Temperature transmitters are widely used. Due to the wide variety of operating environments, field conditions, and supporting instruments, engineers, technicians, and maintenance personnel often encounter various problems during operation. Based on years of practical experience, the author briefly analyzes several major causes of temperature transmitter failures.
I. Faults Caused by Temperature Sensors
These are common and easy-to-diagnose faults. When abnormal output from the temperature transmitter occurs, first check whether the temperature sensor is faulty. Assuming the transmitter circuit is normal, the following situations may apply:
1. Open Circuit of the Temperature Sensor
Temperature transmitters are equipped with a sensor burnout alarm function.Whether the front end is connected to an RTD or a thermocouple, the transmitter output will drop below the standard signal, i.e., below 4 mA.The standard burnout alarm current is 3.75 mA.If the multimeter shows an output current of 3.75 mA and the red LED on the transmitter module flashes, the temperature sensor is open-circuited.The problem can be solved by replacing the front-end probe.
For customers with special requirements for the burnout alarm current due to different host instruments, manufacturers can provide customization.For example, if a burnout alarm current below 3 mA is required, it can be set to 2.95 mA or even lower while ensuring accuracy.
2. Short Circuit of the Temperature Sensor
In this case, the temperature transmitter output is usually unstable and abnormal, similar to "garbled data" in software.Due to the short circuit, the voltage fed into the MCU after constant-current excitation becomes abnormal.After a series of AD conversion, amplification, and DA conversion, the final output will be an abnormal value.If the front-end circuit is well-designed, the transmitter module will not be damaged; otherwise, the module may be destroyed.
3. Loose Connection / Virtual Open / Virtual Short of the Temperature Sensor
This type of fault causes the transmitter to work intermittently.In most cases, it is caused by poor packaging quality of the temperature sensor.Replacing the probe will resolve the issue.
II. Faults Caused by Power Supply
The normal power supply range for temperature transmitters is 9–30 VDC or 8.5–30 VDC.12 VDC and 24 VDC switching power supplies are commonly used in the field.Under normal conditions, the power supply will not damage the transmitter.However, power supply problems are a common cause of transmitter failure.
1. Low Supply Voltage
Transmitter power circuits are generally designed with margins.If the voltage is 2–3 VDC below the rated value (low-power transmitters can even work at 5 VDC or 3.3 VDC depending on the output type), the transmitter can operate normally as long as power consumption is satisfied.If power is insufficient, the transmitter will not function properly but will not be damaged.
2. High Supply Voltage
Generally, the voltage must not exceed 32 VDC.Exceeding this value will almost certainly damage the transmitter.Even if components are not immediately burned out, service life will be significantly reduced.
3. Shared Power Supply Issues
It is common for multiple devices to share the same power supply in a system.Normally, devices with similar power consumption operate without interference.However, high-power equipment or frequently started/stopped devices may cause charge accumulation (interference) or even surges.Therefore, during circuit design, engineers should analyze the equipment and instruments used, and supply power to different types of devices separately to avoid mutual interference.
III. Damage Caused by Surges
Surges are a common hidden danger that damages temperature transmitters.
Surge Definition:A surge, or spike, is a temporary overvoltage exceeding the normal operating voltage.Essentially, a surge is a sharp pulse occurring within just a few microseconds.Common causes include heavy machinery, short circuits, power switching, or large motors.Products equipped with surge suppression devices can effectively absorb sudden high energy to protect connected equipment.
Given the destructive nature of surges, it is understandable that they often damage temperature transmitters.If such conditions exist in your system or equipment, you should not only use isolated temperature transmitters but also implement proper grounding, insulation, shielding, and protective circuits.Other equipment in the system is also vulnerable to surge damage.
IV. Problems Caused by Electromagnetic Interference (EMI)
Large motors, heavy machinery, reactors, electrical equipment, transmission lines, radio devices, and even passing large equipment can generate electromagnetic fields, resulting in conducted or radiated electromagnetic interference.EMI types are diverse and difficult to list completely.
Experienced engineers and technicians must carefully analyze the on-site environment and take necessary measures.Electromagnetic interference should be considered a key prevention point during the design phase to avoid problems in advance and reduce troubles during later operation.