GALEN W. EWlNG Seton Hall University SOU% a a n g e . New Jerrey 07079
XCIII. A Low-Cost Temperature Programmer for Gas Chromatography 1.N. Gallaher, R. C. Atklns, and F. A. Palocsay' Madison College Harrisonburg, Virginia 2280 1
The ability t o carry out programmed temperature changes is a desirable feature in gas chromatography. Unfortunately, temperature programming is available only in the more expensive chromatographs or as an add-on feature a t a cost of over $600 ( 1 ) . Instruments with temperature programming capabilities are often outside the financial reach of small undergraduate chemistry departments or may be too delicate for general student use. The described programmer can be built for a cost of less than $100. In fact, judicious shopping for parts could bring the cost to well below this figure. This programmer may be used with any gas chromatograph that uses a variable resistor to control the oven temperature. Best results are obtained if the chromatograph has proportional oven heater circuitry (e.g., Gow-Mac model 550 or Varian model 920). In its simplest form temperature programming may be carried out by manually changing the oven temperature control, but this is tedious, has very poor reproducibility, and is of limited usefulness. A better solution is t o vary the oven temperature control resistor mechanically, a t a fixed rate. Early electromechanical programmers used a constant speed motor and variable gears to achieve different heating rates (2). Electronically varying the speed of a motor coupled to the shaft of the variable resistor eliminates complex and expensive gear trains. Modern integrated circuit technology has greatly simplified the design and reduced the cast of reliable, low power speed controllers. Electronic control was therefore chosen for the temperature programmer described. Figure 1 is a block diagram of the programmer. The oven control resistor in the chromatograph is replaced by variable resistor R-10, selected by switch 2. This resistor is mechanically connected t o a dc stepper motor (S.M.), whosespeed may bevaried by 'Author t o whom all correspondence should be sent.
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Figure 1 .
Block diagram.
increasing or decreasing the frequency of the energizing dc pulses from the control circuit. The rate of temperature change is determined by the speed a t which the motor turns the variable resistor R-10. The rate is selected by another variable resistor in the timer circuit. This circuit provides a chain of pulses to the J-K flip-flop, IC-2. The J-K flip-flop generates a pair of complementary square waves and divides the frequency of the timer pulses by two. These aqunre waves are ampldled t o d r ~ the e 12.V d r stepper mbtor. Ftaure 2 1s a srhemat~cdiaaram of two posske power supplies either afwhieh could be used for the programmer (3).The first of these supplies uses a 5.5-V, 400-mW, zener diode. The other uses an LM309 voltage regulator to supply the -5 V needed to power the timer and J-K flip-flop. I t should be noted that -5 V is supplied to the ground pin connections of IC-1 and IC-2 and ground is connected to the V, pin connections (see Fig. 4). This arrangement is necessary because the driving transistors switch negative 12 volts to drive the stepper motor. DC stepper motors (Fig. 3) have a permanent magnet rotor of either 15 or 30 poles and a t least two electromagnetic stators. As the (Continued on page A260)
F r a n k A. Palocsay (at the left in the photograph) has been a t Madison College since he received his doctorate a t the University of Arizona in 1968. He is spending the spring semester of 1977 on an educational leave at the University of North Carolina (Chapel Hill), working with Professor T. J. Meyer. Thomas N. Gallaher (center) is Laboratory Supervisor a t Madison College, where he received a B.S. in Chemistry in 1972. Robert C. Atkins (right) received his PhD in organic chemistry a t the University of Wisconsin in 1970. He joined the faculty a t Madison College following a year's p t d o c t o r a l research a t Columbia University. Volume 54, Number 5, May 1977 / A259
Chemical Instrumentation
Figure 2. Two power supply circuits designed to give -5 V regulated and -12 V unregdated. Regulation is supplied (a) by a Zener diode. (b) by an LM309 three-terminal regulator.
Figure 3. Basic stepper motor schematic. The mechanical switch is replaced by the mntrol circuit in the temperature programmer.
