1432
Anal, Chem. 1981, 53, 1432-1436
(14) Wetsel, G. C., Jr., McDonald, F. A. Appl. Phys. Lett. 1977, 30, 252-254. ( 1 5 ) Cahen. D.:Gartv. W. Anal. Chem. 1979. 51. 1865-1867. i l 6 ) Hodgman, C. 0.:Ed. “Handbook of Chemistry and Physlcs”, 44th ed.; The Chemical Rubber Publishing Co.: Cleveland, OH, 1963. (17) Cahen, D.; Gam, H,; Beckerg R, s, J , my,. Chem. 1980, 64, 3384-3389.
(18) Garty, H.; Caplan, S. R.; Cahen, D.,submitted for publication.
RECEIVED for review October 20, 1980. Accepted April 22, 1981. Partial support from the U.S.-Israel Binational Science Foundation, Jerusalem, Israel, is gratefully acknowledged.
Integrated, Microcomputer-Controlled, Adjustable-Waveform Spark Source for Atomic Emission Spectrometry Steven G. Barnhart, Paul B. Farnsworth, and John P. Walters” Department of Chemistty, University of Wisconsin, Madison, Wisconsin 53706
Electrical schematics, constructlon details, and performance examples for an electronic, adjustable-waveform spark source are given. The source is operable under computer control, and an example of such using a PET microcomputer is presented. The source operates at high voltage, is series switched with a hydrogen thyratron, produces long-duratlon, pulsating unidirectional current waveforms, and does not require hlgh-voltage diodes in parallel with the spark gap or thyratron for current wave shaping. The clrcultry and mechanical construction allowlng thls are illustrated.
Spark excitation continues to be a popular method for rapid spectrochemical analysis of metals and alloys without the necessity of sample dissolution. In practical situations, it is important that the spark source be under automatic control and requires a minimum of attention. In research work, the same requirementi9 are added to the need for fine adjustments in the current waveform, synchronization to other experimental events, and mechanical integration with diverse spectroscopic optics. In methods development work, exploration of stabilized discharges attacking electrodes in computer-controlled spark stands a t variable repetition rates requires all of the above, plus an absolute minimum of radio-frequency interference at the spark stand. In addition, the study of unpredictable combinations of samples, e.g., stainless or resulfurized steels intermixed with nonferrous alloys, mandates the source operate a t high voltage so that precise spark formation can occur to “dirty” electrode surfaces. Lastly, to use time-gated methods of detection (1,2) for enhanced spectroscopic sensitivity, the discharge current should have a pulsating unidirectional. waveform and last as long as possible. In all classes of work the maximum source resistance should be in the analytical spark gap (3) to maximize sample erosion. To the best of our knowledge, a source meeting all of the above requirements has yet to appear, including previous work reported from this laboratory (4-9). We report such a development now and emphasize the following features. The circuit is much simpler than previous sources, with shorter leads and better grounding. This minimizes radio-frequency transients. Precise electronic switching at voltages up to 15 kV is accomplished with a rugged and inexpensive type 5C22 glass-envelope hydrogen thyratron. For a long discharge, high-quality, glass-mica transmitting capacitors are used instead of oil-filled or ceramic devices. These are fixed in value, and the current waveform is adjusted inductively (6). By
proper component arrangement and grounding, and by floating both electrodes in the analytical gap, the diodes in parallel with the thyratron and analytical gap are both eliminated, removing substantial resistance from the discharge portion of the circuit. Leads are shortened by building the stand inside the source proper and building the high-voltage ac supply external to it. Lastly, the source diodes, capacitor base-plates, and thyratron cathode are all firmly fixed to a massive, grounded, heat-sink plate, made light by using magnesium and machined with a vee-and-flat on its base for optical alignment on our rail and experiment bed system (10, 11). We have several types of computers in laboratory use for spark-source control. However, it i s clear from economics alone that the new &bit microcomputers are best for controlling analytical instrumentation. To capitalize on this remarkable cost situation, and to meld this source into our research direct-reading spectrometers, we integrated a Commodore PET 6502-based microcomputer into its electronics for firing control (12, 13). The net effect of all of these developments has been to produce a source that is simpler, easier to build, more reliable, and more efficient than any that we have worked with previously, a t about two-thirds the cost of our earlier units. It is analytically significant on these counts alone and is now being tested for practical analysis of steel alloys with positionally stable discharges and time-gated photoelectric detection.
