In fields such as industrial automation, power monitoring, and communication networks, optical fiber sensors have become crucial monitoring devices due to their high precision, resistance to electromagnetic interference, and long-distance transmission capabilities. According to the latest data, approximately 75% of electronic equipment failures are related to voltage transients and surges, which pose a significant challenge to the stability of optical fiber sensors. Within this blog we will combine the protection technology experience of Shanghai Leiditech Electronic, to reveal the key points and solutions for electrostatic surge protection in the circuit systems of optical fiber sensors.

1、The key aspect of protecting the circuit system of the optical fiber sensor

The optical fiber sensor circuit system mainly consists of a light source, the optical fiber sensing part, the photodetector, the signal processing circuit and the control output module. In practical applications, the four major sections are most vulnerable to electrostatic surge threats:

Power supply end: 10-30V DC lines are susceptible to high-voltage pulses induced by lightning strikes (up to hundreds of volts) and switching surge impacts. The peak voltage of 220V AC power supply is approximately 310V, and multiple protections are required;

Signal output end: Reverse intrusion of static electricity from the load end (such as sparking at relay contacts) can cause peak currents of several tens of amperes, which can easily break through transistors or damage signal chips;

Control input end: Contact with personnel can generate ±8kV contact discharge and ±15kV air discharge, directly impacting control chips such as MCUs;

Communication interface: Long-distance transmission such as RS-485 and Ethernet is prone to interference. The common-mode voltage tolerance range of RS-485 transceivers is only -7 to +12V, and when there is no low-impedance return path, electromagnetic radiation interference is likely to occur.

2、Power Supply Port Protection Design and TVS Selection Guide

The power supply end is the first line of defense. Shanghai Leiditech Electronic recommends using the components such as ceramic gas discharge tubes (GDT), metal oxide varistors (MOV), and transient voltage suppression diodes (TVS) for targeted protection. The specific protection scheme is as follows:

Ÿ   AC 220V protection: MOV uses 14D471K (with a breakdown voltage of 470V ± 20% and a rated discharge current of 4.5kA), GDT uses 2R600-8L (600V bidirectional, with a rated discharge current of 10kA), meeting the IEC 61000-4-5 4KV~8KV test requirements;

Ÿ   DC 10-30V: Bidirectional TVS diode SMBJ33CA is recommended (covering a working range of 10~30V, peak pulse power of 600W, response time ≤ 1ns), the third stage is paired with self-recovery fuse HL30-300 (rated current 3A), to prevent overcurrent damage.

3、ESD protection techniques for signal end and control end

The signal end and control end are the key areas for electrostatic protection. Shanghai Leiditech Electronic recommends using ESD diodes as the core and providing targeted protection based on the signal type. For the signal end protection, it is necessary to distinguish between analog input, analog output, digital input, and digital output signals, and adopt targeted protection strategies:

Ÿ   Signal output terminal: It is recommended to use ESD diodes such as the SOD-323 series for protection. This series of diodes covers a power voltage range of 5V to 36V, meeting the IEC61000-4-2 standard at level 4 (the actual parameters can reach 30kV for contact discharge and 30kV for air discharge).

Ÿ   Control input terminal: Select low-capacitance ESD diodes such as ESDA05CP30 (3.3V bidirectional, capacitance 12pF, peak current 8A) to precisely protect control chips like the MCU, meeting the requirements of IEC61000-4-2 level 4.

4、Multi-level Protection Scheme for Communication Interfaces

Communication interface protection serves as the bridge for the connection between the optical fiber sensor and the external system, and it is also the main channel through which static surges can invade. Shanghai Leiditech Electronic has designed differentiated multi-level protection strategies for different interfaces:

Ÿ   RS-485/RS422 interface: Utilizes low-residual-voltage TSS components P0080SC (6V bidirectional, 100PF), with a response time reaching the nanosecond level. It also provides protection against surges and static electricity, meeting the IEC61000-4-2 (contact discharge 8kV, air discharge 15kV) and IEC61000-4-5 (surge 10/700μs, 6kV) standards.

Ÿ   Ethernet interface: The secondary protection scheme ensures no packet loss during high-temperature transmission. The interface layer uses low-capacitance GDT 3R090-5S (response time < 100ns, current capacity 5kA), the transformer layer combines Bob Smith terminal (75Ω resistor + 1000pF capacitor), and the chip layer is equipped with GBLC03C low-capacitance ESD diode (capacitance < 0.3pF), meeting the IEC61000-4-2 level 4 and IEC61000-4-5 6kV standards.

The protection design is not a one-time solution. Leiditech EMC team reminds that engineers need to consider the specific application scenarios, rely on the protection components and technical support provided by Shanghai Leiditech Electronics, make reasonable selections and optimize the PCB layout, and continuously improve the protection plan. Only by fully grasping the threats at each stage and the protection techniques can we build a stable and reliable optical fiber sensing system, laying a solid technical foundation for the industrial intelligent transformation.