Kill A Watt teardown


Come on, folks. If we keep tearing apart everything that’s handed to us, we’ll never get nice things. SparkFun got their mitts on two Kill A Watts and proceeded to plug them into everything and then dismantled them to see how they work. The Kill A Watt keeps track of how much power is used over time. The largest load they found was their soda machine using 500W (should probably add a motion sensor to that). They plugged a meter on either side of a UPS and found out that it uses 5W just to charge. On the inside of the meter, there isn’t anything too substantial. One unlabeled IC runs the whole show.


  1. tony says:

    probably an micro controller with two analog to digital inputs and a shunt. calculate the current drawn from the voltage drop, multiply by line voltage to get watts.

  2. MRE says:

    Im curious about the green ‘brick’ and the u shaped bar..

    Without being able to read the lables from the photo, Im making a stab in the dark that the green brick is the shunt resistor. Id expect the shunt to be rather heavy duty to disipate a lot of heat (usually white bricks with thick leads though). cept, from the other photos it looks as if the traces are putting it across the power spades. could be some sort of stepdown or bridge for the low voltage side.

    Perhaps the u wire is some sort of resistive metal aloy and serves as the shunt?
    Id really like to see a photo of the back end of this, and much more info from the original post.

  3. Brian says:

    The reason the soda machine uses so much is that is is basically a refrigerator. You wouldn’t put a motion sensor on your fridge would you? You’d get warm beverages…

  4. MRE says:

    looking again, it really does look like the u wire connects the left set of spades together (fat traces on the bottom?) with two thin traces on the top side for measurement. It must be a resistive element. the green brick must be for the dc low voltage side. integrated power supply…?
    What are the numbers on it?

  5. tony says:

    the green box looks like a cap. you can see on the board where its says ‘c4′. I’m almost certain the u shaped part is the shunt.

  6. cde says:

    Damn you Tony, I was just about to mention the C4 marking for the green brick. The U bar is also marked at R17, so a resistor.

  7. Taylor says:

    I was thinking this was that ridex thing they constantly advertise on TV. I’d like to see the guts of one of those. Too bad i’m cheap. :]

  8. Taylor says:

    Sorry for not being very clear…

  9. jproach says:

    The U shaped bar is the current sense resistance, you will find a similar design in some multimeters.

    The green box is a cap (notice the C4 on the silkscreen :)). Looks like it forms the power supply (see transformerless power supply:

    brian: Your home fridge doesn’t run all the time, it has a duty cycle (somewhere around 50%). This soda machine is clearly designed to run 100% of the time, for whatever reason. I believe a standard fridge is around 6-800W peak, which is 3-400W average (just a bit less than the soda machine, which has lights, etc.).

  10. Vik Olliver says:

    I’d love to know how to make these things work under 220V. I was given one by a well-meaning American friend and found it most useful – until I came home.

    Yes, I have taken it apart and also just tried it in 220V just for giggles. Screen lights up but the voltage and power readings are waaaay off base.

    Vik :v)

  11. jproach says:

    vik, you could probably swap some component values around the op-amp, OR there may be some way to calibrate it for 220V: (looks like the values may go into that i2c eeprom).

    I would be very careful though, seeing as the thing is rated for 110.

  12. MRE says:

    Waaa! I see the C4 marking. I havn’t seen such a cap much, so I guess I just dismissed it for something else.

    anyone else see the irony of an off-color green block capacitor being labled ‘c4′ ?

  13. cde says:

    @ MRE, thats not irony -_-

  14. Solenoidclock says:

    To reinforce cde, irony is a word that most people use incorrectly. Most folks think it means a humorous coincidence, but it really means an outcome that is the opposite of what is expected.

    So, if a porn star moved to Virgin, Utah, that would be ironic. If the same porn star bought a house in Boner Knob, Montana, that would not be ironic.

  15. Orv says:

    @ brian:

    There’s actually a company that makes motion sensors for pop machines. You might see them every so often if you look carefully — they use a burglar alarm type sensor on top of the machine. It powers the machine on just often enough to keep the beverages cold when no one’s around. The main power savings is in not having the front panel bulbs on all the time when there’s no one’s attention to attract.

  16. riddex says:

    you can buy one of those riddex pest things here: Pretty cheap for 2x – one to take apart and one to test…

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