An Inexpensive Current-Voltage Booster for Electrochemical Instrumentation W. S . Woodward, T. H. Ridgway, and C. N. Reilley Department of Chemistry, University of North Carolina, Chapel Hill, N . C . 27514
THE AVAILABILITY of inexpensive, solid-state operational amplifiers has allowed individual chemists to rapidly build inexpensive electrochemical instrumentation to suit particular requirements as long as the output voltage and current demands fell within the usual bounds of the amplifiers ( & l o volts @ 1 5 to 20 mA) available. Although some specialpurpose amplifiers are commercially available which provide up to *IO0 volts @ 10 mA or 1 1 0 volts @ 100 mA, they are relatively expensive and don’t possess the capabilities rerequired for short-time transient work (high compliance, high current, and rapid slewing) or for bulk electrolysis (high compliance and high current). It should be noted that some commercially available potentiostats possess the desired properties (for example, the P.A.R. Model 173), but the costs are often prohibitively high for many research groups ( I ) , particularly when a number of parallel units are desired, The device (Figure 1) described herein can generally be constructed for a parts cost of approximately $30.00, is self-powered from line voltage, has a 1100-mA capability and a compliance of i- 100 volts with a voltage gain of $10. The stable slew rate depends somewhat upon construction technique but generally is between 10 and 100 voltslmicrosecond. The prototype of this device has been in successful operation for over two years, primarily in electrogeneration studies, and three additional units are now in operation. Construction details are left to the individual, but experience has shown that barrier strip or “Vector board” methods work well for the power supply, while “Vector board“ is preferable for the construction of the booster itself because tight spatial grouping is required for high frequency performance. Since several high-power components are required, heat sinking is mandatory. To date, this has been satisfactorily achieved by bolting the power transistors (MJE 340) to the mounting chassis with the hardware supplied and using a silicon-base stopcock grease to improve thermal conduction while maintaining electrical isolation. The mounting chassis may either be a “Bud” mini-box or an aluminum rack mounting panel. In order to permit a 200-V peak-to-peak output swing using readily available 300-VCEO power transistors and still leave an adequate safety margin, regulation of the booster power supply is necessary. This high-quality power supply also permits good circuit stability and noise performance without undue complication of the booster circuit. For these reasons, the five-power supply transistors are incorporated in a dual-output shunt regulated scheme. Shunt regulation was chosen for its inherent load-limiting and shortcircuit immunity and because it permits both supply voltages to be developed from the same transformer/rectifier assembly. The Triad N-68X step-down transformer (230-V primary, 115-V secondary) is operated somewhat unconventionally in that the 115-V line voltage is applied to the secondary and the output taken from the 230-V primary winding. Other transformers may also be suitable for this application, the sole requirement being 230-V output at above 300 mA; the main virtue of the N-68X is its price. (1) N. Weinberg, Interface, Vol. IX, No. 2, 1972.
POWER SUPPLY DI
BOOSTER lnputp
+I30
> R4
R33
- 100>
1
AI53 1
1 1
Figure 1. Booster and power supply schematic
Resistors in Ohms Semiconductors Capacitors R1 500 @ 25 W D1 1N4004 C1 100pF @ 350V aluminum R2 400 @ 2 O W D2 1N816 C 2 lOfiF@ 150V aluminum R3 18K @ 1 W D3 IN914 C, 10-100pFceramic disk 1000 V R4 27K @ l W Z1 1N715 R5 10K @ 1OW Q1 MJE340 R6 330 @ 5 W Q2 2N5172 R7 120K @ ‘/i W Q3 2N5400 R8 8 2K @ ‘12 W 4 4 2N3702 R9 39K @ ‘ 1 2 W R10 47K @ ‘ 1 2 W R11 27K @ ‘/zW R12 12K @ ’/: W R13 2 2K @ ‘ / 2 W R14 22K @ ‘/i W R15 1K @ ‘/2W T1 2 5K 1 turn T2 25K 1 turn The booster itself consists of a differential input/output driver circuit followed by the “push-pull” output pair. This particular output circuit, common in digital circuitry, permits active “pull-up” of the output signal without requiring “complementary” output transistors. This is advantageous because of the general inavailability of inexpensive 300-V PNP power transistors. The capacitors labeled Ct in the
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY ‘1973
435
schematic are stabilization trimmers and are nominally 20 pf. Exact values are dependent upon the details of wiring practice used in circuit construction and should be experimentally chosen for optimum high-frequency stability. Some operational precautions are required when using a device with these energy levels. Although the output is short-circuit-proof with respect to ground, a short circuit of the output to the other power supply terminals could cause destruction of the output stage. The input stage of the booster exhibits quite high input impedance (approximately 100 Kohms) and will tolerate input voltages of +15 volts; inputs in excess of this range can destroy the input stage of the booster. The booster itself will normally be operated by an operational amplifier (i.e., an inexpensive 741 type for synthetic work) in a DeFord ( 2 , 3 potentiostat configuration; ~~
(2) D. E. Smith, CRC Crit. Rev. A n d . Chem., 2,247 (1971),
under these conditions, it is conceivable that touching reference and auxiliary electrode leads when the potentiostat is at high voltage may damage the follower. Personal safety precautions should, of course, be taken as the circuit can pose a shock hazard (130 V). Our standard operating procedure includes turning offthe booster power supply whenever sample or leads are changed; thus far, no difficultieshave arisen. RECEIVED for review July 3, 1972. Accepted October 6, 1972. Part of this work was conducted under the auspices of the Air Force Office of Scientific Research, USAF(ASFC), AF-AFOSR-69-1625, and by the U.N.C. Materials Research Center under Contract DAHC-15-67-C0223 with the Advanced Research Projects Agency.
