Barton Milligan University of Mississippi University
A Thermistor Actuated Proportional Temperature Controller
The utility of thermistors, which offer sensitivity, small size, and stability, as temperature sensing and controlling devices' has been widely recognized in recent years. Although the advantages of continuous (or proportional) temperature control, in which heater power is continuously adjusted according to need rather than switched completely on or off, have been recognizedz for some time, the cost of continuous coutrollers has been high. The high sensitivity of thermistors compared t o metallic iresistance thermometers and thermocouples nermits thermistor based devices to be relativelv simple. We have assembled, from parts costing about $50, a thermistor actuated continuous control employing only solid state devices. The circuit, shown in the figure, can be easily installed in a 5-X 7-X 2in. or even smaller chassis. The circuit as given, which 1 RATCLIFFE, J . S., J. CHEM.EDUC.,39, 637 (1962); Lewin, S. Z., J. CHEM.EDUC.,36, A199 (1959). 1 STURDEVART, J. M., "Technique of Organic Chemistry," Weissberger, A,, edit., 3rd ed Interscience Publishers, New York, 1960, Vol. 1, p. 1 8. a GUTZWILLER, F. W., JONES,D. V., LOWRY,H. R., SYLVAN, T. P., A N D VANZASTROW, E. E., "Contrdled Rectifier Manual," 1st ed., Semi-eonductor Products Dept., General Electric Co., H. V., ENKE, Liverpool, New York, 1960, p. 90 8.; MALMSTADT, E. C., JR., T l e ~ t r o n i for ~ s Scientists," W. A. C. G., A N D TOREN, Benjamin, Inr., New York, 1962, pp. 384-488.
is derived from suggestions made by a manufacturer of silicon controlled rectifier^,^ will control up to 400 watts, but by means of modifications adding approximately $30 to the cost of parts its capacity can be increased t o over a kilowatt without sacrificing sensitivity. However, a source of detailed information should be cousulted before any changes are attempted. Our controller is less sensitive than some proportional devices2; a change of 0.3'C of the thermistor is required to change the control from "on" to "off." However, it maintains a well stirred 5-gal. water bath a t 10.05"C or better for extended periods Therefore, its compactness, stability and ruggedness are its principal advantages over conventioual controls of comparable cost. The heart of the circuit is the silicon controlled rectifier (SCR), nhich is roughly a solid state analogue of a thyratron tube but has many times the poxer Ports List
SI Switch
A
Fuse 4 amp (because of the sensitivity of SCR's to transients, the fuse should he of the current limiting type, for example, Buss "Limitron" or Chase-Shawmut "Amp-Trap") SCR 2N17i4A. CRI-CR. 6F20 Mounted ou rooline fin
1500 10 watt Ilr Thermistor, 10-20°K in range d use (Veco 41 .4 11 near 25'C), see text It, Multiturn potentiometer to match Rs, low temperature coefficient R4 100 #I/* watt, low temperature coefficient R.. 11. 300 #I. watt. low temuerature coefficient" watt, I?: i i 0 gl &%ti, R8 47 #I/? wait, RI 2.2K I?,. 10002 watt R 250 wirewound (sensitivity rontrol) LI,I.% 3 watt, 120 V panel lamp, ( I red. 1 green)
R,
" The sensitivity of the controller ran be approwmately doubled hy replaring Ra with a. thermlstur smilar to Rz and replacing R. with s fixed reelstor matching Ra
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capacity at comparable cost. Within its design limits the SCR is normally non-conducting in either direction. If current of sufficient m a g ~ t u d epasses between the gate (control electrode) and the cathode while the cathode is negative, the SCR becomes conducting and remains so (even though the gate signal is removed) as long as the current remains above a certain value, called the holding current. Once forward current has fallen below the holding current, as will occur a t the end of an ac cycle, the SCR becomes blocking again and will not conduct until a signal is again applied to the gate while the cathode is negative. In our controller full wave rectified ac, from CRI-CRI, is applied across the SCR and the load in series. This arrangement offers a number of advantage^.^ The control circuit operates as follows: The Zener diode, CR6, breaks down a t 22v, providing essentially a rectified square wave potential across the remainder of the circuit. The power resistor Rl serves to limit the current through CRa. Resistances R2-Rs comprise a Wheatstone bridge. When the temperature of the thermistor (Rz) is too low, the unhalance signal of the bridge is amplified and converted to a pulse by the system Q,, Qz,C,, Rs,and Ro during each cycle. The timing of the pulse during the cycle is determined by the magnitude of unbalance signal. The greater the unhalance the sooner in the cycle the pulse, which fires the SCR, is generated. If the temperature of the thermistor is too high, the unbalance signal is of the wrong polarity to be amplified, and no firing pulses are generated. The resistance R3 is adjusted to set the operating temperature. Any multiturn potentiometer having a reasonably low temperature coefficient of resistance will suffice. Because the temperature setting is seldom altered in our use, XTe have employed a 25 turn miuiature trimming potentiometer having a screwdriver adjustment. The resistance Ra is used to match R8to the thermistor, RI. I t also prevents a runaway condition
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in case Ra is accidentally set to zero resistance. The fixed resistances R6 and REshould also have low temperature coefficients and reasonable values (we have used 300 ohms). One of the great advantages of the high sensitivity of thermistors is apparent here. For this application standard high quality resistances can be employed, whereas in similar circuits employing metallic resistance thermometers special resistances having extremely low temperature coefficients must he employed. The diode CRs serves to compensate for changes in the characteristics of Q,, with temperature. The thermistor can he of any type, but for liquid bath control the bead-sealed-inglass probe type seems to be best. The sensitivity of the instrument is unaffected by the total resistance of the thermistor for a given thermistor characteristic. However, because of the tendency of the therR m mistor to be self-heated by the current passing through it, the higher its resistance the better. We have chosen, as a compromise, to use thermistors having resistances of 1020 kohms at the temperature of the bath. Thermistors as supplied are small and fragile and must be mounted. We have found that a head in glass type thermistor sealed into a 6 or 8 mm glass tube with epoxy cement, as shown in the figure is as rugged and convenient to use as a laboratory thermometer. The relative intensities a t which lamps L, and Lz glow are an indication of the "on-off" proportions. Alternatively, the "on-off" proportions can be indicated by omitting L, and replacing Lz with a small, inexpensive ac voltmeter. Details of construction will be supplied to interested persons. This controller was developed in the course of work supported by the Petroleum Research Fund administered by the American Chemical Society. Grateful acknowledgment to the donors is hereby given.
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