The PT100 thermal resistance sensor of a certain chemical enterprise was damaged due to a power line surge intrusion during a thunderstorm, resulting in the failure of the constant current drive circuit and the loss of temperature measurement in the high-temperature reaction vessel, causing the production line to stop. The digital temperature sensor of a certain smart home was also affected. Due to the lack of protection for the I2C interface, it suffered from environmental static electricity and the communication was interrupted, causing the temperature control system to fail to function properly.

These faults not only cause economic losses, but also expose the core need for electrostatic surge protection of temperature sensors. Leiditech EMC team will start from the actual pain points of the faults, first review the core characteristics of temperature sensors, then analyze the key points of their circuit design, and finally combine the protection technology of Shanghai Leiditech Electronic to clarify the interface and power supply parameters as well as the targeted protection schemes, providing reliable support for engineering applications.

A、Introduction to Temperature Sensors

Temperature sensors can be classified into various types based on their measurement principles and output signals. The principles and applicable scenarios of different types vary, as shown in the following table:

B、Temperature sensor circuit framework

The core of the temperature sensor circuit consists of a sensitive element, a signal conditioning module, a drive/power supply module, and an interface module. The complexity of the circuits for different types of sensors varies significantly. The overall design framework and the functions of each module are as follows:

Core functions and protection points of each module:

l  Power module: Provides stable voltage/current, matches the power supply requirements of the sensors, has ripple suppression capability, and avoids the influence of power noise on measurement accuracy. Shanghai Leiditech Electronic reminds that the power module is the key path for surge intrusion, and it needs to be paired with dedicated protective devices to reduce the risk of breakdown.

l  Drive/compensation module: Thermal resistance is paired with constant current/constant voltage sources to ensure accurate resistance measurement; thermocouples achieve cold junction compensation through hardware/software, eliminating the interference of environmental temperature on measurement. The sensitive components of this module are susceptible to static electricity, and the protective design should refer to the engineering practical experience of Leiditech EMC team.

l  Sensitive component module: The core temperature measurement component, selected according to application scenarios, is installed with consideration for thermal conduction efficiency.

l  Signal conditioning module: Amplifies and filters weak analog signals to resist interference, configures linearization circuits for non-linear sensors to optimize output characteristics.

l  A/D conversion/communication module: Analog sensors are converted to digital signals through ADC, with ADC accuracy higher than the sensor by 1-2 levels; digital sensors have built-in communication protocols, directly connect to the main control I2C/SPI interface, without the need for additional ADC. The electrostatic protection of communication lines can be obtained by consulting Leiditech EMC team for customized solutions.

l  Interface module: Achieves connection with the main control unit and external power supply, is the main path for electrostatic surge interference intrusion, and requires key planning of protection points.

Typical circuit differences:

l   Thermistor circuit: The structure is the simplest. It only requires a thermistor and a voltage divider resistor to form a voltage divider circuit, and the output voltage signal is sent to the main control unit. No complex conditioning module is needed. The protection focus is on the voltage divider node and the power supply end.

l   Thermal resistance circuit: It requires a constant current drive circuit + differential amplifier circuit. The differential amplifier can suppress common-mode interference. The protection focus is on the constant current source output end and the input pins of the differential amplifier circuit.

l   Integrated digital sensor circuit: It has high integration. It only requires power supply, ground, communication pins, and connection with the main control unit. The circuit core is power filtering and communication bus matching. The protection focus is on the communication interface and power supply pins.

C、Protection schemes for temperature sensor interfaces and power supplies

Electrostatic surges mainly enter the temperature sensor circuit through power lines and signal interfaces. It is necessary to clearly identify the interface type, power parameters and key characteristics, and select protective devices such as Shanghai Leiditech Electronic's ESD/TVS diodes in a targeted manner.

