Thermocouples and RTDs are contact temperature measurements in temperature measurement. Although their functions are the same as measuring the temperature of objects, their principles and characteristics are not the same.
First of all, introduce thermocouples, thermocouples are the most widely used temperature devices in temperature measurement. His main features are wide range of kisses, stable performance, simple structure, good dynamic response, and better remote transmission of 4-20mA. Signal for easy automatic control and centralized control. The thermometry principle of thermocouples is based on the thermoelectric effect. Connecting two different conductors or semiconductors into a closed loop, when the temperatures at the two junctions are different, a thermoelectric potential will be generated in the loop. This phenomenon is called the thermoelectric effect, also known as the Seebeck effect. The thermoelectric potential generated in the closed loop consists of two potentials: the temperature difference potential and the contact potential. The thermoelectric potential refers to the potential generated by the temperature difference between the two ends of the same conductor. Different conductors have different electron densities, so the potentials they generate are different. The contact potential, as the name suggests, refers to the contact of two different conductors. Because of their different electron densities, they generate a certain amount of electron diffusion. When they reach a certain equilibrium, the potential of the contact potential depends on the material properties of the two different conductors and the temperature of their contact points. At present, the internationally applied thermocouple has a standard specification. The international thermocouple is divided into eight different scales, namely B, R, S, K, N, E, J and T, which can measure the lowest temperature. Measured at 270 degrees Celsius, up to 1800 degrees Celsius, where B, R, S belong to the platinum series of thermocouples. Because platinum is a precious metal, they are also called precious metal thermocouples and the remaining few are called cheap metal thermoelectrics. I. There are two types of thermocouples, normal and armored. Ordinary thermocouples are generally composed of hot electrodes, insulating tubes, protective sleeves and junction boxes, while armored thermocouples are assembled by combining thermocouple wires, insulating materials and metal protective sleeves. Stretched into a solid combination. However, the electrical signal of the thermocouple requires a special wire for transmission. This wire is called a compensation wire. Different thermocouples require different compensation wires. Their main function is to connect with the thermocouple, so that the reference end of the thermocouple is far away from the power supply, so that the reference junction temperature is stable. The compensation wire is divided into two types: compensation type and extension type. The chemical composition of the extension wire is the same as that of the compensated thermocouple. However, in practice, the extension type wire is not made of the same material as the thermocouple, and is generally used and thermoelectric. Instead of wires with the same electron density. The connection of the compensation wire to the thermocouple is generally very clear. The positive terminal of the thermocouple is connected to the red line of the compensation wire, and the negative electrode is connected to the remaining color. Most of the materials for the compensation wires are made of copper-nickel alloy.
Secondly, we introduce the thermal resistance. Although the thermal resistance is widely used in industry, its temperature range is limited by his temperature range. The temperature measurement principle of the thermal resistance is based on the resistance value of the conductor or semiconductor. The characteristic that changes with the change of temperature. It has many advantages, and it can also transmit electrical signals remotely. It has high sensitivity, strong stability, interchangeability and accuracy, but it requires power supply excitation and cannot instantaneously measure temperature changes. Industrial thermal resistance is generally Pt100, Pt10, Cu50, Cu100, the temperature range of platinum thermal resistance is generally minus 200-800 degrees Celsius, copper thermal resistance is minus 40 to 140 degrees Celsius. Thermistor is the same type of distinction as the thermocouple, but it does not require a compensation wire and is cheaper than a hot spot.
The following is a circuit diagram of a 14-bit 4-20mA loop-powered thermocouple temperature measurement system. This circuit is a complete loop-powered thermocouple temperature measurement system that uses a PWM function of a precision analog microcontroller to control 4 mA. Output current to 20 mA. The higher resolution PWM drives the 4mA to 20mA loop and supports T-type thermocouples from −200° C to +350° C.
Circuit function and advantage
The circuit shown in Figure 1 is a complete loop-powered thermocouple temperature measurement system that uses a PWM function of a precision analog microcontroller to control the 4 mA to 20 mA output current.
Figure 1. ADuCM360 Controls 4 mA to 20 mA Loop-Based Temperature Monitoring Circuitry
Circuit Theory: This circuit integrates most of the circuit functions into the precision analog microcontroller ADuCM360, including dual 24-bit Σ-Δ ADCs, ARM Cortex TM-M3 processor cores, and control loop voltages up to 28 The PWM/DAC feature of V's 4 mA to 20 mA loop provides a low-cost temperature monitoring solution. Among them, ADuCM360 is connected to a T-type thermocouple and a 100Ω platinum resistance temperature detector (RTD). RTD is used for cold junction compensation. The low-power Cortex-M3 core converts ADC readings to temperature values. Supported T-type thermocouples have a temperature range of −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 higher resolution PWM. The PWM-based output provides 14-bit resolution. The circuit is powered by the linear regulator ADP1720, which regulates the loop supply to 3.3 V, powering the ADuCM360, the op amp OP193, and the optional reference ADR3412.
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