IV_004. Sensors (IC and Thermocouple [Reduced]

:: Sensors and conditioning circuits – Chapter 004

Temperature sensors (continued)

1. Termocouple

2. Semiconductor temperature sensors

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3. Integrated temperature sensors


Temperature sensors

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1. Thermocouple

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1. Thermocouple

When two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current which flowsin the thermoelectric circuit. Thomas Seebeck made this discovery in 1821.

If this circuit is broken at the center, the net open circuit voltage (the Seebeck voltage) is a function of the junction temperature and the composition of the two metals.

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All dissimilar metals exhibit this effect. The most common combinations of two metals are listed on next, along with their important characteristics. For small changes in temperature the Seebeck voltage is linearly proportional to temperature:

eAB = αT

Where α, the Seebeck coefficient, is the constant of proportionality.(For real world thermocouples, α is not constant but varies with temperature).


1. Thermocouple

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Output voltages for some popular thermocouples are plotted as a function of temperature.

1. Thermocouple

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Diagrama de variatie a coeficientului Seebeck (variatia tensiunii Seebeck raportata la variatia temperaturii).Atunci cand alegem un senzor de tip termocupla trebuie sa avem in vedere zona in care coeficientul Seebeck al senzorului variaza cel mai putin cu

1. Thermocouple

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temperatura – se va selecta tipul termocuplei in functie de domeniul de temperatura masurat.

De exemplu, o termocupla de tip J are un coeficient Seebeck care variaza cu mai putin de + 1µV/°C in domeniul 200.. 500°C – acesta este domeniul in care se vor folosi termocuple de tip J.


1. Thermocouple

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Tensiunea electromotoare Seebeck va aparea la capetele libere ale celor doua conductoare din material diferite (A, B) la diferenta de temperatura dT.

1. Thermocouple

A

V1T

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(A, B) la diferenta de temperatura dT.

Tensiunea Seebeck este foarte mica (µV pana la mV) ca urmare voltmetrele uzuale nu se pot utiliza.

B

V1T

dV1=S x dT


Un alt aspect important este acela ca pentru a conecta un aparat de masura (un microvoltmetru, de exemplu), se vor forma doua jonctiuni parazite (AC

1. Thermocouple

A

T µV

C

V(µV)

V(AC)

V(AB)

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vor forma doua jonctiuni parazite (AC si CB) care vor functiona ca niste termocuple. Ca urmare, tensiune citita va fi:

V(µV) = V(AB) - V(CB) - V(AC)

B

T

dV(AB)=S x dT

µV

CV(CB)


Ca urmare, daca am masura temperatura jonctiunilor AC si CB (cu un alt dispozitiv) si am calcula V(AB) la aceasta temperatura (de pe grafic),

1. Thermocouple

A

T µV

C

V(µV)

V(AC)

V(AB)

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la aceasta temperatura (de pe grafic), am putea compensa valoarea citita de microvoltmetru:

B

T

dV(AB)=S x dT

µV

CV(CB)V(AB)real = V (µV) – V(AB)calculat

=>compensarea jonctiunii reci (CJC)


O alta metoda de compensare a jonctiunii reci este aceea de a intercala o jonctiune de referinta ca in figura alaturata:

1. Thermocouple

A

B

T µV

C

C

V(µV)

V(AC)

V(CA)

V(AB)

A

V(AB)o

V(AB) – V(AB)o = V(AC) + V (µV) + V(CA)

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Tensiuna V(AC) este egala dar de sens contrar cu tensiunea V(CA), ca urmare suma lor este zero.

B

dV(AB)=S x dT

CV(CA)A

ToV(µV) = V(AB) - V(AB)o


Daca jonctiune BA suplimentara este tinuta la o temperatura constanta, cunoscuta (0°C) se va putea calibra cu usurinta termocupla.

1. Thermocouple

A

B

T µV

C

C

V(µV)

V(AC)

V(CA)

V(AB)

A

V(AB)o

V(AB) – V(AB)o = V(AC) + V (µV) + V(CA)

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B CV(CA)A

0°C

V(µV) = V(AB) - V(AB)o


Daca se foloseste un senzor suplimentar (de exemplu un termistor) pentru a detecta temperatura jonctiunii reci, se poate compensa cu usurinta eroarea de masurare:

1. Thermocouple

A

B

T µV

C

C

V(µV)

V(AC)

V(CA)

V(AB)

A

V(AB)o

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B CV(CA)A

t°CV(AB) – V(AB)t = V(AC) + V (µV) + V(CA)

V(µV) = V(AB) - V(AB)t(Th)

V(CJC)

Th

µAV+


O termocupla de tip K va avea un coeficient Seebeck de aproximativ 41µV/ºC; ca urmare, pentru compensarea jonctiunii reci am folosit un senzor de temperatura intagrat (TMP35) cu un coeficient de temperatura de 10mV/ºC. Acest coeficient este divizat si inversat prin reteaua R1 si R2 pentru a produce o tensiune de jonctiune rece cu un coeficient

1. Thermocouple

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tensiune de jonctiune rece cu un coeficient de – 41µV/ºC. Astfel se compenseaza erorile introduse de conectarea la termocupla (fire si trasee PCB).Aceasta compensare functioneaza corect pentru valori ale temperaturii ambiente situate in intervalul 20ºC .. 50ºC. Pentru un domeniu de masura de 250ºC, tensiunea Seebeck a termocuplei va avea o variatie de 10.151mV.


