Voltage Transducer Circuit: How 110V AC Is Converted to 4-20mA Signal and Sensing Explained

The discussion centers on understanding how a voltage transducer converts 110V AC into a 4-20mA current signal and the sensing mechanism involved. Voltage transducers are devices that convert electrical energy from one form to another, often used to monitor voltage by producing a proportional current output. One example is the IME TM3UL20 voltage transducer, which senses 110V AC and outputs a 4-20mA signal; however, calibration issues such as inability to adjust zero and span potentiometers can cause output errors (e.g., outputting 21-22mA at nominal voltage). A practical sensing approach involves using a ferrite rod coil with hundreds of turns of fine wire to detect magnetic fluctuations from the AC line, amplified by a transistor circuit to generate a 4-20mA current loop. This method requires additional circuitry for calibration and temperature stabilization and a fixed coil orientation for consistent readings. Alternative measurement methods include voltage dividers and isolation techniques (e.g., optocouplers) to safely interface microcontrollers with high-voltage lines. The LV25-P transducer, based on Hall-effect sensing, is mentioned as a current transducer that can be adapted for voltage measurement by using an external resistor to convert voltage to current. Overall, detailed circuit schematics are scarce, and manufacturers often do not disclose internal designs, making reverse engineering or patent research necessary for in-depth understanding.
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