Custom-build IotaWatt PCB Board

Would like to share my custom-build IotaWatt PCB board.

IotaWatt is too big to fit inside the power distribution box. Therefore, I am trying to build an PCB to replicate the IotaWatt and expose the SPI interface out of the power distribution box.

I have added a digital isolator (ISO7241) for interfacing with ESP8266 running out of the box. All the ADC channels are just the same with the IotaWatt, excepted that I modified the port 13 and 14 to “Voltage Transformer ONLY” in order to measure the voltages of 3-phase power.

  • ISO7241 Digital Isolator
  • MCP3208 ADC
  • LM358 Op-amp
  • SRV05 TVS Diode Array
  • CJ431 Voltage Reference 2.5V

Still need to perform more checking and verification before submitting it to PCB and SMT assembly production factory.

Should work. Electrically it’s pretty much the same as the extension unit on the ESP32 prototype.
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The isolation chips would help with electrical certification, but I don’t see where you get isolation for the 3V3 or GND. The 3V3 could be resolved with another power supply, but that wouldn’t resolve the ground issue. I don’t recommend connecting the IoTaWatt ground to equipment ground.

Curious to see what you are doing for the external head.

The digital isolator has isolated powers and grounds (see ISO7421 Digital Isolator Chip Datasheet). The ESP module outside the box will feed in 3.3V and ground through the upper 8 pin connector.

For the rest of the chips, they will be powered by the top-left 2-pin connector from another PCB. This PCB is quite easy to design, and it will contain a 3.3V power supply (HLK-PM03) and three PCB mount hole 12V voltage transformers (Voltage Transformer Datasheet).

Looks interesting, but why not just use a box outside the main panel?

That’s what I did. Only the CTs are in the panel box. They leave via conduit to another box that is big enough for the IotaWatt.

The chip you selected looks fine for isolation, but appears to do nothing to significantly improve the long-distance capability of SPI. I know many people use it over more than a few feet, but it was really designed to be used over much shorter distances. It might work fine, especially for a one-off-home-use device.

Looks interesting, but why not just use a box outside the main panel?

That’s what I did. Only the CTs are in the panel box. They leave via conduit to another box that is big enough for the IotaWatt.

A valid point. Your solution will work fine in my case. The advantage of my design is that only one single cable is need for connection outside the box, so that it is more neat and This design also allows easy changing to a different microcontroller such as ESP32 in the future, without replacing the whole IotaWatt unit.

The chip you selected looks fine for isolation, but appears to do nothing to significantly improve the long-distance capability of SPI. I know many people use it over more than a few feet, but it was really designed to be used over much shorter distances. It might work fine, especially for a one-off-home-use device.

I have to admit that the isolation chip is really not much necessary. The CTs, VTs and power supply are all isolated from mains voltage, and it is not lethal to touch them accidentally. Maybe it is for extra protection.

As a hobbyist, I actually enjoy the progress of making my own PCB.

FYI, I believe the Adafruit huzzah ESP8266 stacked with their datalogger featherwing would have all of the components needed to drive your board. It would be a pretty simple matter to adjust the GPIOs in the firmware to work with that.

My SMT assembled PCB finally arrived :grinning:

Still need some time to solder the through hole components. Hope it will work.