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Simple temperature controller
ANALPTICAL
BIOCHEMISTRY
59,
Simple
316-318
(1974)
Temperature GERT
KARLSSON
Institute of Biological Chemistry, University of Copenhagen, Received
October
Controller
24, 1973;
A, August Copetlhugen accepted
Krogh Instit,ute, 0, Denmalk
November
20. 1973
A dc-operated instrument intended for precise temperature control of small air-filled enclosures is described. An integrated voltage regulator is employed to control the output from the heater elcmcnt in a continuous manner giving a short-term temperature stability better t,han +O.O2”C. The regulated temperature is adjustable. and the exact temperature is determined by the setting of a potentiometer, or alternatively the instrument can be remotely controlled by an external voltage source.
The accurate control of the temperature of experimental setups most often affects design considerations and limit, the accuracy of the data obtained. A practical solution to this problem is to place t’he most sensitive parts of the equipment, or if possible the entire equipment, in an airfilled enclosure of const,ant temperature. Such thermostatic compartments are also quite useful in cases where high stability of electronic equipment is desired. In order to be versatile the adjustment of the controlled temperature should be simple and reproducible. This note describes a simple instrument intended for the precise control of the temperature control of small air-filled enclosures (less than 100 liter). The controller was operated from a de power supply and based on a high-gain amplifier, the input of which was connected to a temperature-sensing bridge. The output from the amplifier controlled the emission of heat from the heater element in a continuous manner. In the instrument described (Fig. 1) a monolithic precision voltage regulator, PA 723,l served as amplifier, and the internal reference voltage of the regulator (pin 4) was used as excitation source for the bridge. The output current capability of the controller was increased by connecting the output of the regulator (pin 6) to the base of a Darlington amplifier, MJ 3000.2 The heater element acted as collector load of the -RIJ 3000, i.e., the heater current was determined by the voltage at the output of the regulator. In this way the control of heater elements rated to more than 100 W became possible. Two microscope lamps rated at 6 V, 5 A ’ Obtained ’ Obtained Copyright All rights
from from
Fairchild Motorola
Semiconductor, Semiconductor,
@ 1974 by Academic Press, of reproduction in any form
316 Inc. reserved.
Mountain Phoenix,
View, Arizona.
California.
SIMPLE
Pos 1 Pns 2 POS 3 POS 4
30-40
c
u-50 50-60 FU-70
c c c
TEMPERATURE
317
(!ONTROLLER
ov FIG. 1. Complete circuit diagram of the controller. RVl is a lo-turn potentiometer of 1OKQ. The thermistor has an R2’ of 150RQ and was obtained from Philips, Eindhewn. The Netherlands (type 635). The pin numbers of the regulator given in thr figuw only apply to the lo-pin mrtal can version: (2) Inverting input; (3) Noninverting input ; (4) V”‘: (5) V: (6) Output: (7) V,; (8) T”.
were used as heater element and the entire instrument was operated from a supply voltage of 12-15 V. Alternatively a 12 V car battery may be used if the controller is to be employed temporarily outside the laboratory. Higher supply voltages may be used when other heater elements are employed, if the maximum safe voltage of the regulator (4OV), is not exceeded and the MJ 3000 is operated within its safe operating area, which is given in the data sheet (1). By connecting a zener diode from pin 5 to pins 7,8 of the regulator and a series resistor from pins 7,8 to the supply, the voltage across the regulator is bounded, and the supply voltage is now only limited by the breakdown voltage of the Darlington amplifier, 60V in the present case. By the substitut’ion of a high-voltage Darlington (e.g., Motorola NJ 3040) this may be increased to several hundred volt,s. From the many types of temperature transducers available commercially (2,3) a NTC-thermistor was chosen as the temperature sensitive component of t.he bridge, in spite of its well-known nonlinearity (2,4), because the large temperature coefficient could be exploited to advantage in this circuit owing to the limited range of temperatures intended. The nonlinearity was nearly eliminated, when the thermistor was connected in series with a resistance (47K) equal to the resistance
318
GERT
KARLSSON
