PowerBlade is the smallest, lowest cost, true-power plug-load power meter, and communicates to smartphones over BLE

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We are interested in the power consumed in our homes, in our offices, and throughout our lives. We purchased several off-the-shelf plug load meters like Kill-A-Watt, and although these devices are highly accurate they are not particularly useful. The main drawback is size - they are simply so large that they block adjacent outlets. Even with 5+ in my apartment it felt overwhelming, how was I going to get the 50 needed to measure every load? And that doesnt even count the cost of these units, both in up-front purchase price and in power consumed while in use.

Instead, we designed PowerBlade. Our true-power plug-load meter is a single PCB 1 inch square and 1/16 inch thick. This diminutive size allows it to sit inconspicuously between the plug and its outlet, measuring the power supplied and reporting these measurements over BLE to nearby smartphones. Our device draws 176 mW in operation, and costs about $11 in large quantities.

The extremely small form factor in PowerBlade is made possible through two insights. First, other plug load power meters make contact with the AC through prongs, and provide another socket for the device (TV, fridge, etc) to plug into the meter. This creates a lower bound on the size of the meter - the size of the AC socket. Instead, PowerBlade allows the original device prongs to pass directly into the outlet through three cutouts in the PCB face. PowerBlade makes contact with these prongs through flexible tabs built into the PCB itself (visible in the picture).

Second, other meters typically measure current by breaking the path and passing one AC line through a sense resistor. Since PowerBlade does not break the path, this is impossible. Instead, we note that the current in the plug will generate a magnetic field. An inductor placed in that field will generate a signal based on the magnetic field strength, and this signal can be used to measure current (after significant amplification). By thinking about power metering in a different way, these two insights allowed us to move past the fundamental minimum size of a power meter to something significantly smaller.

A third insight in this system is that converting 120V AC to 3.3V DC in this extremely small size results in very little power (on the order of 1 mW), and that impacts our choice of data communication. We wanted something that would be usable even at large scale - checking a LCD screen readout on 50+ units is simply intractable in the long term. Instead, PowerBlade communicates to nearby smartphones using a low power BLE radio. This means the system doesnt require gateways or repeaters, it is usable out-of-the-box.

The device is accurate, with an average of 6.5% error in power measurements for a range of household devices like a fridge, television, and microwave. It works well even in dense deployments, 20 or more BLE devices in a room do not greatly affect BLE packet reception rate. Although we have not yet optimized for cost, the current implementation costs $11 to produce.

  • 1 × MSP430FR5738 Low Power FRAM Microcontroller
  • 1 × nRF51822 Bluetooth Smart SoC
  • 1 × OPA333 Amplifier and Linear ICs / Operational Amplifiers
  • 1 × MAX9910 Amplifier and Linear ICs / Operational Amplifiers
  • 1 × OPA2369 Amplifier and Linear ICs / Operational Amplifiers

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  • PowerBlade paper available

    Branden Ghena11/20/2015 at 18:31 0 comments

    A paper detailing the design of PowerBlade and evaluating its performance was recently published at SenSys'15. The paper goes into more detail about the power supply and current sensing methods. It also compares PowerBlade to existing power meters across several household devices.

    If you're interested, the paper is publicly available here:

  • New Contact Method: Pogos

    Sam DeBruin11/10/2015 at 19:41 0 comments

    Having difficulty with the longevity of the flexible tabs, we have implemented and evaluated a more stable method for making contact with the AC. The method is spring loaded "pogo" pins mounted in the plane of the PCB. These pins make contact with the AC prongs as they pass through (as the flexible tabs do as well) but since these spring loaded pins are designed for multiple insertions they seem to last significantly longer.

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solutions4circuits wrote 01/18/2016 at 21:41 point

Hot-neutral swap makes no difference in this kind of half-wave power supply

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alpha_ninja wrote 12/07/2015 at 00:25 point

[verified: no design files missing]

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alpha_ninja wrote 12/02/2015 at 00:47 point

This is your one-week reminder to upload design documents:

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Sam DeBruin wrote 12/04/2015 at 22:16 point

Although those files were available on our GitHub, I went ahead and packaged everything up in a zip and included it as "Design Files" on the left sidebar

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alpha_ninja wrote 12/05/2015 at 00:14 point

Looks good :) Thanks!

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Klima wrote 11/29/2015 at 20:29 point

I must say I am amazed. The way you measure current is really nice!

