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
Contact:Eric Xie
Email: sale@benoelectric.com
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