Power Tips: My personal IoT and how I would power my favorite

The IoT is everywhere!  As my buddy Brian said in his last post, the Internet of Things is here and continuously growing.  As an exercise in my own curiosity, I decided to scour the house and figure out exactly how many things we have connected to the internet.  I was a bit surprised. I am junky for electronics and the latest gadgets, but this is out of control!

34 Products in or around my house are connected to the internet.  Here is the breakdown


  • 6 IP Security Cameras
  • 2Wireless Routers
  • Internet Modem/Gateway
  • Wireless Printer
  • 3 Gaming Systems (PS2, PS3 and Wii)
  • Smart Thermostat
  • AV Receiver
  • 3 IP Cable Set Top Boxes (STB)
  • Smart TV
  • Digital Photo Frame
  • Smart Programmable Remote
  • Home Theater PC (HTPC)
  • Water Meter
  • Power Meter
  • Natural Gas Meter
  • 3 iPads
  • iPod Touch
  • 2 iPhones
  • 2 Dell Notebooks
  • MacBook Pro


How to power all of these devices?  Most of the items listed above come with some sort of external adapter that takes the AC line and converters to an intermediate voltage.  These devices are generally the lower power (up to ~65W) consumers. 

There are a few that are not powered from external adapters.  The smart TV, AV Receiver, STBs, HTPC, PS2 and PS3 all have built in AC/DC power supplies. These devices require more power or multiple outputs, so the AC/DC built into the product makes sense. 

The only devices left are the Nest thermostat and the utility meters. These devices have a unique way of getting power.  The utility meters use the AC line, battery power or flow to power the meter communications. The last device (Smart thermostat) not only has a unique way of getting its power, but it is my favorite.

Thermostats have a unique power system that has been used for many years. The air handler unit is connected to the AC line. This unit has a small transformer used to step the voltage down from the house mains to 24VAC.  The 24VAC is used to power a number of relays that control the on and off functionality of the different modes in the HVAC system.  These relays are controlled by the thermostat.  The 24VAC is fed to the thermostat; this voltage is used as the main source of power. 

In the case of the Smart thermostat, there is also a battery to supplement the AC power.  The battery is necessary to provide peak power for the LCD screen, Wi-Fi transmissions and various other functions.

Here is how I would power a Smart thermostat.  Since I paid $249 for the thermostat, I am not real excited to take it apart and find out what is inside. There are a number of teardowns on the web, but most of those have little or no concern with the power system.  I also don’t have all of the power specifications, but this is what I do know:

  • 24-V AC Input
  • 3.7-V LiPO Battery
  • AM3703CUS Sitara ARM Cortex A8 microprocessor
  • TPS65921 PMIC and USB

I am going to assume that the battery powers the TPS65921, which in turn powers the Sitara processor

So, how do we get from 24-V AC to a voltage that can be used to charge the battery?  The easiest and most practical way is to rectify the AC voltage, store it in a bulk hold up capacitor, then buck down to 5V.  Rectified 24-V AC produces a DC voltage of around 33.8V, if we add 20% to that value, the max DC voltage is 40V.  Again, I am assuming that 5V @ 500mA will be enough power to charge the battery.

Many, Many IC Choices! TPS54140 is just one IC that would work great for this application. This is 42V max input voltage and provides up to 1A output current. PMP4746 is an example of a circuit that could be used in this situation. This circuit has the added advantage of providing some extra input filtering to reduce reflected noise on the input line. The 5-V can then be used with a BQ single-cell charger.  There are a number of chargers that can be used.  Two examples are a linear charger (BQ2408x) and a switch mode charger (BQ2415x).  Not too complicated of a system once it is all broken down.

This was an eye opening exercise for me.  I have no idea if my use case is typical, but I certainly didn’t realize I had that many devices.  How many do you have?