Embedding an electrical energy monitor into very space-limited applications made easy with MSP430i204x MCUs


As more equipment and appliances are equipped with embedded electricity energy monitors to measure and report the devices’ own consumption, I am getting more requests to estimate the real estate occupied by the metering circuit.

When server power supply is the biggest user of embedded energy monitors, the ‘allocated’ metering area is about a square inch with the voltage divider and shunt outside of the board. As time passes, and metering is to be embedded in more applications, this square inch is further squeezed.

To have a more solid idea of how small it can be squeezed and how the performance would be, a board utilizing TI’s ultra-low-power MSP430i204x microcontroller (MCU) as the metrology processor is made to contain all the components for basic metering. The schematic is shown here.

Figure 1: A  board utilizing TI’s ultra-low-power MSP430i204x MCU

Unlike other metering evaluation module (EVM) designs, the three sections of 330 k voltage divider is replaced with R1 and a single 1M-ohm resistor rated at 500V tolerance. As this board is targeted for embedded metering applications, it is assumed that it should already have some kind of surge protection circuit before high voltage is applied to this board. All other parts relevant to metrology are kept the same as the referencing EVM. Using 1.27 mm trace and clearance, I did a layout, as shown, which measures 17 mm by 12.1 mm. I also made the six pins on J6 and a few more MSP430i2040 pins accessible from the board edge, thus when one wants to put this as a module onto a larger board, it needs only minimal modifications.

 
Figure 2: Layout of module

How about the performance? The unit is calibrated at 5A 200V and is then tested with a metering test set. The accuracy is very good. I can get from 10 A down to 20 mA measured power error < 0.1 percent and at 10 mA about the error is a bit worse and is about 1 percent.

Picture 1: Comparison image of module size vs. an electrical plug

Given the size of this module, it is easy to embed metering functionality into applications of very limited space; the level of accuracy achieved is also able to meet requirements of embedded metering applications.