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Difference between thermocouple and thermal resistance and design
Dec 26, 2017

Difference between thermocouple and thermal resistance and design

Thermocouple and thermal resistance are temperature measurement of contact temperature measurement,

 although its role is the same as measuring the temperature of the object, but their principles and

 characteristics are not the same.


First of all, introduce the thermocouple, thermocouple temperature measurement is the most widely

 used temperature device, his main feature is a wide range of kiss test, performance is relatively

 stable, while simple structure, good dynamic response, more remote 4-20mA Signal, easy to 

automatic control and centralized control. Thermocouple temperature measurement principle is based

 on the thermoelectric effect. Two different conductors or semiconductors connected into a closed 

loop, when the two contacts at different temperatures, the circuit will produce thermoelectric 

power, a phenomenon known as the thermoelectric effect, also known as the Seebeck effect. 

Thermocouples generated in closed loops are composed of two potentials: thermoelectric potential

 and contact potential. Temperature difference potential is the same conductor at both ends of the 

potential difference due to temperature potential, different conductors have different electron

 density, so they produce the potential is not the same, and the contact potential as the name 

suggests refers to two different conductors in contact, Because they have different electron 

densities, they produce certain electron diffusion. When they reach a certain equilibrium, the 

electric potential is determined by the material properties of two different conductors and the 

temperature of their contact points. At present the international application of the thermocouple

 has a standard specification, the international provisions of the thermocouple is divided into 

eight different divisions, respectively, B, R, S, K, N, E, J and T, the minimum temperature 

measurement B, R, S belong to platinum series of thermocouple, platinum is precious metals, so they

 are also known as the precious metal thermocouple and the remaining few are known as the cheap 

metal thermoelectricity I. Thermocouple structure, there are two, common type and armored type. 

General thermocouple generally by the hot electrode, insulation tube, protective sleeve and 

junction box and other components, and armored thermocouple is the thermocouple wire, insulation

 and metal protection casing combination of the three, after pulling Stretching from a solid 

combination of body. But the thermocouple of the electrical signal is the need for a special wire 

to be transmitted, this wire we call the compensation wire. Different thermocouples require 

different compensation wire, its main role is to connect with the thermocouple, so that the 

reference end of the thermocouple away from the power supply, so that the reference junction 

temperature stability. Compensation wire is divided into two types of compensation and extended 

type, extension of the chemical composition and compensation of the same thermocouple, but in 

practice, the extension of the wire is not the same material and thermocouple metal, the general

 use and thermoelectric Even with the same electron density of the wire instead. Compensation wire

 and the thermocouple connection is generally very clear, the thermocouple cathode connection 

compensation wire red line, and the negative pole is connected to the remaining color. Most of the

 general compensation wire material used copper-nickel alloy.


Second, we introduce the thermal resistance, thermal resistance, although widely used in industrial

 applications, but because of his temperature range of his application by a certain limit, the 

principle of thermal resistance is based on the resistance of the conductor or semiconductor 

resistance The temperature changes with the characteristics of change. Its advantages are many, can

 also be far from electrical signals, high sensitivity, stability, interchangeability and accuracy

 are better, but the need for power excitation, can not instantaneous measurement of temperature 

changes. General use of industrial thermal resistance Pt100, Pt10, Cu50, Cu100, platinum thermal 

resistance of the temperature range is generally minus 200-800 degrees Celsius, copper heat 

resistance is minus 40 to 140 degrees Celsius. Thermal resistance and thermocouple the same type

 of distinction, but he does not need to compensate for the wire, and even cheaper than the hot 

spots.

    The following is a 14-bit 4-20mA loop-powered thermocouple temperature measurement system 

circuit design, the circuit is a complete loop-powered thermocouple temperature measurement system,

 the use of precision analog microcontrollers PWM function control 4 mA To 20 mA of output current

. The advantage of a PWM-driven 4mA to 20mA loop with a higher resolution supports T-type 

thermocouples with a temperature range of -200 ° C to + 350 ° C.


Circuit functions and advantages


The circuit shown in Figure 1 is a complete loop-powered thermocouple temperature measurement 

system that uses a precision analog microcontroller PWM function to control the 4 mA to 20 mA 

output current.