Daca dorim ca iesirea full-scale a circuitului sa fie de 2.5V, pentru iesirea compensata a termocuplei (10.151mV) avem nevoie de o amplificare de 246.3. Alegand R = 4.99kohms rezulta R5 = 1.22Mohms. Deoarece cea mai apropiata valoare

1. Thermocouple

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Deoarece cea mai apropiata valoare de 1% este 1.21Mohms, se va alege un potentiometru de 50kohms in serie cu valoarea standard, pentru ajustare fina.Desi amplificatorul ales (OP193) este un AO cu alimentare monopolara, iesirea acestuia nu este de tip rail-to-rail si valoarea minima de iesire va fi de aproximativ 0.1V.


Ca urmare, se va aduga rezistenta R3 care sa introduca un offset de 0.1V pentru o alimentare de 5V. Acest ofset (echivalentul a 10°C) trebuie scazut din valoarea obtinuta la iesire. R3 permite si detectarea unei termocuple intrerupte (pentru

1. Thermocouple

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unei termocuple intrerupte (pentru termocupla intrerupta, tensiunea de la intrarea AO va fi mai mare de 3V). R7 adapteaza impedanta de cc si capacitorul de 0.1µF reduce zgomotul de cuplare.


Circuitele AD594/AD595 sunt amplificatoare de instrumentatie pentru termocuple cu compensarea jonctiunii reci, incluse intr-un circuit monolitic. Structura lor contine o referinta de 0 grade Celsius si un amplificator precalibrat, astfel incat la iesire obtinem o senzitivitate de (10mV/°C).

1. Thermocouple

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Circuitele mai contin si o alarma pentru defecte de termocuple (TTL). Pot fi alimentate de la tensiuni monopolare de 5V minim dar alimentarea lor cu tensiuni bipolare permite masurarea temperaturilor negative.

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Tree switching is a method of organizing the channels of a scanner into groups, each with its own main switch. Without tree switching, every channel can contribute noise directly through its stray capacitance.With tree switching, groups of parallel channel capacitances are in series with a single tree switch capacitance. The result is greatly reduced crosstalk in a large data acquisition system, due to the reduced interchannel capacitance.

1. Thermocouple – tree switching

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A filter may be used directly at the input of a voltmeter to reduce noise. It reduces interference dramatically, but causes the voltmeter to respond more slowly to step inputs.

1. Thermocouple – analog filter

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Integration is an A/D technique which essentially averages noise over a full line cycle, thus power line-related noise and its harmonics are virtually eliminated. If the integrationperiod is chosen to be less than an integer line cycle, its noise rejection propertiesare essentially negated.

Since thermocouple circuits that cover long distances are especially susceptible to powerline related noise, it is advisable to use an integrating analog-to-digital converter to measure the thermocouple voltage. Integration is an especially attractive A/D technique in light of recent innovation have brought the cost in line with historically less expensive A/D

1. Thermocouple – integration

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light of recent innovation have brought the cost in line with historically less expensive A/D technologies.


A noise source that is common to both high and low measurement leads is called common mode noise. Isolated inputs help to reduce this noise as well as protect the measurementsystem from ground loops and transients

1. Thermocouple – isolation

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Let’s assume a thermocouple wire has been pulled through the same conduit as a 220 V AC supply line. The capacitance between the power lines and the thermocouple lines will create an AC signal of approximately equal magnitude on both thermocouple wires. This is not a problem in an ideal circuit, but the voltmeter is not ideal. It has some capacitance between its low terminal and safety ground (earth). Current flows through this capacitance and through the thermocouple lead resistance, creating a normal mode signal which appears as measurement error.


This error is reduced by isolating the input terminals from safety ground with a careful design that minimizes the low-earth capacitance. Non-isolated or ground-referenced inputs (“single-ended” inputs are often ground-referenced) don’t have the ability to reject common mode noise. Instead, the common mode current flows through the low lead directly to ground, causing potentially large reading errors.Isolated inputs are particularly useful in eliminating ground loops created when the thermocouple junction comes into direct contact with a common mode noise source.


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In the left figure we want to measure the temperature at the center of a molten metal bath that is being heated by electric current. The potential at the center of the bath is 120 VRMS. The equivalent circuit is shown in the middle figure. Isolated inputs reject the noisecurrent by maintaining a high impedance between LO and Earth. A non-isolated system,represented in the right figure, completes the path to earth resulting in a ground loop.The resulting currents can be dangerously high and can be harmful to both instrumentand operator. Isolated inputs are required for making measurements with high commonmode noise.


