Ronald W. Anderson Universily of Illinois Urbana 61803
Two Low-Cost Solid State Temperature Controllers
A common problem in the laboratory is
the control of temperature to within a degree or two with equipment sufficiently inexpensive that i t can be made generally available. For example, reaction mixtures which should be maintained a t a uniform temperature are often heated by a heating mantle or bath containing a resistance wire coil. Fractional distillations and small scale distillations under reduced pressure are greatly assisted if the heat input is coutrolled. A second kind of problem occurs when the temperature of a bath has to be controlled to one huudredth of a degree as in kinetic investigations. Although commercial equipment which performs these functions is available, its cost is often prohibitively high. Typical older temperature controllers have only two states, on and off, and as a consequence a small temperature fluctuation is introduced. A better device, the proportional controller, supplies power to the beater a t a rate proportional to the drop of the bath temperature below the set point, and thus provides smoother control. The two temperature controllers described below are of this type.
imum gain that does not cause hunting. One user a t the University of Illinois reports that with the proper thermometer and heater, the controller maintains the mercury within a graduation line on the thermometer. A block diagram of the instrument is shown in Figure 1. The clip-on head contains a 500-kc oscillator with a balanced transformer output. A trimmer ca-
Figure 1.
Block Diogrorn of clip-on thermistor sensor controller.
Clipon Thermometer Sensor Controller
The first and lowest cost controller has a sensor that clips onto a mercury thermometer and senses the level of the mercury by measuring the electrical capacitance between the mercury column and a small metal clip. The output, capable of driving a heater of up to 1000 w, goes from full off to full on for about 3 mm change in the mercury level. The price of components for a single unit is about $50. Savings are substantial if several are constructed. Several factors determine the degree of control achieved by these units. Of these, the most important are the coupling of the sensor to the heater and the overall control system gain. In using the clip-on unit i t should be realized that the "gain" is not only a function of the electronic circuits, but also of the degree of expansion of the thermometer scale, and the heater size. Coupling of the sensor to the heater is dependent on the physical spacing, the thermal conductivity and heat capacity of the intervening gas or liquid, and the degree of agitation of the gas or liquid. Highest gain is achieved with an expanded scale thermometer and high wattage heater. Best stability is obtained through maximum agitation of the medium and close proximity of the thermometer and heater. The thermal mass of the heater itself should be small. If the system hunts or "cycles," the sensor-heater coupling must be improved, or the gain reduced by using a smaller heater or less expanded scale thermometer. Best control will be obtained with the max-
pacitor and the thermometer clip complete the bridge circuit. When the trimmer capacitance equals the capacitance seen by the insulated clip, (which depends on the position of the mercury in the thermometer), the output voltage from the center tap of the transformer winding is zero. The output from this point is connected to an amplifier, and the output of the amplifier is rectified so tbat a dc output proportional to the bridge unbalance is obtained. The dc error signal controls a timer tbat is re-started a t the beginning of each half-cycle of ac line voltage. The timer determines the point on the half-cycle a t which a silicon controlled rectifier is triggered so that the average output voltage to the heater is proportional to the error. This is described in more detail below. The capacitance sensor circuit is shown in Figure 2. Q1 and its associated components comprise an oscillator circuit. T1 provides the inductance for the 500-kc tuned circuit, a feedback winding, and a balanced output winding for the capacitance bridge. Q2 is an emitter follower that provides a high input impedance to the amplifier, so the bridge output is not severely loaded. Q3 is a voltage amplifier. C4, C5, D l , and D2 form a peak-to-peak detector circuit and filter to produce a dc output proportional to the bridge unbalance. The controller circuit is shown in Figure 3. When the bridge is neady balanced, the sensor head output is near zero volts, and is more negative than the emitter Volume 44, Number 10, October 1967
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Parts List.
CI C2 C3-C6 Fuse Clips Box Clip Insulator Dl, D2 R1 R2
Figure 2.
