Designers typically default to using low dropout voltage regulators (LDOs) to power industrial sensing and IoT systems designed with 4-20 mA current loops. However, for applications that focus on power consumption and limited space, LDO is becoming increasingly impractical. At this point, designers should consider switching to voltage regulators (also known as buck converters), especially for applications that require high energy efficiency, heat dissipation performance, and extended battery life.
The 4-20 mA current loop is a robust and reliable method for transmitting measurement results from sensors to a programmable logic controller (PLC), and transmitting the PLC's control output to process modulation equipment. This system ensures precise and noise resistant long-distance signal transmission using twisted pair cables, making it an ideal choice for various industrial environments. Regardless of the length of the wire, the current remains consistent, making it a standard configuration for factories, laboratories, and remote monitoring applications.
Evaluating the trade-off between LDO and switching regulators in current loops may help achieve smarter and more sustainable designs.
LDO still has its place in some special situations, where it can provide advantages such as ultra-low noise, simplified material list, or minimal voltage regulation margin. However, they have lower inherent efficiency because they dissipate the difference between the input and output voltages as heat. These wasted energies can lead to an increase in thermal load in applications and greatly shorten battery life in portable or remote applications.
When efficiency, heat dissipation performance, or battery running time are crucial, synchronous voltage reduction may be a better choice. Even under milliampere load conditions, modern synchronous voltage reduction technology can provide an efficiency of 85% to 95%, significantly reducing heat generation, and now also providing low microampere range static current. LDO will dissipate excess voltage as heat, while voltage regulators can effectively convert the extra voltage into usable current, thereby achieving more power consuming functions without overheating or wasting energy.
These features make voltage regulators the preferred solution for any 4-20 mA loop (such as battery powered sensors) with input margins exceeding a few volts, requiring thermal efficiency, or requiring long-term operation at limited power.
If the designed supply voltage is about 6V higher than the voltage required by the current loop transmitter, and there is space on the circuit board to accommodate small inductors and output capacitors, then an efficient synchronous buck regulator is usually the best choice. It can effectively reduce voltage, minimize heat waste, and ensure sufficient current to power other functions in the 4-20 mA loop. Therefore, it is an ideal choice for modern transmitters that require both reliability and energy efficiency in industrial environments.
The heat dissipation advantage of voltage regulators greatly reduces the requirements for heat sinks in high current and high-temperature industrial modules. Even a 5 µ A buck circuit has higher efficiency than LDO, as the latter converts a significant portion of battery voltage into heat.
Drive loop
The 4-20 mA current loop is one of the most common ways to send information between on-site sensors and control systems that use their data. Signals can represent temperature, pressure, flow rate, and even instructions to move valves. It is simple, reliable, and effective for long-distance use.
The current loop (Figure 1) can transmit measurement signals from instruments such as temperature or pressure sensors, or control signals to devices that move or regulate mechanisms such as valve positioners.
Schematic diagram of 4-20 mA current loop
Figure 1: A schematic diagram of a 4-20 mA current loop illustrates how to use current instead of voltage to transmit analog signals in industrial automation, sensor systems, and process control applications. (Image source: Analog Devices, Inc.)
The current loop consists of four main components:
DC power supply: Depending on the settings, it may be 9 V, 12 V, 24 V or higher. The voltage provided by the power supply must be higher - at least 10% higher, which is also the amount of voltage that all components (transmitter, receiver, wiring) in the loop "decrease" when current flows. Then, the local regulator steps it down to power the sensors and electronic devices.
The transmitter on one side of the sensor transmits electrical signals representing the physical world: the sensor generates raw signals related to temperature, pressure, distance, or other physical measurements. If it is an analog voltage, the voltage current converter of the transmitter will convert it into a proportional current of 4 mA to 20 mA. If it is a digital sensor, the output is converted into analog current through DAC. The transmitter has its own power supply, such as LDO or voltage regulator.
Receiver at the control end: The receiver reads the 4-20 mA signal and converts it into a voltage that the control system can measure, display, or execute.
Loop wiring connects the power supply, transmitter, and receiver in series: the loop can be up to thousands of feet long. In a dual wire system, the same two wires simultaneously transmit power and signal currents. The 4-wire system uses different wire pairs to transmit power and signals.
Even in harsh industrial environments with temperatures ranging from -40 ° C to+105 ° C, the components of the current loop must be precise, energy-efficient, and reliable. In addition, they must also support necessary safety and system level functions to ensure the safety and reliability of the loop.