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Sometimes having isolated inputs isn’t enough. In previous figures, the voltmeter inputs are floating on a 120 VRMS common mode noise source. They must withstand a peak offset of ±170 V from ground and still make accurate measurements. An isolated system with electronic FET switches typically can only handle ±12 V of offset from earth; if used in this application, the inputs would be damaged.

The solution is to use commercially available external signal conditioning (isolation transformers and amplifi ers) that buffer the inputs and reject the common mode voltage. Another easy alternative is to use a data acquisition system that can float several hundred


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Another easy alternative is to use a data acquisition system that can float several hundred volts. Notice that we can also minimize the noise by minimizing RS. We do this by using larger thermocouple wire that has a smaller series resistance. Also, to reduce the possibility of magnetically induced noise, the thermocouple should be twisted in a uniform manner. Thermocouple extension wires are available commercially in a twisted pair configuration.


We have discussed the concepts of the reference junction, how to use a polynomial to extract absolute temperature data and what to look for in a data acquisition system to minimize the effects of noise. The polynomial curve fi t relies upon the thermocouple wirebeing perfect; that is, it must not become decalibrated during the act of making a temperature measurement. We shall now discuss some of the pitfalls of thermocouple thermometry.

Aside from the specified accuracies of the data acquisition system and its isothermal reference junction, most measurement error may be traced to one of these primary

1. Thermocouple – sources of errors

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reference junction, most measurement error may be traced to one of these primary sources:

1. Poor junction connection2. Decalibration of thermocouple wire3. Shunt impedance and galvanic action4. Thermal shunting5. Noise and leakage currents6. Thermocouple specifications7. Documentation


2.Semiconductor Temperature Sensors

Modern semiconductor temperature sensors offer high accuracy and high linearity over an operating range of about –55°C to +150°C. Internal amplifiers can scale the output to convenient values, such as 10mV/°C. They are also useful in cold-junction compensation circuits for wide temperature range thermocouples.

All semiconductor temperature sensors make use of the

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All semiconductor temperature sensors make use of the relationship between a bipolar junction transistor's (BJT) base-emitter voltage to its collector current:

where k is Boltzmann's constant, T is the absolute temperature, q is the charge of an electron, and Is is a current related to the geometry and the temperature of the transistors. (The equation assumes a voltage of at least a few hundred mV on the collector, and ignores Early effects.)


2.Semiconductor Temperature Sensors

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2. Semiconductor Temperature Sensors

If we take N transistors identical to the first and allow the totalcurrent Ic to be shared equally among them, we find that the new base-emittervoltage is given by the equation:

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Neither of these circuits is of much use by itself because of the strongly temperaturedependent current Is, but if we have equal currents in one BJT and N similar BJTsthen the expression for the difference between the two base-emitter voltages isproportional to absolute temperature and does not contain Is.

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The circuit shown in the next figure implements the above equation and is known asthe "Brokaw Cell".

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The voltage ∆VBE = VBE – VN appears across resistor R2. The emitter current in Q2 is therefore ∆VBE/R2. The op amp's servo loop and the resistors, R, force the same current to flow through Q1. The Q1 and Q2 currents are equal and are summed and flow into resistor R1. The corresponding voltage developed across R1 is proportional to absolute temperature (PTAT) and given by:

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The bandgap cell reference voltage, VBANDGAP, appears at the base of Q1 and isthe sum of VBE(Q1) and VPTAT. VBE(Q1) is complementary to absolutetemperature (CTAT), and summing it with VPTAT causes the bandgap voltage to beconstant with respect to temperature (assuming proper choice of R1/R2 ratio and Nto make the bandgap voltage equal to1.205V). This circuit is the basic band-gaptemperature sensor, and is widely used in semiconductor temperature sensors.



An innovation in thermometry is the IC (Integrated Circuit) temperature transducer. Theseare available in both voltage and current-output configurations. Both supply an outputthat is linearly proportional to absolute temperature. Typical values are 1 µA/K and 10 mV/K F. Except that they offer a very linear output with temperature, these IC sensors share all the disadvantages of thermistors. They are semiconductor devices and thus have a limited temperature range. The same problems of self-heating and fragility are evident and they require an external power source.

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These devices provide a convenient way to produce an easy-to-read output that is proportional to temperature

=> Used for thermocouple compensation.



Current output

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Proportional voltage output

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Absolute voltage output

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Absolute voltage output

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Digital output

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Digital output

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Interfatarea cu un microcontroller

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Preset setpoint

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Programmable setpoint

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10bit digital sensor

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Temperature Sensors - overview

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Temperature Sensors - overview

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IV_004. Sensors (IC and Thermocouple [Reduced]