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Jared Sanson wrote 11/29/2015 at 00:18 point

Wow that is a really neat design! I've seen clamp-style inductive meters but never would have thought of using a plain inductor adjacent to the wire. What kind of accuracy do you get from the inductor?

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Alex Rich wrote 11/20/2015 at 20:51 point

this is a nice project, the biggest weakness I see is the power contacts, which I gather are only to scavenge power for the device, not for measuring current.  The flex circuit would have been a good solution if it worked, but it didn't.  I see the pogo pins as something that will not last too long either since you are engaging perpendicular to the direction of the pin travel.  I use similar pins in fixtures all the time and they bend/break even when you are pushing in the correct direction of travel.  Have you thought about using little lipo battery spring connectors?  The main issue is size, this one is only 3mm tall, it could potentiallly work:

this is the search category on digikey for this style if curious.  good luck w/ the 1" contest!

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jonsmirl wrote 11/18/2015 at 04:10 point

I would wait until you see what is in the new Espressif ESP32 before going further on this. ESP32 is going to replace five of your chips with a single one for under $2. Full ESP32 details are very close to being announced.

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Norse wrote 11/17/2015 at 15:01 point

I am amazed at the way you were able to downsize the board. I have been working on something very similar in size, and have to ask, how did you step down the 120v AC to 3.3v DC? I can't seem to get mine smaller than about 1 inch cubed for the power supply alone. 

Any help or hints you could provide would be great.

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jonsmirl wrote 11/18/2015 at 03:52 point

I suspect it is a capacitive drop power supply.

The problem with cap drop supplies is making them robust enough to tolerate the garbage found on AC lines. That is hard to do with SMD components.

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Norse wrote 11/18/2015 at 19:34 point

Thank you! I guess it is true that you learn something new every day! I'd never heard of this before, and now I have a lot of reading and research to do. Thank you again!

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Branden Ghena wrote 11/20/2015 at 18:27 point

Good call @jonsmirl, that app note is the same power supply design we are using. You can find more details of the design in our paper:

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K.C. Lee wrote 11/18/2015 at 04:01 point

I am guess the top row of components just above the slots for the power pong is the power supply.  The big 6982 resistor is certainly handling a lot of wattage.

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jonsmirl wrote 11/17/2015 at 12:14 point

What is going to happen with a wall wart power supply? Everyone's favorite thing to measure is a wall wart power supply since we all know that vampire power use by these accounts for 50% of all energy consumption (haha). Seems like a nearby transformer is going to mess up the inductor readings.

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jonsmirl wrote 11/17/2015 at 12:10 point

How reliable is the power supply? Most components rated for AC use are fairly large. What's going happen when the AC line gets a lightning strike? Dead unit?

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Branden Ghena wrote 11/20/2015 at 18:28 point

We haven't had any problems since we've been using them. I'd have to guess that a lightning strike will indeed result in a dead unit though.

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zakqwy wrote 10/07/2015 at 15:59 point

Cool design idea--I'd be worried about certification, though. What about clearance/creepage requirements? Have you looked into any changes you'd need to make to sell this commercially?

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Sam DeBruin wrote 10/07/2015 at 16:22 point

Thanks for your comment! You're raising a great question, we see a commercial future for PowerBlade but we need to make sure its safe to use first. To that end, we have targeted safety from two perspectives. From the design perspective, GND on the PCB is referenced to Neutral (wider slot), so there is no concern about clearance to that prong. For the other prong, Phase (or Line or Hot), no component is closer than the 1.5mm spark gap. We have also sized our components through the standard best practices of power design. 

Also from the design perspective, we conformally coat the unit in a resistive potting. I will add a picture of this conformally-coated unit now. This protects the user, and doesn't affect performance. 

From the evaluation perspective, the device is not yet UL certified but we have self-evaluated it against UL 2735 (Standard for Electric Utility Meters). It passed to our satisfaction, and as soon as we stop making changes I intend to send it for true certification. 

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Nick Sayer wrote 11/21/2015 at 01:20 point

What would happen if this device were connected to a hot-neutral swapped outlet? They are obviously not to code, but mistakes happen...

In fact, I have a hot-neutral swapped outlet in my house that I've been meaning to fix...

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solutions4circuits wrote 01/18/2016 at 21:39 point

Conformal coating makes your sparkgap useless.

I doubt you can ever pass UL with a ceramic cap connected directly to the mains. Certification is why you see film caps there.

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