentiometer was con nected from the plate of T'^to ground* Tha tap on the po- tantiosietar was connected to the grid of a ** magic-eye" tu(ba, a 6B5, and to tha contacta of the "degenerative-control** re lay* Tha magie**eya, or visual "aet" indicator, provided an optical means of adjusting tha amplifier before each sat of anmlyaaa* Wasn all the "eyea" are set with the shadow angle almost eero, all amplifiers will be in Identical electrical balance for tha run* Tha degenerative voltage which is fed to the grid of 7^ is used to make drift between runs negligible, A 6SJ7, choaen to match tha input anqplifiar, utiliaea the earns low-
^Radio Corporation of America, Fhotolithographed taoh- nioal bullatin on Special Red Tubes, (Form SRB-1002) Har- rlaon, N* J*t 1946), pp. 41^^ 75 voltage filamsnt circuit for long life, dependability, and low screen-grid and control-grid currents. The plate reaia tor of TT la the acreen reaiator of Tf so that for effect ive degeneration, the plata current of 7^ nndi be much more than the screen current of "7^ » !Ztoe cathode of the degen erative amplifiers, 7^, and the magie-eye tubes are kept at a poaitlve 75-volt potential supplied by an 0A5AR75 gas eous regulator* The sustaining series resistance for this tuba ia a divider network composed of ^ and ^i^£ ^ * Tha acreen voltage for 7^ ia taksn across ^7, and >fyr 4 ^^^ acrewdriveEr-adJuatabls potentiometers to adjust for toler ance in manufacture of circuit components, so all amplifi ers will operate identically* *Vtm current through ^%s,C is sufficiently large so no interaction between amplifiers ia apparent due to current f3«>m the plate of T^and the I 8Cx*een of 7^ * A separate 75-volt-biaa tube was used in this portion of the circuit to decrease the drain on the power supply 76-volt blaa, luproving its regulation for the more critical potentials cf 7] ^ It waa f ouxid necessary to insert a capacitor Cj a- cross Ts'to prevent relaxation oaclllationa* The center tap of the filamsnt tranaformer waa alao connected at a 75-volt potential to keep the voltage dif ference betwaen cathode and heater of the tubes to less than ninety volta* Fig* 23, (a), (b), and (c) ahow acme photographa of 74 the aequence control and aaqplifiar (Aiaasia* A modified 2000 V, voltmeter haa bean inaerted in a panel and ia plugg^ into the output aooket of the amplifior ehaaaia which haa tha connectiona to thi «»900- end 250-volt auppliea* A yellow ^'aet*' mark haa been painted on the mid- aoale reading ao that tha pointer ia vertical when indicat ing properly* By tdirowing a D,P*D,T, toggle switch, either the 900«» or 250- volt supply may be monitored, or set to the yellow mark which indicataa the proper voltage setting, but ia not oaXibrated to read the exact voltage* 75
(a)- - Front View
(b) Bottom View
(c) Rear View
Fig. 23. Sequence Control and Amplifier
Chassis ^ '• I . '^*«^ * •'
TBE HBCCBDBES 77 The reaordera ueed must operate on 115 V-AC itoich la oontrollad by tha output of the aapHfiar ralaya* Although It would be advantageous to use a calibrated timsr for each element, it waa decided to use aynchronouil clooka with e- lectric brakea as indicators beoause of their versitility* Tha time will than be correlated with Intensity by working grapha determined from known aan^les of material* It ia neoeaaary to study ths nature of coirelatlon be tween material cancentratlon and line intanalty before tha uaafulneaa of tha inatrunent may be known and praetioal work ing gra.2^8 platted* A general diacuaaion is taken from ''Practical Speotro- acopy** At very low concentrations of an elemsnt in a §ma$^lB0 ttka amount of light amitted by that clamant