CONSTRUCTION The source is separated in two physical components: a charging module and a discharge module. The charging module provides the ac high-voltage power, power controls, and safety interlock for the source. The discharge module contains the high-voltage capacitors, diodes, thyratron, inductors, and spark stand. Separation of the charging and discharge functions in this manner permits a unitized design that allows all the discharge components to be grouped together on a single experiment-bed rider (11). In addition to generating less radio frequency interference (RFI), the unitized nature of the source allows it to be completely enclosed thereby further reducing radiated RFI from the spark stand and providing ready portability. The enclosure helps minimize source breakdown from dirty or moist lab air, and the large heat sink provided by the magnesium rider base eliminates the need for circulating outside cooling air to the source. A small vent easily exhausts hot air from the thyratron. The source thus can be used in dirty environments.
00Q3-2700/81/0353-1432~0~.25/0 0 1981 American Chemical Saciety
ANALYTICAL CHEMISTRY, VOL. 53. NO. 9. AUGUST 1981
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TRANSFORMERS
HIGH
VOLTAGE TRANSFORMER
THVRITRON
TRANSFORMER
Flgure 1. Electrical schematic of
Ihe
adjustablewaveformspark source. Components are listed in Table I. Table 1. Components for Figure 1-Adjustable-Waveform Spark Source component description diodes
L1 L2 L3 HV capacitor
Flour8 2. AdiUSIaDewaVelorm dscnarge modue: (11 spark gap wiIh flow .et-tJnQslen ancae ana sample pn. (21 mddctor L2. 13) ndUCtOr L3. (41 5C22 nydroqen myralron. ( 5 ) power mpul jacks from cnarging module. (61 rect ly ng br dqe diodes. (71 energy slaage capacnor. (8) magnesium heal s n* component rider. (9)lnyralron grid connection. Charging .Module. The charging module of the source contains the circuitry necessary to provide the discharge module with alternating current, high-voltage power. Primary power to the module is provlded from a 110-V isolated power source. After being fused and switched, this power is fed to a variable autotransformer, the secondary of which feeds the primary windings of a high-voltage transformer, The highvoltage transformer has a turns ratio of approximately 134 giving an upper limit of 18 kV RMS with the 140-V variable autotransfurmer used. This 18 kV RMS limit cannot be realized at present due to the limitations of the glass-envelope 5C22 hydrogen thyratron used as the switching element in the discharge module, although steady operation at 14 kV is practical. I t prohably is best to limit the charging module voltage to less than 20 kV to keep long-distance transmission problems simple. For full computer control of the source, the ac mains voltage feeding the high-voltage transformer should be switched on with solid-state devices such as a zero-crossing TRIAC. We are not reporting that here because current work is focused on feedhack control over both the line voltage and frequency.
T1, T2 100 k R resistor 5C22 thyratron 50
kR resistor
H V transformer
4 FMC Catalog No. ER*H16,16 k V
max. PRV air core inductor, residual to approximately 85 p H , selected taps are available air core inductor, residual to approximately 87 MH,selected taps are available air core inductor, residual to approximately 137 p H , selected air taps are available 2 parallel-connected 0.007 pF glassmica transmitting capacitors, type 1980-300, peak working voltage: 20 kV, Aerovox Inc. current transformer, Model 110, 0.100 V/A, Pearson Electronics Inc. Ohmite No. 0422, 100 kS2, 50 W I'M 5C22 hydrogen thyratron or equivalent 4 Darallel-connected 200 K. 200-W reststors. Ohmite, Inc unmarkrd surplus component
To characterize its independent performance, we chose to switch the ac primary with a three-pole relay. Both the hot and neutral lines are switched, while the third leg provides a holding loop to the relay coil. Power and safety switches control the actuation of this relay Also contained within the charging module are four, 200.k0, 200-W wire-wound resistors that limit the charging current and isolate the charging module from the discharge circuitry during firing periods. In addition, the module contains a fdament transformer for the hydrogen thyratron, plus two fans for internal cooling, and a volt and ammeter. Discharge Module. The discharge module contains the necessary circuitry to accept the alternating-current highvoltage power from the charging module and produce an adjustablewaveform high-voltage spark. A schematic diagram of the discharge circuit is given in Figure 1with the components listed in Table I. All the components of this module are mounted directly on the large magnesium rider as part
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ANALYTICAL CHEMISTRY. VOL. 53. NO. 9. AUGUST 1981
1 Figure 3. Adjustablewavefamdischarge module component placement: (1) 100-k0 spark gap resistor, (2) rectifying bridge diodes, (3) power input jacks from charging module. (4) magnesium heat sink component rider. (5) power lead to inductor L3, (6) 5C22 hydrogen thyraton. (7) inductor L3. (8) Mucta L2. (9) g!ass-micaenergy stcfage capacitors. (IO) Inductor L1. (11) spark stand.
