Robert C. Badger
University of Toledo Toledo, Ohio 43606
I
A Reliable, Inexpensive Proportional Temperature Controller and Differential Thermometer
The construction of an inexpensive, highly reliable, millidegree temperature controllerand differential thermometer from an extremely stahle thermistor and other readily available materials is~described.Once materials have been collected, the devices can be constructed and in operation in a relativelv short oeriod of time. This Goject was begun as a result of attempts to obtain high nrecision solvolvsis rate data conductimetricallv. We found that a high dezree of temperature con~rolwas necessarv in order to ohrain the desired kinetic orecision. The oubliration of a description of a microdegree temperature coniroller using a remarkahlv stable thermistor stimulated the Durchase of some of these thermistors for use, initially, as tkmperature monitors in coniunrtion with a Wavne Kerr B-331 conductance hridge (f). Their stability i n d reproducibility, as monitored by the conductance bridge, prompted attempts to use them as the sensing element for a proportional temperature controller (2). Before describing the circuit in detail, it might be worthwhile to briefly discuss some external experimental problems that must he addressed before millidegreetemperature control of a hath can he achieved ( 3 . 4 ) .Thorough mixing of the bath medium is essential to remove thermai and good insulation is necessary to reduce the effects of environmental changes. The bath used in these experiments was a Tamson TEV-401150. This hath is equipped with a high-speed stirrer that effectivelv removes thermal eradients in the controlled region and is sifficiently insulatei that considerable cooling is necessarv to maintain the tem~eratureat 25OC. Constant cooling is fairly critical and, it1 this rase, ir was necessary 10 mainwin a second remverature hath at ahout 15.0f 0.I0C for cooling purposes. cold medium was pumped into a constant pressure reservoir from which coolant was allowed to flow at constant rate through the rooling coilsotthe main hath.'l'he c m r r d hentrr shwld hea small (100 IY or IPS\), low thermal lag device li~rstednear the coolingcoili. A constant auxiliary heater is preferable to increasing thesi7eof'therontrol heater if additional heatine caua(it\~is necessarv. It is a h drsirahk to locate the sensorin the h k h proper &me distance downstream from the cooling and heating coils.
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Circuit Description
The circuit (see Fig. 1)for the proportional temperature controller is similar in many respects to that described by
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Figure 1. Proportional temperature controller. Parts List lo00 pf125 V electrolytic. C3 0.5 pf C1 10 $125 V electrolytic C4 1.0 &f/30V C2 general purpose silicon diode*or commercial full wave Dt-D4 bridge rectifier general purpose silicon diode D5 Motorola MC 7805 or equivalent 1Ct 1C2 National Semiconductor LM 324 or equivalent R1 lOK.%W.t% R2 R3 R4 R5 R6-Rll RtZ R13. R 1 4 R15 R16 R17 TI
Triac UJT
IK.%W.l% 100 K. ten-turn
potentiometer. 0.25% lin.. Duncan Electronics 1 K, ten-turn potentiometer. 0.25% Iin.. Duncan Electronics
50 K. 'I2W. 1% TOOK, % W, 1% 1.1 K, % W , 5 %
560 ohm, % W. 5% 10 K. ten-turn trimpot. Bourn Electronics lOK.%W.5% 10 K, ten-turn potentiometer TI 494 Or equivalent SC 250 B. General Electric or equivalent purchased from Radio Shack, No. 276-1624 or equivalent
Volume 55, Number 11. November 1978 / 747
Sarid and Cannell with the addition of several modifications to make it easier to construct and use ( I ) . The thermistor (model GA451'2, manufactured by Fenwal Electronics) is the active element in a high impedance dc Wheatstune bridge. The error voltage from the bridge is amplified hv a high input impedance instrumentation a&plifier-constr&ted from-an Figure 2. Double-ended power supply. LM 324 quad operational amplifier manufactured by National Pans List Semiconductor (5).This integrated circuit was selected beC1. C2 200 ~ 1 1 2 V 5 electrolytic cause all the necessary amplifiers are on one chip and i t can C3. C4 1500 pf ceramic be onerated from either a sinele ended or double ended Dower u C5. C6 1.0 pfI25 V elecbolytlc supply. I t is readily available and relatively inexpensive. D1-D4 general purpose silicon diodes or commercial full wave bridge rectifier Since only three of the four amplifiers are used in this apMolorola MC 1468N dual hacking voltage regulator 1C1 plication, one is left to provide integration or differentiation R1, R2 loohm. % W. 5% of the error signal should this be necessary for a particular Stancer P-8611. 36 V CT secondary or equivalent TR 1 application. The signal from this amplifier is fed to the base of an NPN transistor whose collector is connected to the charging circuit of a programmable unijunction transistor a way that the heater voltage is between 20 and 50 V at the (UJT) oscillator that is operated from an 8-V pulsed dc power desired set point, since this is the most sensitive region of the supply. With no current flowing through the NPN transistor controller. The gain is adjusted by increasing i t until regular (i.e., base at ground potential) the capacitor will rapidly charge oscillation in the temperature is observed and then decreasing up until the voltage at the anode of the UJT reaches its trigger until the oscillation just disappears. The temperature of the potential, at which point the UJT will go into conduction and constant temperature bath descrihed ahove was monitored ra~idlv . discharee " the canacitor throueh the oulse transformer. over a period of several months using a Hewlett-Packard turning the triac on for one-half cycle. Since the UJT is op2801A Quartz Thermometer and was constant to O.OOl°C, eratine from a Dulsed dc s u..