Sunday, November 18, 2018

Another Energy Monitor - Teardown

Looking at other energy-monitor designs has been a past-time of mine and I recently the chance to teardown a energy monitor installed along with many Tesla power-walls in Australia. This one had had some feedback of high voltage over the modbus and had fried itself. Despite best intentions with TVS suppressors etc. it could not take it anymore.

In this blog post I will enrich some of the content I already posted on twitter with some more in-sights in energy monitoring and additional elements regarding current clamps.

 This is the proper blogpost alluded to in the twitter thread. All the image content is already in the thread. Blogs simply allow greater structure. The meter is essentially composed of:

  1. Current samplers with 1ohm burden resisors attached to CT's - recently announced a flexible CT design which can make it easy to install and potentially universal in measuring AC currents via induction and DC currents via hall-effect. I dropped an multi-meter probe on the burden resistors just to check.
  2. Voltage samplers as tiny encapsulated isolation transformers - This approach can introduce some non-linearity due to hysteresis and phase-shifts in the transformer, transformers are also bulky. Since the transformer is under no-load, phase-shifts should be minimal. The advantage is built in LV isolation. There is a bank of x3 transformers to account for x3 phases.
  3. Energy monitor IC's - These are from Cirrus Logic (CS5467), the documentation says the IC is mainly for the Japanese market. seems to be successfully using it in North American and Australian market.
  4. Main processor and wifi - Unfortunately the unit I had was going back to Tesla under RMA, so I did not have chance to take of the shield and probe the processor. However I would love some assistance in poking in there and exploring the possibilities of custom firmwares.
  5. Modbus - The meter has a modbus I/O port to communicate with other systems e.g. Inverter and Battery charge controller.
  6. Power Systems - This is a Recom SMPS (RAC05-02SC) module keeping with my idea of keeping custom subsystems as limited as possible and reusing tested components as much as possible. I have seen a lot of energy monitors include their own power sub-systems including the Sense and WattWatchers. This increases design complexity with perhaps marginal improvements in design flexibility and BOM costs. The module outputs 3W at 5V, giving some head-room for LDO/Cap based noise filtering.
I have come to learn that DIN rails are not that popular in the North American market compared to the European and Australian market. Hence the overall brick packaging of the Sense and the meters. Keeping the DIN form-factor requires a lot of combined mechanical and electronics design work as I have found out the hard-way. It is currently in my pipeline to create break-out boards for the CS5467 and test them out with common micro-controllers.

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