of the optical experimental bed (11). The physical placement of the various components is shown in Figures 2 and 3. This source has one side of the energy-storage capacitor grounded, as well as the thyratron cathode. However, there is neither a diode in parallel with the thyratron nor a diode in parallel with the analytical gap. Instead, the rectifying bridge diodes serve a multiple purpose. They rectify the ac power from the charging module, in addition to acting as a single shunt diode (Figure 4) in parallel with both the analytical gap and the thyratron switching element. The diode shunting action for current waveform generation has been described previously (4.6) and will not be treated here. It is instructive, however, to examine the current paths in this source design during the charging and discharging portions of a line half-cycle. Waveform Generation. During the charging cycle, the 5C22 hydrogen thyratron is not in conduction and effectively produces an open circuit in the loop containing the spark gap (refer to Figure 4). With the thyratron turned off, the 0.014 pF high-voltage capacitor charges from the high-voltage transformer through the 50-kR charging resistor and the full-wave-rectifying bridge. The capacitor continues to charge until its voltage equals the instantaneousrectfied voltage from the high-voltage transformer. This voltage limit can be adjusted with the variable autotransformer in the charging module. Application of a positive trigger pulse to the thyratron grid fires the tube and begins the discharge cycle as an electrical “short” in the circuit loop containing the spark gap. This directly connects the spark gap with the high voltage on the storage capacitor. If the proper conditions exist (primarily electrode geometry, capacitor voltage, and support gas) a spark discharge will form between the electrodes. Once the thyratron has been fired and current conduction has begun, there are two circuit loops by which the energystorage capacitor can discharge. One path is through the analytical spark gap, and the second is through the rectifying bridge. The relative impedance of these two paths determines the magnitude of the current in each. Initially, the oscillatory discharge voltage is positive, placing the diodes of the bridge
BRIDGE DIODE
$.
ilr .....
THIRATRON
FlLIYENT TRANSFORMER
4
Flgun 4. schematic 4aWm Mustrathylhspark gap and dode pafor capacncf discharge. Inset shows a schematic representation of the diode bridge acting as a single shunt diode.
in a reverse bias condition. This creates a high impedance path to ground in the diode loop, and the path through the spark gap is favored on this first half-cycle of the parent oscillatory discharge. On the second half-cycle of the discharge, the polarity changes and the rediying diodes become forward biased, creating a low impedance path to ground to pass a larger fraction of the discharge current. The 50-kQ resistor in the charging module isolates the high-voltage transformer from this shunting action of the bridge. The various inductors in the discharge path can be adjusted to give full unidirectional current wave shaping. Inductor L2 can be balanced against L1 and L3 to make the discharge current continuouslyadjustable from a pulsating to a smoothly decaying unidirectional waveform. In these cases, the thyratron remains in conduction and does not require reignition during the oscillatory,capacitor-dischargecycle. If L2 is made too small, the spark current will attempt to oscillate and be series-clipped hy the thyratron. We understand this to shorten the tube lifetime (6)and avoid this mode. ELECTRICAL PERFORMANCE Due to the combination of low-loss glass-mica capacitors, fewer circuit components, and unitized design, the pulsating-unidirectionalcurrent waveform can he sustained for 200 ps a t 13 kV with the gap shorted and for 165 ps with it open. This is the proper type of waveform damping for analytical work. The long duration with an open gap is presently being exploited for time-gated direct-reading spectrometry and will he discussed in a future article. Microcomputer Control. Because the spark is a pulsed, short duration discharge, many of the measurements of its properties must be made stroboscopically. This dictates that a research spark source must be capable of producing trains of stable, identical sparks. This requires in turn that the
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
r-l
PET MICROCOMPUTER
1435
60 Hz AC
Video
User Prompts
I
Monitor
BASIC Program
6502 IMPU
.