~ .~that l v is in ~ h a s with e the line despite changes in room temperature of 3 to 4'C (8). frequency, the triac will be pulsed on at the same point in each half cycle, thus maintaining a constant voltage to the heater. Differential Thermometer As the temperature rises above the set point, the error voltage It is. a simple matter to conitruct a reliable diftbrential fed to the base of the NPN transistor increases. This turns the thrrmometer by using rhe same basic Wheatstone bridge and transistor on and allows some of the charging current to pass amnliiier confirmratim iunrtim of circuit within dotted lines " throueh it to mound. As a result the capacitor charges - UD- more in Fig. 1) with a well-regulated double ended power supply (see slowl;causi~g the UJT to fire later & the half cycle and eft a strinchart reFie. - 2). and runnine the am~lifiero u t ~ uinto fectivelv reduce the voltage to the heater (6, 7). corder. A single ended power supply was used for the conThe components of thetriac control circuit are not partictroller to sim~lifv . . construction and reduce the number of ularly critical and any general purpose NPN and UJT trancomponents. However, for stripchart recorders capable of sistor should suffice. A programmable UJT is recommended, accepting inputs between 0 and 100 mV with a limited zero since the trigger point can be adjusted to some extent to fit the offset capability, the use of a double ended power supply particular triac. An inexpensive audio output transformer, avoids the difficulties that are encountered with a single ended such as those used in transistor radios, will work as a pulse power supply (amplifier output cannot be adjusted to zero transformer and nrovide isolation from hieh voltaee ac. The output with respect to ground). With the carefully regulated itmplifier feedbadk resistors must he stable. 1% re&torsand double ended power supply descrihed in Figure 2 the bridge the 10 K rain cuntrol sha~uldbe a stable wire-wound tme. The balance condition is indicated by a zero output from the amcoarse a i d fine bridge balance controls are noise-fke, tenplifier and a positive output indicates that the temperature turn, wire-wound potentiometers and are the most expensive is ahove the set point. The gain can be adjusted to give any pen part of the controller. One or both of them could probably be deflection for a given temperature change; ours was adjusted circumvented bv using fixed resistors, hut the use of potento give a 10% (10 mV) deflection for a O.Ol°C temperature tiometers makes the controller much more versatile. The change a t 25.00°C. thermistor is available in several different nominal resistances This circuit was checked out in the same manner as the and so as to have a resistance of about 50 ~should ~ be ~ selected ~ controller circuit and performed quite well over a period of K i n the temperature range of interest. Any commercial fullseveral months. I t has since been used in several other appliwave bridee rectifier or diode will work in the Dower s u ~ ~ l v cations where sensitive temperature monitoring has been and any good quality 5-V regulator will provide &I adequate$ desirable and has been very useful. stable reference voltaee " for the Wheatstone hridee - and amAcknowledgment plifier. Heat sinking of the triac and regulator is unnecessary if a small control heater is used. I wish to thank Mr. Mark Heckler for considerable assistance in designing circuits and procuring parts to test and Operation and Adjustment build the devices described herein. The trimpot in the UJT oscillator is adjusted by tempoLiterature cited rarilv renlacine the thermistor with a 50-K resistor. connectine the keaier t h i t is to he used during operation to the heater (1) Ssrid.D.. and Cennel,D.s..Re~SC~. Insfrum., 45.1052 (1974). (2) Andor8on.R. W..J. CHEM.EDUC..44.569(1967). outout. and adiustine the t r i m ~ oso t that a smooth increase (3) Lowie, Jr,P. H., in "Prfflsion Measurement and Cdibration: NBS Smeid Publication in heater voltafie from 0 110 v is observed when the marre 3W. Vol.3,U.S.GovernmentPrinting Office, Washingt0n.D.C.. 1 9 6 8 , ~114. . (4) Van Swaay,M., J.CHEM. EOUC.,46,A515.A565(1969). bridee adiustment is changed. With the components we used, (5) Graeme. . I .G., ~'Applicatiamof Operational Amplifiem: Thid-Gemration Teehniqu~s." it was oniy possible to observe a smooth inirease between 0 McCrsw-Hill. New York,1973. pp. 53-63. and about 95 V, hut this was found to be satisfactory. I t is ( 6 ) Gallowsy, J. H.. 'Wsing the Trisc fur Control of AC Power." Appliestian note 2W.35. General Electric corporation,p. 9. desirable to independently shield both leads to the thermistor (7) Debm,G..and Burmlu,C. N.."lntegratedCircui~andSemimndudar Owices: Thmw to reduce noise pickup. I t is advisable to adjust external conand Application,"MeCraw-Hill, New York,1971, pp. 539-548. (8) Bonson,B. B.,snd Krsune,Jr.,D.,R e ~ S c iI. w ~ I Y 45,1199 ~., (19741. ditions (coolant temperature, auxiliary heating, etc.) in such
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748 / Journal of Chemical Education