Figure 1. The ADuCM360 controls a 4 mA to 20 mA loop-based temperature monitoring circuit


Circuit Theory: The circuit will be the vast majority of the circuit functions are integrated in

 precision analog microcontrollers ADuCM360, including dual 24-bit Σ-Δ-type ADC, ARM Cortex ™ -M3

 processor core and for the control loop voltage up to 28 V 4 mA to 20 mA loop PWM / DAC

 characteristics, providing a low-cost temperature monitoring solution. The ADuCM360 is connected 

to a T-type thermocouple and a 100 Ω platinum resistance temperature detector (RTD). RTD is used

 for cold junction compensation. The low-power Cortex-M3 core converts the ADC readings to

 temperature values. The supported T-type thermocouple temperature range is -200 ° C to + 350 ° C

, and this temperature range is 4mA to 20mA. This circuit has the advantage of driving a 4mA to 

20mA loop with a higher resolution PWM. The PWM-based output provides 14-bit resolution. The 

circuit is powered by a linear regulator, the ADP1720, which regulates the loop-plus power supply 

to 3.3 V to supply power to the ADuCM360, op amp OP193, and the optional reference ADR3412.


The following analysis of the temperature measurement system constant current source signal conditioning analog-to-digital conversion of the functional circuit works and design basis, and gives the circuit parameters. Temperature is one of the four parameters of the chemical production process, temperature sensor Pt100 has the characteristics of small volume, high accuracy and stability, and is widely used in the temperature measurement below 650 ℃.

Pt100 temperature measurement system to C8051F410 system-on-chip as the control core, the system block diagram shown in Figure 1. Including the C8051F410 system-on-chip constant current source drive circuit signal acquisition circuit signal conditioning circuit shows the circuit circuit and the power supply circuit 7 components.

The constant current source drives the temperature sensor Pt100, converts the resistance signal 

(80.31 ~ 280.98) into the weak voltage signal (0.08 ~ 0.28V), transforms into 0 ~ 2 through the 

signal conditioning circuit. 2V voltage signal, C8051F410 on-chip system A / D on the voltage 

signal is sampled, according to the Pt100 index table and the relationship between sampling

 voltage and resistance, through software processing to obtain accurate measured temperature value,

 in its corresponding Of the digital into the display circuit for display.


Hardware circuit design


C8051F410 On-chip System Basic Peripheral Interface


C8051F410 on-chip system is the temperature control system of the core. The basic peripheral 

interface circuit is to ensure the normal work of the auxiliary circuit, including the C8051F410 

chip system work required JATG interface power decoupling reference filter power-on reset and other

 basic external circuit, as shown in Figure 2.


Figure 2 C8051F410 basic peripheral circuit


Constant current source circuit


Constant current source drive circuit is responsible for driving the temperature sensor Pt100,

 its perception of temperature-dependent resistance signal into a measurable voltage signal, to

 achieve R-V conversion. Constant current source circuit from the OP07 reference voltage source 

resistor and transistor components, in order to avoid the heat generated by the excitation current

 measurement accuracy, the design of the constant current source size of 1mA. Specific

 implementation shown in Figure 3. VZ1 is a 2.5V regulator tube, that U1-U3 = 2.5V; According to

 the principle of virtual amplifier short, the op amp OP07 inverting terminal voltage and inverting

 terminal voltage equal to U2 = U3; The voltage U1-U2 across resistor R31 = 2.5V; the current 

through R31 can be calculated to be 2. 5V / 2. 5K = 1mA. According to the op amp fault principle,

 we can see that the current flowing through the R31 will almost all flow into the composite tube

 without flowing into the reverse side of the op amp, the composite tube collector output current 

of 1mA, to achieve a high precision constant current source design .


Figure 3 constant current source circuit


The key points in the design: LM336 provides a precision 2.5V voltage reference; R31 is a good

 precision wire wound resistance; Flow value is stable.


Add: No. 16, Road 3, Sanjiang Industry Zone, Shengzhou, Zhejiang, China

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Email: boris@benoelectric.com

Contact:Charles Yu     

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