Created it, 06/10/19
Update it, 06/11/02
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6. - FIFTH EXPERIMENT: REALIZATION OF A DIGITAL THERMOMETER
The preceding experiment showed you how to convert an analogical tension into a corresponding numerical value.
However, this type of converter cannot convert other physical sizes such as the pressure or the temperature.
To take a temperature measurement, it is thus necessary, initially, to convert the temperature into tenson and then in the second time, to convert this tension into numerical value. The synoptic diagram of figure 25 represents these two stages of conversion.
The conversion of the temperature into analogical tension is carried out thanks to a transducer.
On the market, there are many types of transducers of temperature. One finds, for example thermisters whose resistive value is a function of the temperature, of the thermocouples generating a tension proportional to the temperature.
The transducers of temperature called also temperature gauges are distinguished according to the range from the temperatures where they can work and also according to the reaction speed to the variations in temperature.
In this experiment, you will use the integrated circuit LM 335.
As shown in the figure 26-a and 26-b, it is encapsulated in the same types of cases as the transistors.
This circuit, represented figure 27, is rather complex and comprises sixteen transistors.
On the other hand, the circuit of use represented figure 28 is relatively simple.
The tension between the terminals (+) and (-) of the LM 335 is a function of the temperature.
With 25° C and with a current of 1 mA circulating in sensor (LM 335), the typical value of the tension is 2,98 volts. The value minimum is 2,92 volts and the maximum value is 3,04 volts.
The value of R4 resistance must be calculated according to + Vcc so that the sensor is traversed by a current of 1 mA.
Calculation of R4 :
R4 = (Vcc - 2,98) / 1 mA
For Vcc = + 5 volts, R4 = (5 - 2,98) / 10-3 = 2,02 kW.
One takes R4 = 2,2 kW which is a value standardized near to that calculated.
The output voltage is proportional to the temperature. It increases by 10 mV per additional degree Celsius.
The relation between the tension and the temperature is given by the following formula :
VT is the output voltage, T the ambient temperature, VT0 is the reference voltage standard for a T0 temperature.
For T0 = 25°C and VT0 = 2,98 volts, one obtains :
To improve the measuring accuracy, one can carry out the calibration of the sensor using a thermometer of precision. With this last, one measures the temperature and one defers the value found in the formula (5), which makes it possible to calculate VT.
It any more but does not remain to regulate the output voltage to the computed value. For that, it is necessary to use a voltmeter of precision and to operate the potentiometer P of 10 kW.
The sensor can work of - 10°C with + 100°C. For larger ranges of temperature, there is the LM 135 (of - 55°C with + 150°C) and the LM 235 (of - 40°C with + 125°C).
6. 1. - FIRST PART OF THE EXPERIMENT
6. 1. 1. - REALIZATION OF THE CIRCUIT
a) While preserving the essence of the preceding assembly, withdraw the potentiometer of 10 kW and the connections indicated into dotted figure 29.
b) Insert on the matrix circuit LM 335, R4 resistance of 2,2 kW and the P2 potentiometer of 10 kW as indicated figure 30-a. Carry out the connections highlighted on this figure.
The electric diagram of the circuit carried out is given figure 30-b.
6. 1. 2. - OPERATIONAL TEST
a) Measure the ambient temperature using a thermometer in order to carry out the calibration of the circuit.
b) Calculate tension VT using the formula (5).
For example, for T = 21°C, VT = 2,98 + 0,01 (21 - 25) = 2,94 volts
It is now necessary to convert 2,94 volts into hexadecimal value. For that, use the tables of figures 20 and 21.
It is necessary to start with the most significant digit. In the table of figure 21, the number immediately lower than 2,94 is 2,8125 which corresponds to the hexadecimal figure 9 on bill-poster DIS1.
The difference between 2,94 volts and 2,8125 volts is worth 0,1275 volt. In the table of figure 20, they is 0,117 which is closest to 0,1275. The hexadecimal figure corresponds is 6.
In conclusion, with 21°C, the terminal voltage of the sensor is 2,94 volts and the bill-posters indicate 96.
c) Put the digilab under tension.
d) Temporarily put pin 5 of the converter at the mass.
According to any probability, the bill-posters will not indicate the number that you calculated. In this case, carry out the calibration of the sensor using the P2 potentiometer to post the correct hexadecimal number.
Circuit LM 335 is rather slow to follow the variations in temperature, its response time is approximately 5 mn.
e) To take a measurement, raise the temperature of the LM 335 by tightening it between your fingers.
f) Observe the bill-posters, the posted number increases slowly and is stabilized after a few minutes.
It is possible to calculate the indication of the bill-posters roughly.
One can suppose that the temperature of the case of the LM 335 rises with approximately 32°C.
On this assumption, the terminal voltage of the LM 335 is :
V32 = 2,98 + 0,01 (32 - 25) = 3,05 volts
Using the two tables previously used, one finds the figure 9 which corresponds to 2,8125 volts.
Difference 3,05 - 2,8125 = 0,2375 volt corresponds to the hexadecimal figure C.
The bill-posters should indicate 9 C on this assumption.
g) The first part of the experiment is finished. Put the digilab not under tension.
This experiment is particularly interesting because it presents a concrete use of an analogical/digital converter.
The temperature measurement revêt a very particular importance in certain industrial systems of control.
The assembly proposed is only didactic and hardly lends itself to a practical application since posting does not indicate the temperature directly in °C.
In the genuine digital thermometers, the reading is done directly in °C. For that, one uses analogical/digital converters which provide information codes BCD of them.
The precision of the assembly is not very high because converter ADC 0804 has a resolution of 19,5 mV, while the tension of the LM 335 varies from 10 mV by °C.
In addition, the LM 335 has a precision of a degree and the converter a precision of ± 1 LSB.