of the thermistor in the center of the t,emperature range used. Conscquently the volt,age at pin 2 of the regulator became an almost linear function of the temperature with a coefficient of about 70 mV/“C. The values of the switchselectable resistors given in the diagram are only approximate due to variations in the resistance of the individual thermistors; exact values must be determined by trial. The controller was designed to operate from 30 to 70°C in four ranges, and within each range the regulated temperature could be adjusted continuously by means of a lo-turn potentiometer. The performance of the instrument was tested by using it to control the temperature of a 60-liter box made from 2 cm thick polystyrene foam. The air in the box was circulated by a small fan, and the temperature was measured by a second thermistor. The short-term (30 min) stability of t,he temperature in the box was measured to +O.O2”C, and the long-term (24 hr) stability was better than +O.O4”C. If a poorly regulated power supply is used! the input voltage to the regulator should be bounded by a 12 V zener diode, as fluctuations in the supply voltage to the regulator resulted in temperature variations of O.O3”C/V (at 12 V). The temperature of the box could be changed from 25 to 40°C in less than 5 min and from 25 to 60°C in less than 15 min. The design lends itself easily to the generation of programmed temperature functions by connecting either a low frequency function generator or a digital-to-analogue converter controlled by a digital programming source to pin 3 of the regulator. With the instrument described a slow ramp decreasing from +5.5 to +2.5 V generated an almost linear temperature sweep from 30 to 70°C. The instrument was designed for use as a temperature controller, but may easily be converted into a highresolution thermometer, as the voltage at the connection between the thermistor and the 47K resistor is an almost’ linear function of the temperature sensed by the thermistor. By disconnecting pin 2 of the regulator and connecting a high-impedance digital voltmeter between this point and ground a visual indication of the temperature is obtained, and the exact temperature can be found from a calibration curve. ACKNOWLEDGMENTS The
author
is indebted
to H. N. Rasmussen
for
valuable
criticism
and
support.
REFERENCES 1. Data Sheet for MJ 3960, Motorola Semiconductor, Phoenix, Arizona. 2. NORTON, H. N. (1969) Handbook of Transducers for Electronic Measuring terns, pp. 581-635, Prentice-Hall Inc., New Jersey. 3. LION, K. S. (1964) in Physical Techniques in Biological Research (Nastuh, ed.), Vol. 5, part A, pp. 259262, Academic Press, New York. 4. KRAIJS, K. (1973) Elektronik 22, pp. 61-62.
SpsW. I,.,
BIOCHEMISTRY
59,
Simple
316-318
(1974)
Temperature GERT
KARLSSON
Institute of Biological Chemistry, University of Copenhagen, Received
October
Controller
24, 1973;
A, August Copetlhugen accepted
Krogh Instit,ute, 0, Denmalk
November
20. 1973
A dc-operated instrument intended for precise temperature control of small air-filled enclosures is described. An integrated voltage regulator is employed to control the output from the heater elcmcnt in a continuous manner giving a short-term temperature stability better t,han +O.O2”C. The regulated temperature is adjustable. and the exact temperature is determined by the setting of a potentiometer, or alternatively the instrument can be remotely controlled by an external voltage source.
The accurate control of the temperature of experimental setups most often affects design considerations and limit, the accuracy of the data obtained. A practical solution to this problem is to place t’he most sensitive parts of the equipment, or if possible the entire equipment, in an airfilled enclosure of const,ant temperature. Such thermostatic compartments are also quite useful in cases where high stability of electronic equipment is desired. In order to be versatile the adjustment of the controlled temperature should be simple and reproducible. This note describes a simple instrument intended for the precise control of the temperature control of small air-filled enclosures (less than 100 liter). The controller was operated from a de power supply and based on a high-gain amplifier, the input of which was connected to a temperature-sensing bridge. The output from the amplifier controlled the emission of heat from the heater element in a continuous manner. In the instrument described (Fig. 1) a monolithic precision voltage regulator, PA 723,l served as amplifier, and the internal reference voltage of the regulator (pin 4) was used as excitation source for the bridge. The output current capability of the controller was increased by connecting the output of the regulator (pin 6) to the base of a Darlington amplifier, MJ 3000.2 The heater element acted as collector load of the -RIJ 3000, i.e., the heater current was determined by the voltage at the output of the regulator. In this way the control of heater elements rated to more than 100 W became possible. Two microscope lamps rated at 6 V, 5 A ’ Obtained ’ Obtained Copyright All rights
from from
Fairchild Motorola
Semiconductor, Semiconductor,
@ 1974 by Academic Press, of reproduction in any form
316 Inc. reserved.
Mountain Phoenix,
View, Arizona.
