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DC on your doorstep

  1. May 11, 2005 #1
    Another intersting point on home electronics raised was - why does every device you purchase have it's own transformer. Would it not have made sense to implement a series of 12V DC plugs in modern homes?

    That way minor electronic internals could shape it as they want and we would not all be carrying these clunky transformers with us.

    Just a thought.

    Thanks
    Warrick
     
  2. jcsd
  3. May 11, 2005 #2

    chroot

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    It would be a brilliant idea, in fact. Maybe a bit higher than 12V, though -- 18V would be better.

    Microelectronics didn't exist when the 60 Hz AC system was invented; back then homes mostly had only motors and resistive-heating devices.

    - Warren
     
  4. May 11, 2005 #3

    DaveC426913

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    Nope.

    There is an excellent reason why they use AC in homes. It is way safer. Electrocution from DC is much more deadly than from AC.
     
  5. May 11, 2005 #4

    chroot

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    Uh, Dave.... 12 VDC won't do anything to you at all.

    Voltages lower than about 50 V are not harmful to people.

    - Warren
     
  6. May 11, 2005 #5
    Are you infering that the house is supplied with 12V dc?, or that plugs be adapted to carry transformers.

    On the 1st issue, kettles, cookers, etc. would not be able to function, and the power supply has become an international standard. This change would cause a lot of agro.

    If you are suggesting just changing the plug itself, isn't this just as simple as having a transformer in the appliance???
     
  7. May 11, 2005 #6

    chroot

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    I assumed he meant an additional plug, above the 120VAC plug, which supplied 12VDC from a large transformer which supplied the entire house.

    - Warren
     
  8. May 11, 2005 #7

    russ_watters

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    Are we also assuming that the myriad of DC devices that exist and use about 20 different voltages would all start coming at 12v...?
     
  9. May 12, 2005 #8
    dave,
    why is ac safer than dc?
     
  10. May 12, 2005 #9

    Integral

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    It is current which is fatal, not voltage, so if you get ~50ma through your heart your life will be in danger, whether the current is generated by 120VAC or 12VDC. So the question is can 12VDC generate 50ma through the heart. Certinally not through dry or even wet skin, but what if your skin is broken, if you were to get 12VDC under the skin and into the blood stream I would think that there exists a good chance that the blood would carry sufficient current to cause some troubles. This seems pretty unlikely, so I do not think safety is a reason not to do this.

    The best reason I can come up with is cost. Specialized wiring is not cheap. Losses, It would be difficult to maintain a consistent voltage thought the system. As was suggested above, a higher voltage would be better. I would think that 24VDC would be even better, it is a industry standard so there are many 24VDC devices on the market and that voltage gives some head room for regulation to the needed voltage. This of course is the key problem. There is no industry standard Voltage for running small electronics devices.
    So you would need a regulator for each device you plugged in, where is the gain? You still need a special plug for each device, and until every house hold in the developed world has a DC distribution you would still need to supply a
    AC-DC conversion.

    Seems simplist just to assume a AC source.
     
  11. May 12, 2005 #10

    Integral

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    AC goes to 0V twice every cycle. so it "lets go" giving you an opportunity to free yourself , DC never lets go.
     
  12. May 12, 2005 #11

    chroot

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    The advantage to DC is that there are myriad tiny surface-mount regulators available with efficiencies reaching 99%. These regulators also work with a wide variety of input voltages. By contrast, an AC-to-DC converter is clunky, has a transformer, and ends up being relegated into a wall-wart. A DC-to-DC converter is miniscule and could be integrated directly into the device without even changing its external appearance.

    - Warren
     
  13. May 12, 2005 #12
    A guy was electrocuted in downtown San Diego a couple years ago from the AC current that went to the lighting in a bus shelter. The wiring was shorted to some metal in the shelter frame, and he was grounded through a rain puddle on the sidewalk. Once the circuit was complete he could not let go. A witness tried to pull him free, first insulating his own hands with his jacket, but could not. If you're grounded 110 v AC is deadly. You can't let go.

    Theoretically, since the point of 0 voltage is there, you have the opportunity to let go, but in practise the human muscle cannot unclench in 1/60 of a second.
     
  14. May 12, 2005 #13

    DaveC426913

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    Really? Like to test that next time you open the hood of your car? I dare ya.
     
  15. May 12, 2005 #14

    DaveC426913

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    1] You sure it was AC? Fluorescent lights convert the power to DC (though that's internal). Don't know if they were fluor. or not.

    2] The idea is not so much that you have time to let go, as your muscles alterantely flex and distend, throwing you off.
     
  16. May 12, 2005 #15

    DaveC426913

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    Please.


    The phrase "...the myriad of..." is redundant.

    Correct usage is "...we also assuming that myriad DC devices ..."

    (See chroot's correct usage.)



    Apologies. Nothing personal. Pet peeve.
     
  17. May 12, 2005 #16

    chroot

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    I'd be quite happy to touch both terminals of a 12VDC battery with my hands. I have no idea why you think 12VDC is dangerous, because it isn't.

    - Warren
     
  18. May 12, 2005 #17

    chroot

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    No, they don't.

    - Warren
     
  19. May 12, 2005 #18
    I wouldn't do this. A car battery can deliver enormous current. My battery can deliver something like 800 cranking amps.
     
  20. May 12, 2005 #19

    chroot

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    I'll do it and sing a song at the same time, zooby.

    Your car battery can deliver 800 amps to a very small load, but a human body is not a very small load.

    12 VDC is utterly, completely, totally safe for humans. The only dangers are shorting things and causing sparks, catching things on fire, etc. -- not electrocution.

    The automotive industry regards 50V as the threshold for safety. There is considerable incentive in the industry to move to 40V automotive electrical systems to improve efficiency.

    - Warren
     
  21. May 12, 2005 #20
    You will wet your hands with salt water and measure your resistance with an accurate meter, then work out how much current you'll recieve BEFORE you try this. I just don't like the sound of it.
     
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