In industrial field environments, 4-20mA loop-powered transmitters occasionally experience signal jumps, inaccurate readings, or even complete crashes and burnout. After ruling out wiring errors and sensor faults, we often find that the root cause is electrostatic discharge (ESD) and surge. Industrial environments are complex, and events such as the start/stop of large motors, lightning-induced surges, or human contact can instantly generate voltage spikes as high as thousands of volts. Since transmitters are often mounted on metal pipes or equipment housings, they easily become discharge points for static electricity. Without effective protection circuits, these surges can directly damage sensitive internal chips, causing irreversible hardware damage. For such precision chips, Shanghai Leiditech's TVS devices can be used for surge protection.

I.What is a 4-20mA Loop-Powered Transmitter?

1.Definition and Working Principle

A 4-20mA loop-powered transmitter is an industrial measurement device that converts non-electrical physical quantities (temperature, pressure, flow rate, etc.) into a standard 4-20mA DC current signal, using the same two wires to simultaneously provide power and transmit the signal.

Its core working principle is: the transmitter draws a minimum loop current of 4mA as its own operating power, linearly converts the physical quantity collected by the sensor into a current signal within the 4-20mA range, and superimposes it on the loop. The receiving end (PLC/DCS) converts the current into a 1-5V voltage through a sampling resistor (typically 250Ω) for acquisition and processing.

2.Why Choose the 4-20mA Standard?

4-20mA is the most commonly used analog signal standard in industrial control.

l Strong anti-interference capability: Current signals are not sensitive to electromagnetic interference (EMI), making them suitable for long-distance transmission (up to several hundred meters).Broken wire detection:

l Self-powered design: Only two signal wires are needed to simultaneously transmit the signal and power the transmitter, simplifying wiring.

l A standard 4mA typically represents the minimum measured value (zero point), while 20mA represents the maximum value (full scale). Current values below 4mA (e.g., 0.0-2.2mA) are typically reserved for diagnostics and alarms, used to indicate whether a sensor is disconnected, faulty, or in a warm-up state, allowing the system to distinguish between "reading zero" and "device failure." Current values above 20mA are used to indicate that the measured value has exceeded the sensor's maximum range.

3.System Block Diagram and Module Functions

A typical block diagram of a 4-20mA loop-powered transmitter system is as follows:

System block diagram: The workflow of a 4-20mA loop-powered transmitter can be vividly described as a closed-loop energy and data cycle. The entire process begins in the physical world of the industrial field and ends in the digital system of the control room. The specific steps are as follows:

l Physical quantity sensing: Everything begins in the industrial field. For example, the temperature inside a reactor or the pressure inside a pipe under test — these measured physical quantities are the source information of the entire monitoring system.

l Signal conversion: The sensor, acting as the system's "sensory organ," directly contacts or senses these physical quantities and converts them into weak, non-standard electrical signals (typically millivolt-level voltage or resistance changes). This raw signal is very fragile, susceptible to interference, and not suitable for long-distance transmission.

l Core processing and conversion: This weak electrical signal is sent to the transmitter's core circuit. Here, it first passes through a signal conditioning module, where it is amplified, filtered, and linearized, making it stable and accurate. Subsequently, the processed voltage signal is precisely modulated by the V/I conversion circuit into a standard 4-20mA current signal.

l Loop transmission and power supply: This 4-20mA current signal flows out of the transmitter via Signal Wire 1 (+), travels along the cable to the control room. In the control room, the current flows through the input of the PLC/DCS/display instrument. The instrument precisely reads the current value by measuring the voltage across a precision resistor connected in series in the loop, thereby determining the physical quantity at the field.

l Energy cycle: After flowing through the instrument, the current does not disappear but continues to flow back to the negative terminal of the 24V DC power supply through Signal Wire 2 (-), forming a complete closed loop. This 24V power supply not only powers the PLC/DCS but, more importantly, provides energy for the entire loop. The transmitter "draws" the small current it needs from this loop to maintain the operation of its own circuitry (sensor, core circuit) — this is the essence of "loop power."

