With the increasing maturity of microcontroller (MCU) technology, traditional analog transmitters have gradually been replaced by smart transmitters that incorporate microcontrollers as their core data processing and control units. These smart transmitters expand upon the capabilities of analog transmitters, improving measurement accuracy and operational reliability, while also providing features such as linearization, temperature compensation, automatic zero and span adjustments, and digital communication. When designing a low-power intelligent two-wire transmitter, the internal micropower supply design is crucial. Firstly, the smart transmitter equipped with a microprocessor must provide sufficient power for the microcontroller, A/D converters, D/A converters, and communication circuits. This requires more power than conventional analog transmitters, necessitating an efficient internal power supply. Additionally, for capacitive sensors and thermocouples, the ground or sensor may come into contact with the casing (grounding). Therefore, the designed transmitter circuit must ensure isolation between the input and output signals to maintain the normal operation of the subsequent control system and exhibit strong common-mode interference resistance. Given that the external circuit provides only 4mA of working current for a two-wire transmitter system, these specific requirements present significant challenges for the design of the power supply system. 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 design, offering simplicity, stability, and low cost. The input voltage range is 16~32VDC, and it employs a buck converter topology to deliver two groups of isolated 5V power supplies. At an input voltage of 24VDC, one group of power supplies can handle a maximum load of 10mA without isolation from the input, while another group can support up to 4mA of load capacity isolated from the input. Under these conditions, the 24VDC bus current is less than 3.5mA, achieving an efficiency of over 85%. This design fully meets the power requirements for input and output isolated two-wire smart transmitters. The overall design utilizes a multiplexed power line and signal line for the smart transmitter. When the RF admittance level transmitter operates normally, it outputs a 4~20mA current signal based on the level being measured, with the circuit's power consumption current not exceeding 4mA of the loop current. There is also a need for a fault alarm function, requiring a bus current of 3.6mA. To allow for some margin, the power consumption current of the RF admittance level transmitter itself must be less than 3.5mA. Now, let us 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, which typically includes a 250Ω resistor, converting the signal into a 1~5V DC voltage signal sent 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 include the voltage drop in the input circuit or other factors. Figure 1 illustrates the composition of the smart transmitter and its power supply requirements.
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