Temperature transmitters are used in a wide variety of applications, because the environment, the field, and the associated instruments vary widely, and engineers, technicians, or maintenance personnel have encountered various problems during use. Here, the author combined with years of work experience, briefly analyze the main causes of temperature transmitter failure.

1. Fault caused by temperature sensor

This is a common fault that is also a good judgment. During use, once the temperature transmitter output is abnormal, first check if the temperature sensor is faulty. In the case where the temperature transmitter circuit is normal, there are the following cases.

(1) The temperature sensor is open. The temperature transmitter has a temperature sensor blown alarm function. At this time, regardless of whether the front end of the transmitter is connected to a thermal resistor or a thermocouple, the transmitter output value is less than the standard signal, that is, less than 4 mA. At present, the standard fuse alarm current is 3.75mA. When the temperature transmitter is tested, the multimeter displays a current value of 3.75mA. At the same time, the red light of the transmitter module flashes to determine that the temperature sensor is open, and the probe at the front end is replaced. Can be solved.

Some customers have special requirements for the fuse alarm current because of the difference of the upper instrument. The manufacturer can customize it. For example, if the fuse alarm current is less than 3 mA, it can be 2.95 mA or even lower with the accuracy guaranteed.

(2) The temperature sensor is short-circuited. At this time, the value output by 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 MCU 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 a badly processed circuit will damage the module.

(3) The temperature sensor is “virtually short and short”. In this case, the temperature transmitter is normally normal and not normal. Most of the reasons are due to the quality of the temperature sensor package, and the replacement of the probe can be solved.

2. Fault caused by power supply

The normal temperature transmitter power supply range is 9~30VDC, or 8.5~30VDC. The 12VDC and 24VDC DC switching power supplies are used by customers. Under normal circumstances, the power supply will not damage the temperature transmitter. If there is a problem with the power supply, it is very likely to damage the temperature transmitter.

(1) The supply voltage is low. The design of the temperature transmitter power supply circuit is generally left with a margin, if it is lower than the standard supply voltage 2~3VDC (of course, the low-power temperature transmitter can do 5VDC power supply according to different output, even 3.3 VDC power supply), the temperature transmitter can work normally while ensuring the normal power consumption of the temperature transmitter. Even if the power consumption required for the temperature transmitter to operate properly is not met, the temperature transmitter will not work properly and will not be damaged.

(2) The supply voltage is too high. Under normal circumstances, the power can not exceed 32VDC, more than the basic damage to the temperature transmitter. Even if there is no component burned out in the power circuit, it will reduce its service life.

(3) The problem of shared power. In a system, it is common for most devices to share the same power source. Under normal circumstances, equipment of the same power consumption level will basically be safe, and it is feared that there will be high-power equipment or equipment that constantly starts and stops in the system, which will cause interference caused by charge accumulation, and will cause surge. Therefore, when designing the circuit, the engineer specifically analyzes the equipment and instrumentation used, and separates different types of equipment and instruments from separate power supply so that they do not interfere with each other and do not affect each other.

3, the disaster of the surge

Surge is a common black hand that damages temperature transmitters. The definition of surge is as follows. Surge is also called a surge, as the name suggests is a transient overvoltage that exceeds the normal operating voltage. In essence, a surge is a violent pulse that occurs in just a few millionths of a second. Possible causes of surges are: heavy equipment, short circuits, power switching, or large engines. Products containing surge arresters can effectively absorb sudden bursts of energy to protect connected equipment from damage.

After reading the above definition, I don't need to elaborate on the lethality of the surge. It is estimated that you should feel that the damage to the temperature transmitter is normal! If the above conditions exist in your system or equipment, you should not only use isolated temperature transmitters, but also various protection measures such as grounding, insulation, shielding, and protection circuits. Because in addition to the temperature transmitter, other devices in the system may not be spared in the event of a surge.

4, the trouble of electromagnetic interference

Large motors, large machinery, reactors, power equipment, transmission lines, radios, and even large-scale equipment that accidentally pass through can generate electromagnetic waves. There are basically electromagnetic wave conduction or radiation. There are many kinds of electromagnetic interference, and there is no way to explain them. Therefore, experienced engineers or technicians must carefully analyze their own site environment and take necessary measures at the site. At the beginning of the design, electromagnetic interference is the focus of prevention, so as to prevent problems before they occur, and strive to reduce the trouble in the subsequent use.

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