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• Typical design decissions

  Anton Verburg • 12/07/2024 at 16:19 • 0 comments

  So what decisions did I make during the design of the ControlBox, and why?

  
  Well, when building hardware a lot of decisions have to be made. During the development of this piece of hardware, I always tried to choose the most-robust option. So here we go:

  • For mains outputs, we selected panasonic AQG22105 solid state relays with integrated snubber circuit and zero-crossing detection.
    • The disadvantages of electromechanical relays are their slow response
      time, high maintenance, and limited functionality. They are prone to wear and tear, contact erosion, and mechanical failures. So we wanted to use solid state relays.
    • According to the datasheet it should be able to supply up to 2A. But when testing the fuses, it was proven that a fast fuse of 2A would break the switch, so we decided to limit the current to 1.5A.
    • Utilizing the integrated snubber circuit and zero-cross detection, unexpected current peaks and other harmful signals are omitted.

  • For the 24VAC outputs, I had the following reasons:

    • This 24VAC is typically consumed by installation devices in an environment where leakage is possible. Think about the thermal wax valves of the radiant floor heating, but also 24VAC zone valves exist. The reason why these devices do not work on mains power, is safety. Imagine what happens if water is leaking over your 230VAC powered devices....

    • 24VAC thermal wax valves are very common, and can be bought anywhere on the world.

    • An unforeseen benefit of using the 24VAC devices is the connector space on the board; because the lower voltage, a smaller distance between the pins is allowed. And that allows denser connectors, so that we can put more IO on a smaller footprint.

    • Switches for 24AC are cheaper than those for 230VAC as well.

    • During my search for controlling the thermal wax valves I also saw that you can control them with 24VDC. But when I continued my search, I concluded that a 24VDC power supply is always more expensive than an on-board mains-to-24AC transformer.

  • Reading the 1-wire (temperature) sensors happens via two DS2482S-100 drivers. They are robust, and they have a function that enables to provide parasitic sensors with power via a push-up FET.
  • You might have asked yourself the question:
    • Why a RS482 half duplex port?
      The reason is simple; I have seen several devices utilizing such a port. An example is our own GoodWe solar panel transformer, or the Hewalex heat pump. And as our idea was to combine all available hardware via Home Assitant, in order to make the smartest possible decisions, we needed an RS485 port.
    • Why 2kW LED strip outputs?
      Just because it's fun. We had the idea to provide some kind of 'anti-theft' light around our house. Because we where preparing the renovation, we had the freedom to plan it like we preferred. And because I had done some nice projects with LED lights before, I thought 'well, it would be nice to put that around our house too'. And that's why I integrated it. When bringing this device to the public, I have considered to leave it out. But as the component costs are not that large compared to the rest of the unit, they are still there.
    • Why the RPi 5 + NVMe solid state disk?
      The raspberry pi is a common used hardware platform for Home Assistant, and can be easily installed with Home Assistant Operating System. This will give you all benefits of Home Assistant, including automatic system maintenance via the supervisor, and easy installation of the HACS. In future, the raspberry pi can easily be swapped for a faster or better single-board computer. The only disadvantage of the raspberry Pi till now was that it runs from an SD card. As internally Home Assistant uses database software, this is not a stable solution. But from version 5 this is solved; the pi can now run on a fast NVMe solid state disk, that contains a good controller to prevent the disk from corruption.
    • Why do you prefer wired connections? The newest wireless protocols are good, isn't it?
      Well, the newer protocols are getting better. But I have seen too...

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