Figure 4. Schematic diagram of the temperature programmer.
stators are alternately energized the rotor moves in precise steps of 15 or 7.5". By switching the stators electronically with a variable frequency oscillator, the rate of rotation may be varied over a wide range. Figure 4 is a schematic diagram of the programmer. IC-1 (an NE-555timer), generates 5-V dc pulses from a steady dc power supply. The frequency of the generated pulses can be increased or decreased by (Continued on page A262) A260 / Journal of Chemical Education
Chemical lnstrumentatian varying resistor R2, allowing the operator t o select the rate of temperature change of the programmer (4). When these pulses are received by the "clock" input of IC-2 (an SN7476, J-K flip-flop), i t generates a pair of complementary square waves a t the outputs (pins 10 & 11). When the voltage a t pin 10 is zero, the voltage a t pin 11is -5; when pin 10 is a t -5 V, pin 11 is a t 0 V. These complementary square waves are amplified by transistors Q-l and Q-2 whieh alternately energize the fields of the stepper motor. Figure 5 is a mechanical diagram which may he visualized as a top view of the mechanical assembly of the temperature programmer. The stepper motor is connected through a pair of 5:l reduction gears (A and RI to the shaft of the variable resistor (R10) which &ace% the oven temperature control. Spring clutch ( 8 ) alhms manual adjuqtmcnt
operation. The value of R2 has been chosen t o provide a range of heating rates from less than 1°C/min t o lS°C/min. The upper limit a t whieh linearity can he achieved will, in part, be determined by the design of the rhn~matographh e w used As noted m Ftgure 6, lmearity and reprod u r ~ b l l mof the Droarammer are excellent. overshoot a t the dndif the programmed run is absent, and lag a t the start is minimal. In conclusion, a temperature programmer has been designed which allows programmed operation of a variety of chromatographs far a minimal investment in time and money.
Figure 6. Oraph of temperature versus time. Results obtained ona Varian Model 920 Gas Chroma. tograph.
FROM CONTROL
TO CONTROL
T4N'T T-F'N Figure 5. Mechanical diagram, top view.
of the variable resistor so that initial temmrature mav be selected. This clutch consists uf a spring compressed againat gear LI held in podition by stop-, ( h and C, clsmped to the res~rtorshaft. 'l'here is no slippage of gear R on the shaft ofrhe var~ahleresatm when gear A drives gear ti (i.e.,gearR rotates theshaft of the variable resistor!. Gear C is free tu rotate around the resistor shaft. GearD meshes with gear C and holds the position of gear C steady as the shaft of the resistor turns. This is accomplished by slip clutch d (a spring compressed against the hack of the u-shaped steel chassis by stop e ) . Gear D permits final temperature selection by moving the magnetic reed switch mounted securely on gear C. A bar magnet is fixed to stop b on the variable resistor shaft. As the motor drives the resistor it also moves the magnet until the magnet passes under the reed switch, stopping the stepper motor and holding the temperature a t the selected upper limit. When the programmer is in operation, the oven control resistor instillled in the chromatograph is bypassed to R10with a DPDT switch allowing a choice of either isothermal or programmed operation. Oven temperature controls may be wired in several different ways. The Varian 920 control is as shown in Figure 1; however, the Gow-Mac 550 uses a three wire configuration. The control in the programmer should have the same resistance, oower rating" and be wired eaactlv as the built-in resi,tor it replaces. Thedesi~mof the temperature prugrammrr allows operation with several rhrumatugraphs t h n w h replacement of R10. Initial and final temperature settings and programming rates must be calibrated before
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A262 / Journal of Chemical Education
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PARTS LIST
C5 22 p C6,7 2 pF,25V ICl NE-555 timer IC2 SN-7476 J-K Flio-Floe IC3 LM309 regulato;5-v ' (All resistors %watt unless otherwise indicated) RI 190 n R2 lkCl2W R3 50 kCl R4 3 kR R5,6,7,8 330 0 ~9 220 n -~~ R10 (see text) Ql,z GE-67 D1,2 IN4001 diode D3 5.1V zener diode 400mW TI 12 volt CT 1A transformer S1 Maenetic reed switch (normallv closed) Light emitting diode Stepper Motor (North American Phillips Control Part #B45129-M2 -12 volts d.c., 0-60 CPS, 0-3 RPM.Available from Heath Company, Bentan Harbor, Mich. 49022 a t #42072) ~~
LITERATURE CITED
