edited by DENNIS SIEVERS Central Cornunity High School Route 50 West Breese. IL 62230
Inventory Control A Simple. Low-Cost Programmer for a Temperature Controller H. R. Alzabet and J. A. Barbero' Deeartamenta Quimicade Reactores ~eknclade lnvertlgaciones ComiSi6n Nacional de Energia At6mlca Avda. del Libertador 8250 Capital Federal (1429). Argemina A great number of experiments in analytical, physical, or inorganic chemistry require temperature control. A programmed temperature controller is expensive (about $1,000 or $1,500), but in many labs there are old, proportional temperature controllers that can be easily modified to useful programmable instruments. We describe here an economical, easily made programmer for use in standard proportional temperature controllers. The present device uses a previously developed proportional controller2 with minor electronics improvements. The block diagram of the instrument in operation is indicated in Figure 1.There are four basic sections in the circuit:
active elements of a saw-tooth generator, and an output section. The ZCS can he used either as an on-off control or as a proportional control with the use of an internal saw-tooth generator. In the present instrument i t was used in its proportional mode. Tha Programmer There are three main parts in the programmer, a multiple connection hoard, a step relay, and an electronic timer. The multiple connection board is shown schematically in Figure 2. The grid is defined hy copper connectors that determine the voltage (horizontal dotted lines in Fig. 2b) and time (black strips in Fig. 2b). Voltage strips are on the upper face of the board and time strips on the lower face. Intersection points in the grid are connected in such a way as to define properly the temperature-time program. T o join points we
1) The temperature sensor. A thermmuple placed inside the furnace to be controlled.
2) The programmer. A variable voltage offset generator.
3) A differentialamplifier with variable gain for driving a trigger. 4) A zero-crossing switch (ZCS). It is a combination threshold de-
Figure 1. Programmed temperature controller block diagram,
tector and zero-crossingtrigger, intended primarily for ac powercontrol circuits. Tha Components of the Proportional Controller The Temperature Sensor
The present design uses a type-K thermocouple (nickelchromium as opposed t o nickel-aluminum or Chromel-Alumel) with a positive Chrome1 wire and a negative Alumel wire. I t is recommended for use in clean, oxidizing atmosnheres. I t was used in this device because the operating ;ange for this alloy is about 1300 OC, depending on t h e wire sizes. and an acceotahle voltage-to-temperature ratio (about 4 , L V ~ Cwhich ) is important in the d e & n of the controller.
1
Temperalure
% 1 STEP RELAY
I I1
The Instrumentation Differential Amplifier
This circuit is a low-set differential amplifier, and it is designed to compare the reference signal (from the programmer) with the output of the thermocouple, and then to amplify the difference between the two. The voltage output is zero when both signals reach the same value. The Zero-Crossing Switch (ZCS)
I t allows a triac or SCR to be fired when the ac input signal crosses through zero volts, thereby minimizing undesirable electromagnetic interference. In this manner, the load utilizes full cycles of line voltage as opposed to partial cycles typical with SCR phase-control power circuits. The circuit includes a zero-voltage detector, a differential amplifier, the
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Present address: Centro At6mico Bariioche, San Carlos de Bariloche (8400), Rio Negro, Argentina. Benkiki, M.; Penchia, C. M. J. Phys. E. Sci. hst. 1978, 9, 95.
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Journal of Chemical Education
Figure 2. Multiple mneclion board with an example of a tempsrature program. (a) Voltage dlvlra and step relay schematic.(b)Detail of construction of the grid (copper clad).
Frequencies Obtalnedwith Potemlometer 1 Range of time between steps (minutes)
MC14521B Stnoes
Terminals
R3=0 R
to
R3=50 kR
I
Figure 3.Elecnonic timer and step relay schematic.
used brass 3/32-in screws and nuts through holes drilled in the crossing points of the board. An example of a temperature program is shown in Figure 2a. The circuitry for the electronic timer and the step relay is shown in Figure 3. For the former we used a 24-stage freauencv . .divider. I t consists of a chain of 24 flip-flops with an input circuit that allows three modes of operation. In our case the input works as an RC oscillator. Each flip-flop divides the frequency of the previous flip-flop by two, consequently this part will count up to 224=16,777,216.The count advances on the negative going edge of the clock. The outputs of the last seven stages are available to fire the step relay (see Fig. 3). In the oscillator mode, this integrated circuit has a freauencv. .f .. eiven bv. .f=2.3.R.C. With ~otentiometer1we were able to adjust the frequency in the range shown in the table. The outDur from the treauencv divider is, led into thecircuit to fire a b s t a g e step relay. 1t consists of two transistors and a double contact relay. Parts LISP 2.2 K 1% 100 K 5% 50 K 10-turnpotentiometer 4.7 K 5%
R1 R2 El3 R4 RS
1K
1%
LLl seven-stageswitch LL2 nush button switch LL3 k-~~ o f f switch 1 MC145'21H 24-stagefrequency divider 1 BC547 transistortNPN) 1BD131 transistor(NPN) 1double-contact relay (manufacturedin USA hy Potter & Brumfield;tvpe PRIAY. 25 Amp, 1201140 V ac) 1N5002 hiode 140-staee steo relav (manufacturedin USA bv Guardian Electric: type 657705.30 Amp, 115 v ac, 60 cy) ~~
~
1 LM725 differential amplifier
1 1.M301 differential amplifier 1SN72440 zero-voltage switch
1TL430 zener diode 1TIC263D triac 25 Amp, 220 V
Usage In order to start the preset temperature program, the following steps should be carried out in sequence. 1) Position the step relay.
a) Open LL3 switch (stand-by position). b) Press the push button LL2 as many times ss required. 2) Clwe LL3 switch (run position). Immediatelv the temDerature vroeram will start We used the programmer in many experimental studies, such as a hieh-tem~eratureelectrochemical studv of iron oxides. We carried but some solid state coulome~rictitrations with aalvanic cells. A samde of iron oxide witha predetermined 0 l ~ ratio e was placed in the cell and the temperature raised up to a value T I (600-1100°C). Applying apotential +V (0-300 mV) to the electrodes, the stoichiometry of the sample was changed in the one-phase field. Stability in the emf readings meant two-phase boundary at TI. With the programmer we increased (or decreased) automatically the temperature to a new value Tz=Tl+AT. Reversing the potential (-V) in the electrode compartment we were able to find, in a short time span, a new point of the boundary onephaseltwo-phase field. Further detailed information including complete schematics, examples of its application, and-a more detailed theory of operation may be obtained by contacting one of the authors (JAB).
Fixed resistors are 0.25 w.
Volume 64 Number 4
A ~ r i 1987 l
381