California.
SIMPLE
Pos 1 Pns 2 POS 3 POS 4
30-40
c
u-50 50-60 FU-70
c c c
TEMPERATURE
317
(!ONTROLLER
ov FIG. 1. Complete circuit diagram of the controller. RVl is a lo-turn potentiometer of 1OKQ. The thermistor has an R2’ of 150RQ and was obtained from Philips, Eindhewn. The Netherlands (type 635). The pin numbers of the regulator given in thr figuw only apply to the lo-pin mrtal can version: (2) Inverting input; (3) Noninverting input ; (4) V”‘: (5) V: (6) Output: (7) V,; (8) T”.
were used as heater element and the entire instrument was operated from a supply voltage of 12-15 V. Alternatively a 12 V car battery may be used if the controller is to be employed temporarily outside the laboratory. Higher supply voltages may be used when other heater elements are employed, if the maximum safe voltage of the regulator (4OV), is not exceeded and the MJ 3000 is operated within its safe operating area, which is given in the data sheet (1). By connecting a zener diode from pin 5 to pins 7,8 of the regulator and a series resistor from pins 7,8 to the supply, the voltage across the regulator is bounded, and the supply voltage is now only limited by the breakdown voltage of the Darlington amplifier, 60V in the present case. By the substitut’ion of a high-voltage Darlington (e.g., Motorola NJ 3040) this may be increased to several hundred volt,s. From the many types of temperature transducers available commercially (2,3) a NTC-thermistor was chosen as the temperature sensitive component of t.he bridge, in spite of its well-known nonlinearity (2,4), because the large temperature coefficient could be exploited to advantage in this circuit owing to the limited range of temperatures intended. The nonlinearity was nearly eliminated, when the thermistor was connected in series with a resistance (47K) equal to the resistance
318
GERT
KARLSSON
of the thermistor in the center of the t,emperature range used. Conscquently the volt,age at pin 2 of the regulator became an almost linear function of the temperature with a coefficient of about 70 mV/“C. The values of the switchselectable resistors given in the diagram are only approximate due to variations in the resistance of the individual thermistors; exact values must be determined by trial. The controller was designed to operate from 30 to 70°C in four ranges, and within each range the regulated temperature could be adjusted continuously by means of a lo-turn potentiometer. The performance of the instrument was tested by using it to control the temperature of a 60-liter box made from 2 cm thick polystyrene foam. The air in the box was circulated by a small fan, and the temperature was measured by a second thermistor. The short-term (30 min) stability of t,he temperature in the box was measured to +O.O2”C, and the long-term (24 hr) stability was better than +O.O4”C. If a poorly regulated power supply is used! the input voltage to the regulator should be bounded by a 12 V zener diode, as fluctuations in the supply voltage to the regulator resulted in temperature variations of O.O3”C/V (at 12 V). The temperature of the box could be changed from 25 to 40°C in less than 5 min and from 25 to 60°C in less than 15 min. The design lends itself easily to the generation of programmed temperature functions by connecting either a low frequency function generator or a digital-to-analogue converter controlled by a digital programming source to pin 3 of the regulator. With the instrument described a slow ramp decreasing from +5.5 to +2.5 V generated an almost linear temperature sweep from 30 to 70°C. The instrument was designed for use as a temperature controller, but may easily be converted into a highresolution thermometer, as the voltage at the connection between the thermistor and the 47K resistor is an almost’ linear function of the temperature sensed by the thermistor. By disconnecting pin 2 of the regulator and connecting a high-impedance digital voltmeter between this point and ground a visual indication of the temperature is obtained, and the exact temperature can be found from a calibration curve. ACKNOWLEDGMENTS The
author
is indebted
to H. N. Rasmussen
for
valuable
criticism
and
support.
REFERENCES 1. Data Sheet for MJ 3960, Motorola Semiconductor, Phoenix, Arizona. 2. NORTON, H. N. (1969) Handbook of Transducers for Electronic Measuring terns, pp. 581-635, Prentice-Hall Inc., New Jersey. 3. LION, K. S. (1964) in Physical Techniques in Biological Research (Nastuh, ed.), Vol. 5, part A, pp. 259262, Academic Press, New York. 4. KRAIJS, K. (1973) Elektronik 22, pp. 61-62.
SpsW. I,.,