Temperature transmitters are widely used. Because of the huge differences in the use environment, site, and supporting instruments, engineers, technicians, or maintenance personnel have encountered various problems during their use. Here, the author combines years of work experience to briefly analyze the main reasons for the temperature transmitter failure.1. Failure caused by temperature sensorThis is a common and easy-to-judge fault. In the course of use, once the output of the temperature transmitter is abnormal, first check whether the temperature sensor is malfunctioning. When the temperature transmitter circuit is normal, there are the following situations.(1) The temperature sensor is open. The temperature transmitter has a temperature sensor fuse alarm function. At this time, regardless of whether the front end of the transmitter is connected to a thermal resistance or a thermocouple, it will show that the output value of the transmitter is less than the standard signal, that is, less than 4mA. The current standard fusing alarm current is 3.75mA. When testing the output of the temperature transmitter, the current value displayed by the multimeter is 3.75mA. At the same time, the red light of the transmitter module flashes to determine that the temperature sensor is open. Replace the probe at the front end. Can be resolved.Some customers have special requirements for the fuse alarm current due to the difference of the upper instrument. The manufacturer can customize it. For example, if the fuse alarm current is required to be less than 3mA, it can achieve 2.95mA or even lower under the condition of ensuring accuracy.(2) The temperature sensor is short-circuited. At this time, the output value of the temperature transmitter is generally irregular, which is an abnormal value, which can be understood as “garbled” in the software. In fact, due to the short circuit of the temperature sensor, the voltage flowing into the microcontroller after being excited by the constant current source may be an abnormal voltage value. After a series of AD conversion, amplification, and DA conversion, the final output is an abnormal value. If the front-end circuit is handled well, the temperature transmitter module will not be damaged, and the circuit that is not handled well will damage the module.(3) The temperature sensor is “virtually broken and short”. In this case, the temperature transmitter is generally normal and abnormal from time to time. Most of the reasons are the quality of the temperature sensor package, which can be solved by replacing the probe.2. Failure caused by power supplyThe normal power supply range of temperature transmitters is 9-30VDC, or 8.5-30VDC, and 12VDC and 24VDC DC switching power supplies are used more frequently by customers on site. Under normal circumstances, the power supply will not cause damage to the temperature transmitter. If there is a problem with the power supply, it is likely to damage the temperature transmitter.(1) The power supply voltage is low. The design of the temperature transmitter power supply circuit generally leaves a margin, if it is lower than the standard power supply voltage 2～3VDC (of course, the low power temperature transmitter can achieve 5VDC power supply according to different output, or even 3.3 VDC power supply), the temperature transmitter can work normally under the condition of ensuring the normal power consumption of the temperature transmitter. Even if the power consumption required for the normal operation of the temperature transmitter cannot be met, the temperature transmitter will not work normally and will not be damaged.(2) The power supply voltage is too high. Under normal circumstances, the voltage cannot exceed 32VDC, which will damage the temperature transmitter. Even if there are no components in the power circuit burned out, it will reduce its service life.(3) The problem of shared power supply. In the system, it is very common that most devices share the same power source. Under normal circumstances, devices of the same power consumption level will basically be safe, and there are high-power devices or devices that constantly start and stop in the system, which may cause interference due to charge accumulation and surge. Therefore, when engineers design circuits, they analyze the equipment and instruments used under specific analysis, and separate different types of equipment and instruments to supply power so that they do not interfere with each other and do not affect each other.3. Surge disasterSurge is a common black hand that damages temperature transmitters. The definition of surge is as follows. Surge is also called surge, as its name implies, it is an instantaneous overvoltage that exceeds the normal operating voltage. Essentially, a surge is a violent pulse that occurs in just a few millionths of a second. The possible causes of surges are: heavy equipment, short circuits, power switching or large engines. The products containing surge blocking devices can effectively absorb sudden huge energy to protect the connected equipment from damage.After reading the above definition, I don’t need to elaborate on the lethality of surges. I guess you should think that it is normal to damage the temperature transmitter! If the above conditions exist in your system or equipment, not only choose an isolated temperature transmitter, but also take various protective measures such as grounding, insulation, shielding, and protective circuit. Because in addition to temperature transmitters, other devices in the system may not be immune to surge disasters.4. Trouble with electromagnetic interference
Large motors, large machinery, reactors, power equipment, transmission lines, radios, and even large equipment that pass by accidentally can generate electromagnetic fields. Basically, there will be conduction or radiation of electromagnetic waves. There are many types of electromagnetic interference, and there is no way to describe them. Therefore, experienced engineers or technicians must carefully analyze their on-site environment and take necessary measures. At the beginning of the design, electromagnetic interference was taken as the focus of prevention, to prevent problems before they happened, and strive to reduce the trouble in the subsequent use process.
Post time: 21-09-21