In-depth article: Analysis of soft breakdown faults in thermal resistors

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Temperature plays an extremely important role in the control of industrial production processes. Temperature measurement systems provide accurate data for monitoring and automatic control during industrial production, reflecting the temperature levels at the measurement points and ensuring the safe, economical, and efficient operation of production. A temperature measurement system mainly consists of a temperature-sensitive element, a conversion unit, and a display or control unit. Generally, the conversion unit and the display or control unit are implemented using electronic circuits, and their fault analysis follows the principles of conventional circuit analysis. This article provides a brief analysis of the common faults in thermal resistance temperature elements. Soft breakdown of thermal resistors: yunrun.com.cn/tech/2058.html. Working principle of thermal resistor elements: The most common types of thermal resistor elements are copper thermal resistors and platinum thermal resistors. Since the electrical conductivity of these two metals changes in a nearly linear manner with temperature, the resistance value of the element is measured in order to determine the temperature. The resistance value of a temperature element at 0°C is known as the element’s calibration value; the most common types today are Cu50 and Pt100. Classification of common faults in thermistor temperature sensing elements. Thermistor temperature measurement systems operate in a relatively stable and reliable manner, with the average service life of thermistor elements reaching 2 years. Some common faults can be classified into the following 4 categories: 1. Open circuit – The thermistor element is disconnected, and the fault is typically manifested as an overflow in the displayed data. The reason may be an open circuit in the temperature element, or poor contact or disconnection of the connection wires. Generally, large vibrations at the measurement points or loose fixation of the component protection sleeves can cause the components to break due to vibration or the connections to become loose during operation of the system, leading to this fault. 2. Short circuit of the temperature element: This refers to a complete or partial short circuit of the temperature element, and the fault is typically manifested by a negative temperature reading. A poor quality temperature element, excessive vibration in the operating environment, or a short circuit in the connection wires can cause this fault to occur. 3. Wear and tear refers to the soft breakdown of thermal resistors; the fault is manifested by the display being normal at times and abnormal at other times. The abnormal conditions include fluctuations or inaccurate readings that deviate from the correct values. 4. Penetration occurs frequently in temperature measurement environments containing flowing solid particles; under negative pressure, the displayed value deviates from the actual value and approaches room temperature. This is because the protective sleeve of the component is worn through by the flowing solid medium, resulting in air leakage that causes the measured temperature to lean towards room temperature. If the actual temperature is higher than room temperature, it will display a lower temperature, and vice versa. Under positive pressure, contamination of the sensing element by the testing medium can affect the measurement. If the medium is conductive, a short circuit will occur ； If the medium does not conduct electricity, the phenomenon is not obvious. The phenomenon of soft breakdown in thermal resistors: After a soft breakdown occurs in a thermal resistor, the situation is rather strange. When the fault arises, the maintenance interval is sometimes as short as 1 week; fixing the problem requires simply replacing the component, and the replaced component turns out to be completely normal upon testing. Even without removing the components, it is normal to use a multimeter to measure the resistance value in the operating environment; the temperature value obtained from the resistance measurement corresponds to the actual temperature. The entire system can return to normal when power is restored. When the system malfunctions again, it sometimes shows an overflow, sometimes instability, and sometimes low readings. Characteristics of soft breakdown in thermal resistors. In summary, soft breakdown of temperature sensors has the following key characteristics: 1. This fault is likely to occur when the quality of the platinum resistance element is poor. Statistics show that such failures account for 86% of all failures. It can be seen from this that the poor quality of platinum resistance elements is the main cause of soft breakdown in these elements. Poor quality of platinum resistance elements is manifested in low insulation of the frame, poor quality of the platinum (copper) metal, and low manufacturing standards. 2. This fault is likely to occur when the operating temperature of the thermal resistance temperature element exceeds the upper limit of its operating range or is near that limit. If a Pt100 is frequently operated in an environment of 500°C, the likelihood of it failing is very high. 3. The thermoresistive element can return to normal after being removed from its operating environment, it can function properly at low temperatures, and the soft breakdown fault disappears; this is an important characteristic of soft breakdown in temperature sensors. The electrical mechanism of soft breakdown in thermal resistors: Once the temperature element of a thermal resistor suffers soft breakdown, it loses its original electrical properties. The thermistor element should possess the basic characteristics of a normal resistor at any temperature within its normal operating range. After the thermal resistor undergoes soft breakdown, its U-I curve is no longer linear; when the current reaches a certain value, the insulation quality declines, the thermal resistor breaks down, its resistance value drops, and there is no longer any correlation between the resistance value and temperature. The actual UI characteristic curve of the thermistor soft-breakdown element is shown in the figure below. Based on actual measurements, the breakdown temperature of the thermistor that has suffered from soft breakdown failure is approximately 350°C, with the minimum breakdown current ranging from 0.6 to 0.8 mA. The measurement current in the typical measurement circuit is 0.06 mA; in some cases, to increase sensitivity, the current can even exceed 1 mA. Therefore, for temperature sensors of lower quality, soft breakdown failures are likely to occur when the current in the measurement circuit exceeds 0.06 mA under high operating temperatures. When the loop current is less than 0.6 mA, the soft breakdown phenomenon disappears; since the resistance setting of a multimeter measures the loop current at around 0.1 mA only, it is difficult to detect faults in components suffering from soft breakdown using a multimeter. Solutions to thermal resistance temperature measurement faults: Appropriate actions are taken based on the various principles behind these faults, as shown in Table 1. Fault classification; Solutions to thermal resistance temperature measurement faults: Open circuit – Strengthen the protective casing around the thermal resistance element, select measurement points with minimal vibration, use vibration-damping components, and secure the signal line connections. Short circuit – Use high-quality, long-lasting externally wound, fully sintered platinum resistance elements to reduce vibration. Interference – Replace with high-quality externally wound, fully sintered platinum resistance elements, and choose the appropriate model of platinum resistor. Leakage – Use wear-resistant casings, adjust the installation angle, and install protective devices. Author: Xu Fengquan, Zhongzhou Branch of China Aluminium Corporation