ia direotly proportional to the nuBft>ar of its atoiaa P9p»«iiitt if all other factera are kept ocnatant* Thia linearity provides a very convenient baala foor tiuwatitatlvt amilywia by tht enlaalon apoatrum* * * The apectrograi^io method can be applied quant 1- tatiwlj to tha dattrmination of any elaannt than ean be oetooted qualitatively* As a reault more than 70 aliManttt cdT tha pariodla table are auaoap- tibla to a method that la more rapid than diemioal vat Biathoda and ean ba carriad out on aiueh amallar aajqplca, 10 wg uaually being aufficlent for datar- mination* Tha method ia alao axtraaaly aanaitiva, being effective in some oaaea down to conoentrationa of 1 part in 100 adllion# Spectrographic quantitative analyaia provides fairly unifona praoiaicn at all conoentrationa. Thus, it ia aa easy to meaaure the dlfferanoe bev twaen 0*0010 and O^OOH par cent content of an im purity aa tftiat batwa«3 1*00 and 1*10 per oant* At low aoMantrationa tha praaiaion of tha apaotro- granhio sMthod ia auparior to that of ohaialoal wat BMtnodaj but it baoomaa inferior at concantrationa 78 of about 5 per cent and over. At concentrations be low 5 per cent it is possible to reduce the average deviation among successive determinations on the same sample to less than 2 peir cent,^ The constant-per-cent error, as Indicated in the pro ceeding analyals, suggests that the scale for percentage compoaition be logarithmic on the working curve. Also, since Important time values will be a fraction of that for the standard element, the normallaed time scale should be logarithmic also* Thia log scale will also allow the curve of compoaition veraus time to be nearly linear for small element concentration* A slldlzi^ log scale calibrated in absolute recorded tlms may be used In conjunction with the working curve to allow interpretation of contposition with absolute time. A representative of such a working curve is ahown in Pig. 21*. Thla la not an actual curve but Is us ed to illustrate the type that would be of most value for rapid analyaia* For practical use, t^e scale would probably be expanded for more accuracy, or it could be calibrated directly on the recording mechanism ao that per compoaition may be read directly*
%arrison. Lord, and Loofbourow, 0£. cit., p. 12, 79
Hg. 2h* TS^TID OFMIATIOW MB SXiOOlSTSB XHFROVKIIEIITS
'. ^.- 81 A photograph of the completed electronic portion of the automatic apectrograph is shown in Fie. 2$. 'Simulated conditiona of light input have been tried and all circuits observed have worked quite well. The sequence control, power supply, visual "set" control, output relays, and clock recorders have all been tested. The optical arrange ment for directing the spectral lines to the phototube cathodes have not been completed at the time of this re port. Therefore, no definite results have yet been record ed, Olie developrasnt of the completed instr\ament is in tended to be carried oA from tixoe to time as components be come available, and a larger instrument Is needed. The spectroscope is not represented to be in the final form now, and several factors for securing improved and reliable operation are to be suggested. As an explanation of why theae Improvementa have not been incorporated In the pre- aent Instrumsnt, the fact should be noted that all funds and parts orders for such a research project are allotted for each school year. Hbe advantages of the suggestions have only been determined since thia original unit was con structed* Therefore, the suggestions are primarily to serve as a guide for any future construction. The tube sockets of t;he phototubes have been found to get hotter than