With the maturation of microcontroller (MCU) technology, traditional analog transmitters have gradually been replaced by smart transmitters featuring a microcontroller as the core for data processing and control. These smart transmitters enhance the capabilities of analog transmitters, improving measurement accuracy, operational reliability, and enabling features such as linearization, temperature compensation, automatic zero and span adjustments, and digital communication. When developing a low-power intelligent two-wire transmitter, designing an efficient internal power supply is crucial. Firstly, a smart transmitter with a microprocessor must provide power for the microcontroller, A/D converters, D/A converters, and communication circuits. This requires more power than conventional analog transmitters and demands higher efficiency from the internal power supply. Additionally, for capacitive sensors and thermocouples, which may share a common ground or be connected to the casing (ground), the designed transmitter circuit must isolate the input and output to ensure the proper functioning of the subsequent control system and maintain strong anti-common mode interference capabilities. Given that a two-wire transmitter system provides only 4 mA of working current, these specific requirements present significant challenges in designing the system's power supply.
The isolated two-wire transmitter power supply with micro input power is specifically designed for RF admittance level transmitters. It uses a fully integrated circuit approach, offering a simple structure, stable performance, and low cost. The input voltage range is 16~32VDC, and it employs a buck converter mode to deliver two sets of isolated 5V power supplies. At an input voltage of 24VDC, one set supports a non-isolated load capacity of up to 10mA, while the other supports an isolated load capacity of up to 4mA. The total bus current at 24VDC is less than 3.5mA, with an efficiency exceeding 85%. This solution fully meets the power requirements of the input and output isolated two-wire smart transmitter.
The overall design leverages the multiplexing of the smart transmitter's power line and signal line. When the RF admittance level transmitter is operating normally, it outputs a 4~20mA current signal based on the level reading, with the circuit's power consumption current not exceeding the 4mA loop current. A fault alarm function is also required, with a bus current requirement of 3.6mA. To ensure stability, there must be a margin, meaning the power consumption current of the RF admittance level transmitter itself must be less than 3.5mA. Based on this, we can roughly estimate the maximum power consumption of this transmitter. Assuming the voltage from the control room to the transmitter is 24V, the 4~20mA DC signal is sent to the transmitter and converted into a 1~5V DC voltage signal via a typical 250Ω resistor before being sent back to the control room. In theory, the maximum power consumption within the transmitter should not exceed (24-1) × 3.5 = 80.15mW. This does not account for voltage losses in the input circuit and other factors. Figure 1 illustrates the composition of the smart transmitter and its power supply requirements.
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Furthermore, the importance of power efficiency becomes even more apparent when considering the practical application scenarios of such transmitters. In industrial environments, minimizing power consumption is essential for reducing heat generation, extending device lifespan, and ensuring long-term reliability. The isolated power supply design ensures that sensitive components remain protected against potential electrical noise or interference, making the transmitter suitable for demanding applications where high precision and robustness are paramount.
In summary, the development of an efficient, reliable, and compact power supply for two-wire smart transmitters represents a significant advancement in modern industrial automation. By leveraging advanced power management techniques and innovative circuit designs, engineers can overcome the inherent challenges associated with limited current availability and stringent isolation requirements. This not only enhances the functionality of transmitters but also opens new possibilities for optimizing entire systems. As industries continue to embrace digital transformation, solutions like these will play a pivotal role in driving innovation across various sectors.
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