All these errors are added and it results from this a relatively weak precision from the digital thermometer. However, if one restricts the range of temperatures of circuit LM 335 and that one modifies the range of tension in which the converter functions, it becomes possible to post the temperature directly.
It is what we will do in the second part of the experiment.
6. 2. - SECOND PART OF THE EXPERIMENT
6. 2. 1. - REALIZATION OF THE CIRCUIT
a) While preserving the essence of the preceding assembly, withdraw resistances R1 and R2 and the six connections indicated into dotted figure 31.
b) Insert on the matrix the P1 potentiometer of 1 kW and R5 resistance of 82 kW as indicated figure 32-a.
Carry out the connections indicated highlighted on this figure.
The electric diagram carried out is given figure 32-b.
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Compared to the preceding assembly, this one makes it possible to vary the beach of tension in which the converter functions. For that, there are two potentiometers P1 and P2.
The cursor of P1 connected to VIN (-) makes it possible to regulate the zero. For a temperature of 0°C, the bill-posters will indicate 0 0.
The cursor of P2 connected to VREF / 2 makes it possible to regulate the scale factor of the converter, i.e. the width of a LSB expressed into mV.
Thus, for a variation of 1°C, hexadecimal posting varies from a unit.
Let us consider the two temperatures 0°C and 100°C. The two terminal voltages of the sensor are worth respectively :
V (0°C) = 2,98 + 0,01 (0 - 25) = 2,73 volts
V (100°C) = 2,98 + 0,01 (100 - 25) = 3,73 volts
So that the bill-posters indicate 0 0 to 0°C, it is necessary to apply a tension equal to 2,73 volts to the entry VIN (-) using the P1 potentiometer.
In addition, it is wished that with each rise in temperature of 1°C, the indication of the bill-posters increase by a unit. However, the tension delivered by the sensor increases by 10 mV by °C additional. It is necessary thus that the LSB corresponds to 10 mV. As the characteristic of transfer has 256 stages, the scale of tension must correspond to 10 mV x 256 = 2,56 volts.
Consequently, it is necessary to apply a tension of 2,56 / 2 = 1,28 volt to entry VREF / 2 using the P2 potentiometer.
6. 2. 2. - OPERATIONAL TEST
a) Lay out the controller on the gauge 3 V and place the test probes between pin 7 of circuit ADC 0804 (red test probe) and masses it (black test probe).
b) Regulate the P1 potentiometer so as to read on the galvanometer 2,73 volts.
c) In the same way, regulate the P2 potentiometer so that pin 9 of circuit ADC 0804 is to 1,28 volt.
With these two adjustments, the thermometer should be theoretically calibrated. In practice, it is not thus.
Indeed, the two calculated values of tension are theoretical.
In addition, there are tolerances on the characteristics of the sensor and the converter which one cannot neglect.
Lastly, the precision of the controller also intervenes.
Therefore a second calibration proves to be interesting in order to increase the precision of the digital thermometer.
For that, you will use two temperatures of reference: 0°C and the ambient temperature.
d) Put some cubes of ice in a perfectly tight plastic sachet or use an aluminum foil.
e) Put the sachet or the aluminum foil containing the ice in direct contact with the sensor. Take care that no water drop falls on the digilab. If that occurred, dry the matrix with a hair drier.
The indication of the bill-posters decrease. Wait a few minutes until it stabilizes. This moment, regulate the P1 potentiometer so that the indication of the bill-posters tends to pass from 0 0 to 0 1.
f) Move away the sachet from ice, the indication of the bill-posters increases slowly. Wait several minutes again, then regulate the P2 potentiometer so that the indication of the bill-posters is the same one as that given by the thermometer (ambient temperature).
Do not forget that the indication of the bill-posters is in hexadecimal code.
For 21°C, you must read 15 on the bill-posters.
Indeed, 1516 = 2110
g) Repeat the two described operations of calibration as of the phases e) and f) in order to increase the precision of the digital thermometer.
NOTE :
To obtain a better precision in the calibration, it would be necessary to increase the beach of temperature of adjustment. For example, it would be necessary to regulate with 0°C and 100°C instead of 0°C and 21°C.
In addition, the sensor can be at a temperature of 2 with 3°C higher than the ambient temperature since it is located above the food of the digilab.
Maintaining the thermometer is calibrated, you can take temperature measurements.
For that, you can cool the sensor with a ventilator or overheat it with a lamp.
h) When the experiment is finished, put the digilab not under tension.
7. - GENERAL CONCLUSIONS
This last experiment enabled you to note that it was possible to better exploit circuit ADC 0804 by modifying the beach of the tensions for which it functions.
For that, it is enough to apply half of this beach of input voltage VREF / 2.
In addition, it is possible “to fix” this beach of tension between the extreme values 0 volt and 5 volts thanks to the entry VIN (-).
In the preceding experiment, these two adjustments make it possible to obtain a resolution of 1°C.
The beach of tension was worth 1 volt (of 2,73 volts to 3,73 volts), it would be possible to increase the resolution of the digital thermometer.
For that, it would be necessary to make correspond the 256 “steps” of the converter to the beach of 1 volt. In other words, the LSB should be worth 1 / 256 = 3,9 mV.
In this case, one would lose the advantage of the direct reading and it would be necessary to draw up tables of correspondence between the indication of the bill-posters and the temperature.
Nevertheless, since the LSB would correspond to 3,9 mV, one would obtain a thermometer with a resolution higher than a degree. For each variation of a unit on the bill-posters, there would be a variation in temperature of 0,4°C approximately.
With this last experiment the practical part of the numerical electronics is completed which enabled you to carry out many assemblies illustrating the fundamental principles described in the theoretical part of the digital synopsis of electronics.
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