Capocitonce sensor circuit
of Q1. For this reason Q1 is turned on, and its collector remains near zero volts. When the bridge is unbalanced, as the mercury level drops, Q1 starts to turn off. As Q1 turns off, current through R2 begins to flow into the emitter of Q3. Q3 is connected as a current source to discharge the timing capacitor C1. Q7
Figure 3.
Copacitonce Sensor Circuit (Fig. 2)
500 Pf Ceramic 100 v minimum Piston Trimmer JFD VC 57G 1-12 Pf .O1 pf Cerrtmic 100 v minimum Littelfwe 101001 BUD CU 3000-A ,020-,040 Polypropylene IN 482, 1N 662, 1N 663, 1N 458, ete. Nearly any general purpose silicon signal diode will do. 3.3K ' / z w 5% 750 0 I/, w 5%-May need to adjust up or down, i.e., 820, 910, 680, 620 for proper oscillation level W 5% 100 K 'Ir " 10K " 4.7 K " " 2N 3638 2N 3567. 2N 3568. 2N 697 See ~ i & e 2
is an amplifier, whose output is low except very near the zero crossing of the ac line voltage. As the voltage crosses zero, Q7 turns off, its collector going negative, charging C1 through R19 and the base of Q4. When the collector of Q7 goes to ground, the base of Q4 goes 12 v positive with respect to the emitter. Current
Power contml circvil
Parts List. Power Control Circuit for Capocitonce Sensor (Fig. 3)
C1
cz
C3
C4,C5 Dl-D3 D4-D7 D8, D9 D10, Dl1 M Q L Q5Q7 &Z &3 &4
570
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.47 wf 100 v Pttper or Mylm .22 pf 100 v " .O1 hf Ceramic 100 v minimum 1000 d / l 5 v ELECTROLYTIC 1N 482. 663. 662. 456. 45&Almost any general silicon diode 1N 3193 (RCA) 1N 3210 Motorola MCR 23054 Motorola (now called 2N 4170) 0-10 MADC Meter 2N 3638 (Fairchild) Not Used 2N 3568 (Fairchild) 2N 404
Journol o f Chemicol Education
RH ~ 1 6 R17 S1 T1
T2
75n 1 w 5% " 39n 100 K '/B w 5% SPST Toggle Switch 10 amp Rating Triad F41X Transformer or Equivalent 24 to 25.2 v Ct 600 mamp to 2 amp Pulse Transformer, Spragne l l Z 12 (1:l) or 11213 (1:l:l) (see text)
from Q3 charges C1 toward ground. When the base of Q4 reaches the emitter voltage, Q4 turns on, Q5 off, and Q6 on, generating a trigger pulse which through C2 and T1 turns on whichever of the two SCR's is forward biased. The SCR remains on until the end of that half-cycle. If the input error voltage is large, Q3 supplies a large current to charge C1 quickly, turning the SCR on early in the power half-cycle. If the error is small, Q3 supplies a small current, charging C1 more slowly, turning the SCR on late in the half-cycle. The output voltage from the controller is pulsating dc, whose average value is proportional to the temperature error. Operating lnrtructions for Clip-on Temperature Controller
Maximum heater rating 900 w. Do not use a Variac between controller and heater; output i s dc. Clip head onto thermometer with cord away from bulb end (see Fig. 4). With the thermometer in place in the environment it is to control, slide the head so that the mercury is a t the top of the single clip. The power output
Figure 4.
just reaching zero when the mercury reaches the top of the clip. The power meter should indicate at least 7 mamp when the mercury is at the bottom of the clip. and should remain zero for any level of mercury above the top of the clip. Head adjustment will be necessary when switching control from a nou-conductive oil or air bath to a conductive bath or solution, or when thermometer bulb is in close proximity to a heater or metal base. Heads are interchangeable between control units, but adjustment may be necessary. Thermistor Bridge Controller
The second unit uses a thermistor as a sensor. A ten-turn potentiometer and 5-position range switch are used to set the temperature over the range of 0-25O0C. A single unit has a parts cost of about $75. The design of the thermistor probe is important. Our best results have been obtained by flattening and soldering the end of a length of '/4-in. copper tubing and inserting the thermistor bead in this with some silicone oil. With this probe, the controller maintains a well-stirred kinetic bath within =t0.02' and a 25-gal bath at 100°C to within *O.OlO. In this unit, the thermistor, some b e d film resistors, and a precision ten-turn potentiometer are connected in a bridge circuit, (see Figures 6 and 7 )and the output connected to a dc differential amplifier. The output of the amplifier is a dc voltage proportional to the bridge unbalance. The controller from this point on is identical with the one described above.