-
(3) A. A. Pilla, in “Applications of Computers to Analytical Chemistry,” H. Mark, Jr., J. S. Mattson, and H. C. MacDonald, Ed., Vol. 2, Dekker, New York
Filter Unit for Ion Exchange Resin-Loaded Papers K. A. H. Hooton and M. L. Parsons Department of Chemistry, Arizona State University, Tempe, Ariz. 85281
Tm USE OF ION EXCHANGE resin-loaded filter papers has become established as a concentration technique employed in X-ray fluorescence and neutron activation analysis ( I , 2). Filtration of the sample through the resin-loaded paper disks has to be repeated a number of times before all the cations or anions have been exchanged because of the thinness of the paper ( ~ 0 . mm) 3 and consequent short length of theimpregnated ion exchange “column.” Various types of filter units have been used by workers in the field. Campbell and coworkers (3, 4 ) described two versions of their filtration apparatus for a 35-mm diameter disk where a polyethylene cap with a 30-mm diameter hole was sealed into a filter funnel, the disk held in the cap by a vial with the bottom cut off. Hayden (5) used a disposable unit in which a 25.4-mm resin-loaded disk was placed in the cap (with 22.2mm hole) of a polyethylene bottle cut off at the shoulder. The top section of the bottle, which acted as a funnel, screwed into the cap containing the disk and the bottom section of the bottle was used to collect the filtrate. Bergmann and coworkers (6) mounted a 14-mm resinloaded disk in a Teflon (Du Pont) holder clamped in an aluminum-glass pipe coupler. The inlet side of the coupler was sealed to a 25-ml capacity funnel and the outlet side to a Teflon stopcock. Hakkila et al. (7)used a 23.8-mm disk in a filter unit consisting of a 125-ml Erlenmeyer flask with a glass (1) W. J. Campbell, E. F. Spano, and T. E. ween,
ANAL. LHEM..
38,987(1966). (2) D. E. Becknell, R. H. Marsh, and W. Allie, Jr., ibid,,43, 1230 ,1(1.11\ \L,,L).
(3) E. F. Spano, T. E. Green, and W. J. Campbell, US.Bur. Mines Rept. h e s t , 6308(1963). (4) E. F. Spano, T. E. Green, and W. 3. Campbell, ibid., 6565 (1964). ( 5 ) J. A. Hayden, Talonra,14,721 (1967).
(6) J. G. Bergmann, C. H. Ehrhardt, L. Granatelli, and J. L. Janik, ANAL.C H E M . ,1258 ~ ~ ,(1967). (7) E. A. Hakkila, R. G. Hurley, iind G. R. Waterbury, ibid., 41, 665(1969).
._- ......_.^..
450
e
^_.RY,
HlYHLY I IGHL t i H t M I > I
Figure 1. Lexan disk holder chimney clamped over a modified Hirsch funnel. Tackett (8) seated a 32-mm resin-loaded disk in a machined Plexiglas (Norton Co.) block and held the disk in place with a Plexiglas tube which fitted snugly into the block. Malissa and Marr (9) clamped 5-mm disks between two rubber washers and two glass plates, one plate fused to a funnel and the other plate fused to an outlet tube. AU of the filter units described above passed the sample unidirectionally through the disk a number of times until cation or anion exchange was complete. However, due to the nature of the ion exchange resin-loaded disk, filtration can be performed through the disk in one direction and back through in the reverse direction, the cycle being repeated an adequate number of times until all the cations or anions have been exchanged. Using this principle, we have constructed a number of the following filter units for 50-mm resin-loaded disks giving an active filtration area of 29-mm diameter for use in a Philips PW1410 vacuum X-ray spectrometer. (8) S. L.Tackett,ibid.,43,972(1971). (9) H. Malissa and I. L. Marr, Mikrorhim. Acta, 1971,241.
VOL. 45, NO. 2, FEBRUARY 1973
