Wednesday, November 25, 2015

Dedicated Energy monitoring with the ADE7763

After building the NodeMCU based energy monitor with an external 16 bit delta-sigma ADC i.e. the ADS1115 I started looking into solutions which were more accurate and cheaper.

I came across some work by Skye Sweeney with the ADE7763. He had built a prototype on breadboard and wanted to monitor lots of channels so the cost of sensors became prohibitive and he set the project on hold. I am attempting to measure only a single phase and disaggregate the data later by post processing, so a single ADE7763 should serve my purpose.

I set about building a breakout board for the sensor using the reference design in the datasheets and ordered it from Oshpark. This is where I made an error in my rush. I did not check the footprint the element14 library provided against the real chip. So I ended up with a board with TSSOP footprint for a SSOP chip. Should have printed the PCB 1:1 and checked with a real chip. At least the error was found while prototyping and not in production. I bent the package pins a tiny bit to squeeze it in and proceeded to assemble the breakout.

I could not quite get the SPI bus to work on the ESP8266 NodeMCU, it kept resetting the Watch dog timer. With the ESP it is often a power issue. So for a quick test I hooked up the breakout to a Teensy 3.1 and loaded Skye's code without the multi-sensor bus stuff. It worked on the first go and started dumping RMS voltage and current registers. The only thing to be careful about is the rather limited 1v peak-to-peak input range. Otherwise this chip is great with dual 16bit d/s ADC's and all the code for RMS and registers for offset removal. At about 1/2 to 1/3 the price of the ADS1115. I have uploaded some standalone  breakout testing code to github - Arduino ADE7763.

For accurate measurements this sensor can be connected to AC line using Shunt resistors for current measurement instead of a CT sensor and a direct volatge divider ladder to measure the voltage waveform instead of an isolation transformer. This makes the chip MCU unfriendly and subject to mains voltages. The SPI bus should be isolated with optocouplers when using this mode.

Meanwhile I have tested the sensor using my basic load bulb and CT clamp and voltage and current waveforms are picked up just great. At some point I will build up the courage to create a high voltage / phase accurate measurement rig.

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