The Envelope Engineer - Smarter Plugs

December 11, 2018 Simon Holt, Mouser Electronics

Everyone should have an automated home. That isn’t just the geek in me speaking, it’s an observation based on where we are as a species. We now generate and consume around 25TW of energy yearly, or just over 3kW for every single person on the planet. The process of generating electricity produces greenhouse gases, which are measured in terms of grams of CO2 or equivalent per kWh (written as gCO2eq/kWh) of electricity produced. Not surprisingly, governments want to see this figure come down - with targets for the carbon footprint of generating electricity expected to see it dropping to around 500 gCO2e/kWh by the 2030 timeframe. By reducing our personal demand for power we could help reduce the overall carbon footprint for energy generation almost overnight. Home automation could help us curb that personal demand, simply by remembering to turn things off for us.

Let’s face it, nobody turns everything off all of the time, but technology could clearly help here. Digital assistants are great for this and through wireless technologies, like Zigbee and Z-Wave, we are already moving in the right direction. Any home appliance that is mains powered could be switched on and off remotely. The problem is, however, that most of our current home appliances are dumb, so we need to use a smart plug to facilitate the automation a bit.

Today, a smart plug is simply a wirelessly enabled adapter that goes between the existing plug and the socket. Some use Wi-Fi to connect to a cloud service, which is in turn accessed by a smartphone or digital assistant, while others are Zigbee-enabled and can be controlled directly by a home gateway. A few digital assistants now integrate a Zigbee home hub for this very purpose.

So here’s the idea folks: develop a smarter plug, not an adapter (although these are cheap, simple to use and easily retrofitted - it’s just a plug and socket, after all) but a proper OEM moulded power lead that can be sold in large volumes to appliance manufacturers and fitted at the factory during final assembly. It would make all appliances smarter by design, but without forcing the manufacturer to make any design changes to the appliance itself.

There are a few things that would need to go on inside the plug to achieve this, such as generating a low DC voltage to drive the smart functionality, and adding in some form of wireless connectivity - although power line communications (PLC) is also potentially an option to consider. Lastly, it would need to be capable of isolating the mains power from the socket to the appliance, without cutting off its own power supply. This means we are looking at AC/DC conversion, possibly DC/DC buck conversion with regulation, a power switch and wireless compatibility with modern home hubs. This isn’t going to be a single-chip solution, so we’re probably talking about specifying a handful of components. Since RF can prove tricky it would make sense to use a wireless MCU, and as there’s quite a lot of power management going on here it would be nice to use one that could potentially handle everything.

If we assume that we are targeting Zigbee-based systems, then the options are numerous. There are a couple of potential solutions that I like the look of, including the EM341 Zigbee SoC from Silicon Labs and the JN516x from NXP. The Silicon Labs device integrates an ARM Cortex-M3 processor, while its NXP counterpart relies on a proprietary processor that can also host application code. Both are competitively priced and feature-rich, so I suppose it might come down to processing architecture preferences.

The power conversion could be handled in a number of ways, but the overarching requirement here is to keep it as small as possible. Ideally it wouldn’t be apparent to the consumer that the plug is smart in any way. Modern USB power adapters are now available that meet this requirement, so it shouldn’t be too difficult to mimic that. A quick web search has turned up the VIPer01 family of high-voltage converters from STMicroelectronics. High up on the list of target applications for these devices is low-power SMPS for home appliances, along with building and home control. Based on that, I think we may have a winner!

The only other part of the puzzle is really the power switch that would cut off power to the device when it is not needed. I’m thinking that something from the ACST range for triacs, again from STMicroelectronics, would do the trick.

Now that we’ve got the basis for a design it’s time to start tinkering. Each of the devices selected requires a minimal number of external components to accompany it, so I am hopeful that I can squeeze everything into the size of a regular plug. This will mean there are now aesthetical or functional disadvantages associated with utilising it.

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