II.Transmitter Wide Voltage Range Design

1、Mainstream transmitters: Wide voltage range design

For two-wire transmitters that use 4-20mA signal transmission (such as pressure, temperature, and level transmitters), their operating voltage range is typically DC 12V to DC 36V. A 24V DC power supply is commonly used in system design.

Actual range: The transmitter itself can operate normally within this wide voltage range. For example, a pressure transmitter's datasheet may specify its operating voltage as "DC 12-36V," meaning it can run stably on DC power supplies of 12V, 24V, or 36V. This design enhances its adaptability to different field environments.

2、Special type: Low-power transmitters

In addition to mainstream two-wire transmitters, there are also low-power models designed for specific scenarios.

Power supply method: This type of transmitter is specifically designed for battery power and is commonly used in IoT, fire hydrant pressure monitoring, and other applications where wiring is inconvenient.

Operating voltage: Their operating voltage is very low, typically 3V, 3.3V, 5V, etc.

III.Sensor Transmitter ESD and Surge Protection Circuit

Under normal conditions, the input voltage is provided by a dedicated module of the PLC/DCS or by a separate power supply.

1、How is dangerous overvoltage generated?

l Voltage overshoot during power supply startup;

l A sudden change in high voltage or high current on other cables adjacent to the signal line interferes with our signal line through inductive coupling;

l Surges, electrical fast transients (burst), or electrostatic discharge (ESD) — these can create voltage differences between signal lines. (These tests are commonly used for EMC compliance certification.)

2、How does the Shanghai Leiditech protection circuit protect the sensor transmitter?

This is a very classic industrial interface protection circuit design. For surges on the 4-20mA signal line, this circuit achieves protection through a combination of clamping, current limiting, rectification, and filtering.

Below is a detailed explanation of the specific function of each component when dealing with surges:

l TVS Diode Array (D2)

Function: Voltage clamping ("Shield")

Function: Voltage Clamping ("Shield") When a high-voltage surge (such as a lightning strike) occurs on the line, it instantaneously conducts, limiting the voltage to a safe range (e.g., 24V or 36V) and diverting the large surge current away, preventing high voltage from damaging downstream chips. Common choices include GBLC24C and GBLC36C.

l Current Limiting Resistors (R1, R2)

Function: Current limiting and voltage division ("Buffer valve")

Function: Current Limiting and Voltage Division ("Buffer Valve") When a surge occurs, these two resistors share most of the voltage, limiting the current flowing through the TVS diode and downstream circuits, preventing the TVS diode from burning out due to excessive current, while also reducing the residual voltage entering the downstream circuits. Typical values range from tens to hundreds of ohms.

l Rectifier Bridge (D1)

Polarity protection and path guiding ("Directional valve")

Ensures that the downstream circuit always receives the correct polarity voltage, regardless of whether the external signal wires are connected in reverse or correctly. When a surge arrives, it unifies interference pulses of different polarities and guides them to the following circuit for processing.

l Filter Capacitor (C1)

High-frequency filtering ("Reservoir")

Primarily filters out high-frequency noise. When a surge occurs, it absorbs part of the high-frequency energy, smooths out voltage fluctuations, and protects downstream circuits from high-frequency interference. Typical values are 10nF or 100nF.

The entire circuit works as follows: D2 clamps the voltage, R1/R2 limit the current, D1 corrects the polarity, and C1 filters high-frequency noise — working together to protect the "sensor transmitter" from surge damage on the 4-20mA signal line.

IV.Recommended ESD and Surge Protection Components

Recommended as follows in the table below:

V.Application Industries and Equipment

4-20mA loop-powered transmitters are widely used in the following industries and equipment:

Petrochemical: Pressure, temperature, and level monitoring, such as reactors, storage tanks, and pipelines.

Power generation: Generator temperature, transformer oil level monitoring.

Water treatment: Flow meters, water quality analyzers.

Pharmaceuticals: Reactor temperature and pressure control.

Manufacturing: Sensor signal transmission for machine tools and automated production lines.

The 4-20mA loop-powered transmitter is a core component of industrial automation, and its reliability directly affects production safety. By adding TVS protection components, it can effectively withstand surge impacts and ensure stable system operation. Shanghai Leiditech's TVS devices, with their advantages of high reliability and fast response, have become an ideal choice for industrial sensor protection.