Stirring apparatus with clip-on temperature contmller in ploce.
will vary with the setting of the trimmer capacitor in the head as shown in Figure 5 . Locate the range of adjustment for which there is no power output, then set the screw to the clockwise (higher capacity) end of this range. This should result in the power output
Figure 6.
Thermistor bridge circuit.
Parts List.
RI R2 R3 R4 - ~
R5 R6 R7 R8
R9 Figure 5.
Voriotion of power output with retting.
Thermistor Bridge Circuit (Fig. 6 )
1200 n 11, w 5% 1200 a '/, w 2% IRC "Metal Glaze" 36 K " " 0 -K " " -1 -~ 2.4 K " " 620 a " " 150 D " " 200 D " " 1000 n 10 turn 5% Res. 25% linearity pot," or equivalent with &lo00 D i d Volume 44, Number 10, October 1967
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Figure 7.
Power control circuit.
Parts List. Power Control Circuit for Thermistor Bridge (Fig. 7)
Ql, QZ 2N 3638 R1, R2 1 K '/% w 5% R3. R4 3.9 K " " ~2'2 1K Potentiometer Fenwal GA 45 P2 50 K a t 25'C, 3.996/T RT1 All other parts same as capacitance sensor controller All parts listed are avdable from Allied Electronics Catdog 670, except 2N 3638, 2N 3568 obtainable from m y Fairchild Semiconductor distributor.
Figures 6 and 7 show the thermistor bridge and controller circuit. The voltage output of the bridge is zero when the thermistor is a t the set temperature. Q8 and Q9 are connected in a differential amnlifier circuit.
Approximate Temperatures for Settings of the Ten-Turn Potentiometer ond Range Switch for the Thermistor Bridge Controller Range A B C D E
DIAL 000 100 200 300 400
-4 3 9 14 17
600 700 800 900 1000
23 26 28 31 32
ROO ...
Figure 9.
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Wove forms.
laurnal o f Chemical Education
~n. 46 . 53 56 59 61 64
55 65 73 79 85
96 108 118 126 134
~i~~~~ 8.
circuit
145 164 178 190 198
qn ..
140 . ..
2n6 ..
94 98 101 105 108
146 151 156 160 164
213 220 226 230
.:. ,-.
\\'hen rhe rcmpcraturc falls t)elow the set p ~ t i n t ,the rolltfct~wr u r r u . 1 of Q9 is rcduccd rind currcur through
R21 flows into the emitter of Q3 causing the SCR's t o fire as described above in the description of the clip-on sensor controller. An incandescent pilot lamp of the 120-v type may be connected across the heater output to give a rough iudication of the relative power output, eliminating the meter and its associated resistor. The original design of these units incorporated the bridge power switching circuit so that the two SCR's could have their cathodes
20 -.
21 30 36 42
2'35
common and a two-winding trigger transformer that was available could be used. By using a three-winding transformer, two rectifiers are eliminated, and the outac which can then be run through a put is Variac to control the maximum heater power. Refer to either controller schematic for the locations of uoints x, y, and a. The circuit shown in Figure S may be substituted. Figure 9 shows voltage wave forms at points
A and B of Figures 3 and 7. These are included for troubleshooting purposes. Acknowledgmen,
I wish to thank Dr. S. Smith for assistance in the design of a suitable thermistor probe and patient testing of the prototype units.
Volume 44, Number 10, October 1967
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