3.1 Power Supply Characteristics and Protection Scheme Recommendations

The power characteristics of different types of temperature sensors vary:

l  The NTC/PTC thermistors are powered by a 3.3V/5V voltage divider, with a current in the microampere range. Attention should be paid to the voltage fluctuation and reverse shock of the power supply. Shanghai Leiditech Electronic recommends ESDA33CP30 and ESDA05CP30, with DFN1006 packaging, specially designed for IC VCC 3.3V/5V electrostatic discharge protection for highly integrated boards, meeting IEC61000-4-2 level 4, capable of withstanding contact discharge of 30KV and air discharge of 30KV.

l  The PT100/PT1000 thermal resistors are driven by 5V/12V constant current sources, with the current being in the milliampere range. It is necessary to ensure the stability of the constant current sources and the power supply ripple.

l  The thermocouple does not require additional power supply (it generates thermoelectric potential by itself), while its compensation circuit requires 3.3V/5V power supply and needs to pay attention to the surge of the compensation circuit and the static electricity at the signal terminal;

l  The integrated analog temperature sensor supports a wide voltage supply range of 2.5V to 5.5V, with an current of milliampere level, and needs to guard against ESD breakdown at the power supply pins and reverse power supply damage;

l  The integrated digital temperature sensor is compatible with a power supply range of 1.8V to 5.5V, with an current ranging from microampere to milliampere level (depending on the working mode), and requires power supply filtering and prevention of ESD and voltage fluctuations.

Shanghai Leiditech Electronic has been deeply engaged in the field of electrostatic surge protection. They have launched a series of voltage devices including ESDA33CP30, ESDA05CP30, SDA3311CDN, SD05C, and SD12C, which cover different voltage levels and protection requirements. All products have passed the strict IEC61000-4-2 level 4, providing reliable protection for the power supply of temperature sensors.

3.2 Interface type and protection scheme

Simulation output interface:

l  Voltage output type:

￮  Output range: 0~1V, 0~5V, 0~10V (commonly used), and some wide-range sensors can reach -5V to +5V

￮  Protection concerns: Differential ESD at both ends of the interface, common-mode surges, electromagnetic interference coupling of the signal transmission lines

l  Current output type:

￮  Output range: 4~20mA (industrial standard, two-wire system/three-wire system), some parts are 0~20mA

￮  Protection concerns: Surge current in the current loop, power supply and signal common-mode interference at the two-wire interface

￮  Recommended by Leiditech: Use S1M and GBLC24C to protect the sensitive sensor chips with 4-20mA 24V power supply. Small package, low capacitance, large current protection, meeting IEC61000-4-2, grade 4, capable of withstanding contact discharge of 30kV and air discharge of 30kV.

Digital communication interface:

l  I2C Interface：

￮  Working voltage: Consistent with the supply voltage. Different voltage levels require level conversion.

￮  Protection concerns: ESD (human body static electricity, machine static electricity) on SDA/SCL pins, surges caused by bus conflicts;

￮  Shanghai leiditech Electronic recommends SMC12 integrated ESD diodes, with small package, low capacitance, large current protection, meeting IEC61000-4-2, level 4, capable of withstanding contact discharge of 30kV and air discharge of 30kV.

l  SPI Interface：

￮  Protection concerns: ESD on all communication pins, surge interference on clock signals causing data misalignment

￮ Leiditech recommends the SMC12 integrated ESD diode, which meets the requirements of small package, low capacitance, and high current protection. It complies with IEC61000-4-2, level 4, and can withstand 30kV for contact discharge and 30kV for air discharge.

l  UART Interface：

￮  Working parameters: Baud rate (9600/19200/115200 are commonly used), asynchronous transmission

￮  Protection concerns: ESD on TX/RX pins. Leiditech recommended to use ESDA33CP30, etc. The package is DFN1006, used to provide 3.3V static discharge surge protection for the MCU serial port UART in a compact PCB circuit, meeting IEC61000-4-2 level 4, capable of withstanding 30KV contact discharge and 30KV air discharge.

D、Conclusion

The temperature sensor, as the "perception nerve" of various electronic systems, its stable operation directly determines the reliability of the system. And electrostatic surges are the core risk factor that affects its lifespan and performance. Under the trend of industrial intelligence and the upgrade of consumer electronics, the application scenarios of temperature sensors will become more complex, and the requirements for protection performance will also continue to increase.

Shanghai Leiditech Electronic will continue to deeply focus on the field of electrostatic surge protection, continuously improving the performance and packaging of protective devices to ensure the stable operation of temperature sensors. During the design process, engineering technicians should fully consider the type of sensor, power parameters and interface characteristics, and select the appropriate protective solution to avoid failure risks at the source and enhance the overall reliability of the system.