For many years, measuring applications have used current transformers (CT), but these come with a number of problems: interference (EMI), large size, and possible core saturation at high current level. Low resistance current shunts solve many of those issues.
Due to their low cost, reliability, and relative freedom from any EMI problems, low resistance current shunts have been quite useful in current and voltage measuring applications. These include industrial motor drive control, photovoltaic inverter applications, and in energy metering devices. The one main problem in using this device is its resistive element. The power dissipated in the device (in the form of heat) is a factor of the square of the current (P=I2R).
Typical Low Resistance Current Shunts
TI has introduced an Isolated Shunt-based Current and Voltage Sensing Module with a Delta-Sigma modulator, the TIDA-00080 . The offering implements both an isolated current sensing and voltage sensing method using the AMC1304M25 isolated delta-sigma modulator in conjunction with the TMS320F28377D microcontroller for digital filtering of the output. The system can provide a current reading accurate to less than one percent error un-calibrated.
Products such as the AMC1304M25 has one isolated current input –with an isolated 5v power supply. Current channels can be connected nose-to-tail to provide a significantly higher current reading capability. The digital output is isolated from the analog input via a capacitive barrier that is highly resistive to EMI. The barrier has been certified for isolation to 7000Vpeak in accordance to UL1577 and IEC60747-5-5.
The module also has three voltage inputs. However, in this case, all three channels share a common isolated 5-volt power supply. The input frequency range for the unit is 45Hz to 65Hz, thus making it suitable for the majority of commercial frequencies in use worldwide. Its module uses an external clock for synching the data transfer, typically set to 8.192MHz. The analog input signal is sampled on a continuous basis and compared to an internal reference voltage. The isolated output provides a time average, which is proportional to the analog input. Below is the block diagram of the AMC1304M25 .
AMC1304M25 Block Diagram
The TIDA-00080 implements the isolated current/voltage sensing system using the AMC1304M25 , together with the TMS320F28377D microcontroller, as part of the digital filter module.
The digital filter module provides the 3.3V and 5V power to the AMC1304M25 module, as well as the clock signal. The digital filter module receives the digitized current/voltage signal from the delta-sigma module, converting it to a 16 bit digital signal that the user can download to via a USB connection.
Digital Filter Block Diagram
Closed Loop Control System Block Diagram
This device is used in Smart Grid applications, providing current and voltage sensing for protective activations of relays and circuit breakers if a variety of electrical faults are sensed. The system can continuously monitor electrical parameters such as voltages, currents, and frequencies, issuing trip commands to appropriate circuit breakers depending on the type of fault detected. Typical fault protection includes generator protection, overvoltage protection, and over current protection.
Texas Instruments has sponsored this post. They have no editorial input to this post - all opinions are mine. Ray Floyd