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Running electric blanket on DC?

Discussion in 'Electronic Basics' started by Eric R Snow, Dec 3, 2004.

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  1. Eric R Snow

    Eric R Snow Guest

    Greetings All,
    I know this is silly but here it goes:
    My wife and I keep our bedroom un-heated so it gets cold in the
    winter. An electric blanket warms the bed before we get in. My wife
    would like to keep it on all night instead of using all the extra
    covers but is concerned that the "electrical magnetic field" emanating
    from the blanket will give us cancer or something. I mentioned that
    it's low frequency AC that people are all worried about and we are
    subjected to it all day from wires in the walls etc so why worry about
    the blanket? I know it's closer so any effect AC might have would be
    greater from the blanket but sheesh! Then, I foolishly said that if
    the blanket were run on DC we wouldn't need to worry. Now she wants me
    to fix the blanket to run on DC. I took apart the control and it seems
    like the heat control is just a bi-metal switch and the light is a
    little neon bulb. Is there any reason why it couldn't be run on DC? I
    envision a full wave rectifier and a capacitor. And it looks like
    there is enough room in the control for these extra parts. Is there
    anything inside the blanket itself that precludes Dc operation?
    Eric R Snow,
    Machinist, electonics hobbiest
  2. Don Kelly

    Don Kelly Guest


    It could be done but I wouldnt recommend it. The thermostat switch would
    likely fail quickly and start a fire as it is much harder to interrupt DC.
    What you would be doing is trading an insignificant (if present) hazard for
    a much greater one. Much safer to leave it alone.

    I suggest that you direct your wife to John Moulder's site.
    and to stop worrying. Either that or get a good down duvet.
    Also :
  3. Tim Williams

    Tim Williams Guest

    Meh. Take it apart, look like you're doing something, then put it back
    together as it was. As long as she knows you did something, but not exactly
    what, she'll buy it.

  4. amdx

    amdx Guest

    Meh. Take it apart, look like you're doing something, >then put it back
    together as it was. As long as she >knows you did something, but not
    exactly what, she'll >buy it. Tim

    Perception is reality!

  5. Bob Masta

    Bob Masta Guest

    I recall reading some years ago that in response to the
    alleged problem, manufacturers were rearranging
    the heating wire routing inside the blanket to
    largely cancel the fields. I believe the new wiring
    is just one big "hairpin" that is then routed around
    the blanket. There is no loop to be inside of.

    I might add that the original studies on the "problem"
    were of ridiculously poor quality, both on statistical
    and logical grounds. They probably would have
    never gone anywhere except that a reporter wrote
    up a big scare-mongering story in the New Yorker,
    and suddenly eveybody figured this was a real
    problem. Things like this can easily happen in
    a scientifically illiterate society.

    Just my 2 cents' worth!

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  6. I agree and would like to mention a further problem:
    If you just rectify (without filtering by a condenser) you get the same
    heat, but you still have a lot of alternating current (and field).
    If you filter to get pure DC, you need a very large condenser and ir
    will give the peak voltage (165v for 117V AC) and the blanket will
    Greetings R.Z.
  7. Eric R Snow

    Eric R Snow Guest

    Yeah, I know the study was crap. But my wife prefers to believe the
    Eric R Snow
  8. Kim Clay

    Kim Clay Guest

    Be a hero & convert it :)

    Just a FWB (no cap) & its DC - _definitely_ not AC anymore!

    Valid data snipped...
    She wants you to convert it to run on DC? Put your full wave bridge in
    the circuit. Presto - its DC - Yes its unfiltered DC but it _is not_ AC
    anymore! I don't know of any studies that have shown low levels of
    pulsating DC causing any problems :)

    Thermostat shouldn't be stressed because of the pulsating nature of the
    current - heat output should be the same - the neon light may be
    somewhat dimmer (only if FWB is between AC cord & controller) because
    just one side is flashing - that will prove its running on DC.

    Depending on how much room there is in the control box & the wiring
    arrangement, the preferred place for the bridge would be where the
    actual heater connects to the controller - the thermostat & neon would
    still be conducting AC but the heating element is running on DC. If that
    is difficult (maybe because of the small wires?) -
    Second best (but still very acceptable) would be at the AC input (cord)

    Please think "Safety" when working with AC mains.

    Everyone comes out a winner :)

  9. Does the blanket generate a strong EM field? WHy not find out?

    If you hook a coil to the input of an audio amp, then listen to
    the amplified output using headphones, you can easily hear the 60Hz
    magnetism detected by the coil. For instance, wave the coil around
    an AC wall clock motor, or around an old-style (non-electronic)
    fluorescent light ballast. BUZZZZZZ! Once you can detect strong
    AC magnetic fields, wave your coil around the electric blanket and
    see if it's stronger than, say, standing under a fluorescent ceiling

    Here's a great little $12 amp. All hobbyists should have one
    of these (I have several. They have lots of uses.) :

    Mini audio amplifier w/speaker, 9V

    That, a couple of mini-jacks, and a telephone pickup coil, and
    you're ready to scan the environment for those lethal 60Hz hums.

    Telephone pickup coil,44-533,&iGo=

    When I did this years ago at home, I discovered that the whole
    house was full of 60Hz b-fields. It was wired with old-style
    knob-and-tube wireing, where all of the conductors are several
    feet apart, rather than being twisted together inside a steel pipe.

    ((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
    William J. Beaty Research Engineer
    UW Chem Dept, Bagley Hall RM74
    Box 351700, Seattle, WA 98195-1700
    ph206-543-6195 http//
  10. Steve Evans

    Steve Evans Guest

    Good idea! wimmin will buy anything.
    it's not a good idea to convert hte blanket to dc, anyway. All the
    time its running on AC, there's an even distribuion of blood cells in
    your body. If you change to DC, all you blood cells will either get
    drawn to your head or your feet depending on which way around the
    polarity is. ;-)
  11. steamer

    steamer Guest

    --Getcher self a couple of warm doggies; works wonders, heh.
  12. Rich Grise

    Rich Grise Guest

    I'd be worried about any modification to an electric blanket. How does the
    thermostat actually sense the temperature of the blanket? Isn't the
    resistance of the element itself the sensor? If so, then any messing with
    it will upset its operation.

    I agree with that other poster - leave the AC, because a DC magnetic field
    will cause your blood to pool. ;-) Tell the wife you did some research and
    found out that AC is actually beneficial! We're engineers, we ought to

    Good Luck!
  13. Eric R Snow

    Eric R Snow Guest

    Greetings William,
    Thanks for the link to the amp and the ideas to use it. I can already
    think of other uses for it.
    Eric R Snow
  14. Eric R Snow

    Eric R Snow Guest

    Perfect answer Rich! I'll just say that the engineers on line have
    conducted a study and found that ac is actually good for you as it
    keeps the blood evenly distributed.
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