Crossing Detector
Decimal D t l o y o
Keyboard
1
I
I
Machine Language Subroutine
Interrupt Handler
Binary Delays
Binary Delays
6522 Interrupt Requests
i
Figure 5. Schematic representation of the PET microcomputer operation for spark source control.
ignition of the discharge be synchronized with the ac charging circuit, so that the breakdown of the spark gap will always occur at a predictable set,of voltages. In a separate paper (14) we describe how this is accomplished by a laboratory minicomputer interfaced to a zero-crossingdetector and a digital delay generator. For this spark source the functions of the minicomputer and the delay generator are combined in the form of a Commodore PET microcomputer. While this substitution introduces some minor limitations in discharge timing, which will be described later, it provides satisfactory performance in most situations at a price that is less than one-tenth that of the minicomputer system. The cost of the microcomputer is low enough that it becomes practical to dedicate the computer to control of the spark and integrate it into the spark source. The operation of the PET computer is outlined schematically in Figure 5. A simple 1/0 routine written in the machine’s resident BASIC prompts the user to enter the pertinent burn parameters. These include the number of breaks per half cycle of the ac charging voltage, the delay of each discharge from either the zero-crossing reference or the previous discharge, and the duration of the burn. The burn duration, in the form of the number of half cycles in the burn, and the delay times are converted by BASIC into two-byte binary numbers and are stored iin a table in memory for later access by a machine language subroutine and an interrupt handler. Once the discharge parameters have been specified, control is passed to the machine language subroutine, which sets up the interrupt logic for tlhe VIA chip and also monitors the half-cycle count as the 8ource is fired. The P E T logic board includes one 6522 VIA that is only partially used by the computer. Complete descriptions of this chip are available in ref 15 and 16. (Both ref 15 and 16 contain errors. The bit assignments for the auxiliary control register are incorrect in ref 15 arid correct in ref 16, whereas the bit assignments for the peripheral control register are incorrect in ref 16, but correct in ref 15.) This application uses the following chip features: (1)a latched 16-bit counter (timer 1)that is decremented by the 1-MHz computer clock, (2) an edge sensitive input, CAI, that can generate an interrupt on
either the leading or trailing edge of an input signal, (3) one pin of the eight-bit A port, and (4) the four interrupt and control registers. CA1 is configured to generate an interrupt on the leading edge of the zero-crossing signal. The service routine for this interrupt loads the first delay time into time 1,starts it, and decrements the half-cycle count. Timer 1is operated in the one shot mode, which means that upon timing out it will generate an interrupt and then remain inactive until loaded with another delay. The interrupt from the timer is serviced by a routine that fires the spark, loads the next delay time, and restarts the timer. This process is repeated until all of the stored delay times have been loaded. The machine then waits for the next zero crossing pulse. All hardware interrupts for the PET are vectored through a section of code that saves the 6502 registers on the stack and then performs an indirect jump to a location stored at hexadecimal locations $90 and $91 ($219 and $21A in the 2001 series machines). Normally this address is that of a keyboard and clock routine. This routine is activated every sixtieth of a second by an interrupt generated on the PET logic board. In order to use the interrupts for accurate timing, it is necessary to make two changes. First, the 60-Hz PET interrupt is disabled. This interrupt is not synchronous with the line voltage and could thus cause unpredictable interference with the spark timing. The interrupt is disabled by storing 0 in hex location $E813. Because disabling this interrupt also disables the keyboard, it must be done after all keyboard 1/0 functions are completed. Second, the interrupt handler for timer 1 and CA1 is stored in an open section of memory (we used the machine’s second cassette buffer), and the vector at locations $90 and $91 is changed to point to it. Such an interrupt handler must include statements pulling the accumulator and the index registers off the stack. Once the burn is complete, normal machine operation is restored by storing the original PET vector ($E62E) in locations $90 and $91 and storing $3D in location $E813. Several machine language instructions are executed between the occurrence of an interrupt and the loading of a delay time. This makes it necessary to subtract a constant value from the delays as they are entered and dictates the mimimum value
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ANALYTICAL CHEMISTRY, VOL. 53. NO. 9. AUGUST 1981
EARTH GROVND
PET GROUND
Flgure E.
