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variable electromagnet

Discussion in 'Electronic Basics' started by archi, Dec 22, 2004.

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  1. archi

    archi Guest

    dear all,

    i would like to build a variable electromagnet that is directly
    connected to a computer controlled (220v AC) dimming unit. i suppose
    that dimming the electric current will lower the magnetic field and
    thus its attractive force? the (somewhat creative) goal is to
    gradually attract light weight suspended metal parts depending on the
    dimming unit output.

    - is this possible? if so, could you give any tips what to do or where
    to look?
    - dangerous? (e.g. to nearby computers or other electric equipment)
    - what would be the range of such a electromagnet? the suspended
    objects will be very light but some object-magnet distance should be
    kept.

    thankx beforehand for any help!

    - Andre.
     
  2. You are not making this easy on yourself by starting with a 220 volt
    AC dimming unit. It is not trivial to link this to a computer, and it
    involves obviously lethal voltage.

    I think you should read up on the concept of a buck switching
    regulator that is operated in current control mode. This could be run
    from any DC supply you might have, including an automotive battery
    charger.

    Here is a link to a tutorial on the concept of a buck regulator. The
    inductor in the regulator would be your lifting electromagnet.

    http://www.national.com/appinfo/power/files/f5.pdf

    To operate a buck regulator in current control, you need a way to
    measure the coil current, and a low value resistor in series with the
    ground end is one way to produce a signal that represents the
    current. For starters, you can ignore the current and just control
    the on off duty cycle.
     
  3. andre

    andre Guest

     
  4. andre

    andre Guest

    thankx for your rapid answer. the computer-controlled dimmer is the
    'easy' part, as i received one 2nd hand. this equipment is already
    fully functioning, and thus the reason for the 240v AC constraint.
    however, i don't know where to buy or how to make such electromagnets,
    if they can be used for long time spans in safe conditions and whether
    they are able to attract light metal objects about 10 inches away.
    - Andre.
    ---
     
  5. But from the sophistication of your question, I fear for your safety
    if you use it to start your experiments.

    Some generalities:

    The reach of an electromagnet follows its size. If you want to
    attract metal from 10 inches away, you will need to think in tens of a
    coil with this scale of dimensions. Before you build a big one, scale
    the whole thing down and learn with a small model.

    Coils run with AC are not as useful at attracting things than coils
    run on DC (they make things hum and make iron cores hot from eddy
    current), so you will probably need a rectifier between your dimmer
    and the coil. This may make the dimmer work strangely.

    A safer way to use the 240 dimmer would be to drive a step down
    transformer with it and do all your experiments on the low voltage
    (say, below 30 volts) side after you tape up all the high voltage
    terminals. Don't forget to add a fuse on the line side to keep an
    oops from making the dimmer and/or transformer burst into flames.

    An iron core through the coil will concentrate the magnetic field but
    also retain some field when the current is off.
     
  6. John, what about a variac? Then, all he needs is a beefy diode ( and the
    magnet coil, of course ).

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  7. The OP seems determined to use a computer controlled dimmer.
     
  8. D Akers

    D Akers Guest

    "i would like to build a variable electromagnet that is directly
    connected to a computer controlled (220v AC) dimming unit. i suppose
    that dimming the electric current will lower the magnetic field and thus
    its attractive force? the (somewhat creative) goal is to gradually
    attract light weight suspended metal parts depending on the dimming unit
    output.
    - is this possible? if so, could you give any tips what to do or where
    to look?
    - dangerous? (e.g. to nearby computers or other electric equipment)
    - what would be the range of such a electromagnet? the suspended objects
    will be very light but some object-magnet distance should be kept."
    ___________________________________
    Re;
    Not sure about the AC dimmer control part; but I would suggest that you
    use a full wave rectifier to provide DC current to the magnet coil. I
    would also recommend that you use a soft iron core to minimize magnetic
    "memory". You can scavenge them from discarded AM radios; they are used
    in the "antennae coil". They are also found in some large inductors.
    Assuming that would be meet your needs. Just remember that the magnetic
    field strength will be proportional to the number of turns of wire you
    wind around the core and the magnitude of the DC current flowing through
    it. The more turns the greater the field strength. Thus, even a very
    small current can create a powerful field with a large number of turns.
    The "range" of the field will depend upon it's shape and that in turn
    depends on the geometry of the coil/core assembly. If the ratio of the
    length to diameter is greater, then the range of the field will be
    greater.
    You may also need a power resistor between the rectifier and coil to
    provide the current control you desire. The characteristics of the AC
    controller will determine this.
    Yes, strong DC magnetic fields can do strange things to the electron
    beam of cathode ray tubes; but they usually recover during the auto
    degauss at start-up.
    I hope that helps in some way...

    -Dan Akers
     
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