would be desirable for low dark current. It la suggested that the voltage dividers for these tubes 82 85 be removed from the bases and that circulating-air blowers or a refrigerant be used, Aa mentioned before, increasing the value of the dividing resistors to 50,000 ohms each would also be desirable from the atandpoint of ahunt cur rent on the 900-volt supply and for subsequent reduction in heat dissipation. The tubes should be protected from ex cessive stray light by light-shields anS from heat by proper atorage. For more dependable operation at a reduced sen sitivity, a small bulb shouM be used to keep the photo tubes "fatigued•" The operation of the 900-volt phototube supply could ba improved by increasing the value of realstanca in series with the NB-51 glow tubes to approximately 300,000 ohms* Thia would limit the current through them to apparaxlmate ly 1 ma* providing longer life and more dependable operation, Thia would alao Increase the current reserve available for the load by 2 ma. If the bias voltage on the input tube, 7^ , of the aa^lifier section were increased, tiie plate resiator could ba Increaaad, and the screen-grid and control-grid currents, if any, would be decreased. This would Improve the ampli fication of the stage, and provide better degenerative-bal ance anplif ioation* Also, by increasing the values of /Xj and 'y at the in put from tha phototubes, the sensitivity will be increased, tha capacitor voltage will be more nearly the tine-integral 84 of tha input currant, and tho discharge time will be longer, allowing more aoourate time measurements by the recordera* Znoraaaing the spring tension on the output relays will allow a alightly higher plate voltage on tha inpOb tuba, 7/ , and will improve the control of the aenaitlvity po- tantioflMitar 4^* The parcentage deviation betwaen tha anargislng and de-energiaing currenta will alao decrease, pro^idi^g a more accin^ate output control* A brilliant light aource, sudi aa a high voltage a park, will allow the phototubea to be operated at reduced sensi tivity and will give longer recorded tlms intervals for more acourata analyaia* This type of source la particularly auited to oontrol by tdie arc-control-relay contacta. Although the accuracy of the apectrograph may not be natarially improved, a timing relay controlled by a aynoh- ronoua clock, inatead of a thermoatatic elemsnt, would allow batter oonqpariaon between successive runs, inqproving tha convenience of analyaia* Finally, although all amplifiers for the different apactral line intenaitiea were daalgned to ba Identical, batter aacuracy would be inherent if the meaaurement a for any given element were always nade by the same phototube- aiqplifi^r-tiaer circuit* BIBLIOORAPW 86 Abate, A* "Basic Theory and Design of x::iectronically Hegu- lated Power Supplies," Proc. I.R.B., Vol. 53, July, 1945, """^ Applied Research laboratories. Spectrographer's Hews Letter, Vol. II, No, 11. San Francisco: April, 1949. Bell, P. H, "Quantitative Spectrographic Estimation of Trace Elements in Biological Ash,"^ Ind. L^nr;. Cham., Anal. M»* Vol, 10, 19S8, "-^ Bareakin, A. B, "Voltage Regulated Power Supplied," iroc, !•£•£•# Vol. 31, February, 1943* Boettner, E. A., and Brewington, 0. P. "The Application of Multiplier Photo-Tubes to ^quantitative Spectrochemi- cal Analysla," Jour* opt. Soc. o£^ Amer., Vol, 54, January, 1944, Bouaquet, A. B* "Improving Regulator Performance," Electron- ica. Vol, 11, July, 1938* Brode, W* R, Chemical Spectroacopy, New York: John ? lley and Pons, 1950* Camrpbell and Ritchie* Photoelectric Cells, London: Sir Isaac Pitaaan and Sons, Ltd, Central Scientific Company. Cenco Gratin^ Spectrograph, Cat, Ho* 87102 and Bulletin 16V. Technlcal Literature, Chicago* Dieke, G, H*, and Croestrtiite, H. 14, "Direct Intensity Mea surement of Spectrum Lines with Photo-MultlpHer Tubes," Join** Opt* Soc, of Amer,, Vol, 35, July, 1945, • (Use of Photo-Multiplier Tubes for Intensity "•"" Seas\iramenta). Jour, Opt. Soc, of Amer., Vol, 36, 1946, p, 192, Eastman, A. V. Fundamentals of Vacuum Tubes. 