Electrical schematic of the
PET
isolation circuil
Flgure 7. Examples of source operation at 1. 2, 3. and 4 breaks per half-cycle. The top traces represent the capacilor vohage with an ordinate of 5 kVldNiswn and an abscissa of 2 msldivisbn. The bottom traces illustrate #?e curen1 waveform in me spark gap with an ordinate of 50 Aldivision and an abscissa of 20 psldivision. of a delay time. The minimum values are 140 ps for the first delay and 90 ps for subsequent delays. Once corrected, the delays are accurate to *3 ps for the first delay and 1 ps for subsequent delays. The jitter in the first delay time of f 3 ps results from the fact that the zero crossing interrupt can occur a t any time during the execution of an instruction and is not serviced until the instruction is completed. Later delays are jitter free with respect to the fimt. A l l factors considered, such noncumulative jitter is acceptable for most practical source uses. In those research applications where more exact synchronization of other parts of the experiment with source firing is needed, a minicomputer and external programmable timer are used. Because this microcomputer employs CMOS integrated circuitry, it is highly sensitive to radio-frequency interference. For stable operation of the computer and spark source an optical isolator was placed between port A of the PETS 6522 and the thyratron driver (EG and G type TM-27, Boston, MA). A schematic of this isolation circuit is given in Figure 6. This circuit completely isolates the PET signal from the thyratron driver, as well as from the power supplies and grounds. All circuit components on the PET side of the MCT-2 optical isolator are powered hy a power supply derived from the PET power supply. Those components on the source side of the circuit are powered by a separate power supply originating from the isolated power used to power the thyratron driver and spark source. Model CD 4050 CMOS buffers are used to couple between CMOS and TTL circuit components, and a 74132 Schmitt trigger is used to sharpen the trigger signal and to help eliminate noise pulses. The com-
plementary transistor pair totem pole driven are used to boost the respective signals. Example Waveforms. Some representative waveforms which illustrate the operation of the source under control by the PET are given in Figure 7. Waveforms for one, two, three, and four breaks per half-cycle are illustrated. All traces were taken by using an argon flow-jet with a tungsten pin as the anode and an iron pin as the cathode. The top trace in each case represents the voltage a t the capacitor using a 2 ms per division time base. The bottom trace shows the current through the spark gap using a 20 ps/division time base. Note in the multiple break cases that the break times have been set such that the source is fired at the same capacitor voltage each time. This is further illustrated by the current waveforms, where every discharge waveform is superimposed upon the previous one. A wide variety of break patterns can he achieved by software manipulation of the number of breaks per half-cycle and the time at which each occurs (e&, ref 13). ACRNOWLEDGMENT The assistance of Russ Riley and Ken Spielman in fabrication of portions of the hardware is appreciated. Discussions with David Coleman, Alex Scheeline, John Bernier, Bob Watters, and John Norris during the design phases of this work were valuable and are acknowledged. LITERATURE CITED (1) Waners, J. P.: Malmstadt H. V. Anal. Chem. 1965. 37, 1484. (2) Waners, J. P. Anal. Chem. 1968, 40, 1672. (3) Famswwth, P. 8.: Waters. J. P. Specrrochim. Acta. Part B 1980, 358. 315. (4) Waters. J. P. Anel. Chem. 1966, 40, 1672. (5) Waters. J. P. Anal. Chem. 1969, 41. 1990. (6) Coleman. D. M.; Waners. J. P. Speckchim. Acta. PartB1976. 318. 547 (7) Rentner. J. R.; Uchlda. T.; Waters. J. P. Spectrochh. Acta. Part 8 1977 . ~328~125 ~ , (8) Coleman. D. M.; Waiters. J. P.: Waners. R. L.. Jr. Spectrochlm. Acta. Part B 1977. 328. 287. (9) Araki. T.; Waners, J. P. Appl. Specrmsc. 1979. 33. 463. (10) Waters. J. P. In "Contempwary Topics in Analytical and Clinical ChemirW; Hercules. D. M.. Hiellie. G. M.. Snyder, L. R.. Evenson. M. A,. Ed$.: Plenum Press: 1978; Vol. 3. pp 91-151. (11) Coleman. D. M.: Waners. J. P. Specrrochim. Acfa. Part B 1978. 338, 127. (12) Waners. J. P.; Coleman. D. M. US. Patent 4055783. Nov 18. 1977. (13) Scheeline. A,; Coleman. D. M.: Waners. J. P. Appl. Specnosc. 1978.
"-. ,,c- .". ?I
(14) Ekimoff. D.; Matbws. S. E.; Waners. J. P.. in prepsrallon. (15) Osborne. Adam; Kane. Jerry "An lnkoducllon 10 MicrocompU1ws"; Adam OSbMne: Berkeley. CA. 1928: Vol. 2. (16) Zaks. Rodney. '"6502Applications B o d " , Sybex. 1979.
REcElvn, for review January 26,1981. Accepted May 12,1981. Portions of this work were funded by the National Science Foundation under Grant CHE-79-15195 and the Graduate School of the University of Wisconsin.