2d ed. New York: McGraw-Hill Book Company, inc*, 1941* Engstrom, R. W. "Multiplier Fhototube Characteristics! Ap plication to Low Light Levela," Jour. Opt. Soc, of Amer,* Vol, 37, June, 1947* Fo3*sythe, V, E.(ed,). Measm^menta of Radiant gnerp;y* New York: McGraw-Hill ^ok Company~Inc,, 19377 87 ^^^^^^Jtll^ aafl«>8cliwait«ar, E* Foundationa aa3 Method a of
^^•^t^^iJIfl."^ S*I^**^ ^ *^ l>«valoxaBant of Senaitiva Thotatiiiaat* Proo> of 1^*£*, Vol* 29, Auguat^ 1941* -yrAy ^llP Barriaana 0* B» (ad*). Proceedinga of tha Suzmaer Ccmfaran- #fI ] 5g| Sa SpaotroacopyiafT^^ Held at lftt¥ ^ 15 ^fSAuMtta'^Mnnrtuia £rTechnoa»gy> H^ ^forkt t 11^ John tfllay and Sana* ^ ^-HSi'l^***^"?^-!* ^•^ ^'^^ ^» ^•t «^nd Loofbourow* Practical ' W0f' anaa?roaaQnY> 1^ Yorki Pranttoa-Hall, l!no*, 1MB* lllfitHaalart »« Ft* and 0iatartt H* w, "A Diraot Reading In- atruoanl for Spaotrochomieal Analyaia*" Jour* Opt* &HL* SL iSSaL^^ V<>1* **• I3»aairibert 1944* ""^ Hillt W» R^, jrrt "Analyaia of voltage Ragulatdr Operation^" J2S&* fi£l«I^ta Valt 55^ January, 1945* Bughaa and 0uat»riaga« Phatoelaetrio PheiKMaam* New Yorki MoOraw^Hill Boolfeo*^ Xne« ^ Janaat R« B«t and Olavari A« x^^ "Recent Developments in Photo-Tuibaa," Ra Rayiaw, Vol* 6* Ho* 1, July, 1941* Xailmanf Hartnut* "Quantitativa Xaaaurananta with Seintil* lation Countarat" ttiyaical Revlaw* Vol* 75, No* 4« Fatn^uary* 1949* Knowlton« A* S* (ad*) * standard Handbook for glaotrioal Bn- fiijaaai^ ad ad* B8^ Torki fecSraw-MH look Coi^pany; TSSrTiwWIte Xdtkanhouat B* s*, Johttfltent ^« B«t <^ Hartwiok, J* P* "Tha Dat^nnlnation af tha Major Mineral Conatituanta af Grain Aah," Aiasrioan Wine and Liquor Journal* Ootabar* 1940* Maakf J* B*# KoA Martini M* J* Tha Photograidaio Prooaaa * Haw XcDPki licOraw*Hill BooTToiB^MmTil Zno*, WSfli llull«P# R* H«» l3arsMna H« L«^ and DroSf M* E, S»P iMflTfHlff* 1^ ^^ki PraBtiaa.«all» x^fH Piaroa, W* 0*, Torraa, o* R*, and ifaraball* W, w* "Qualita- iJ^*^*t*^**'2«**?=^^« Analjmla in itoa Aro, with Oraphita fi^JSS^** -fifi* auL* am^$ iaa* »•# •©!• 12, HO* ajLa xirai/* PrtaaadiMa, Raporta, and standard Speaifioationa of Com* adttaa S«£ (Spaetragraphic Anilyaia) of tha Ajnariaan Saaiaty far Tasting Materiala* Radio Oorparation af Anariaa* Photolithogyapha 1P22-8-45* igglXaa* f»i>4>-s^^* gao-10-4fl7'^eggl7R^— Tae&nioai iditarawSFa* Bamaon, New jeraeyi KCA Viator Diviaion* Radio corporation af Anariaa* Technical Information on Spaoial Red Tt8>aa* Photolithogranh SRB-1002, Har- rlaoni I^eirjg?iy> RCA Vlctor^filvlalon, IBte* Rajofaaainf J* A** and Sn|dar« R* L* "An ElaetrioallT Foaua* ad Multipiiar Phototube," glaetroniaa* vol* 13, Dacanbar» 1940* Raidhf H* J* Thamr| .fBa!^ AnpHcationa of Electron Tubaa* 2d i^^9 Waw ISiSifficiraw-riiiul feooE company, ine*t 1944*
• .1 ' • RaynoldBf Z* D* "HHU»61 Olow Tubaa*" Tftipu(bliahed Slainar' rapcnrt, Texaa Technological Collage, Spring, 1949* Saundaraon» J* L*t'0a3daaourtt V* J,, and Peteraon* E, w* Jour* Opt* sga* 4»«ir** Vol* 35, 1945, p* 681,
Londont 1953*
Sullivan* H* M* "Quantitative Determlnaticm of Cadmium and Lead in Zlaa 0aing a Orating Spectrograph with a saator Diak*" IpiAm Bnpf, Cham*, Anal* Bd*» vol* 8, 1936, p* $SB« Tormant P. B* Rfdif SfiiBS?}^* New Yorki McGraw-Hill Boak Ooaqpanyt ^i