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AC fan speed control by periodic on/off switching

Discussion in 'Electronic Design' started by ectoplasm, May 3, 2007.

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  1. John Fields

    John Fields Guest

    ---
    No, you should use a fuse which blows (or a circuit breaker which
    breaks) if the steady-state current into your load is exceeded for
    some amount of time.

    ISTR that your fan dissipates 60 watts, so the current it needs from
    the mains will be:

    P 60W
    I = --- = ------ = 0.5 ampere
    E 120V

    However, the inrush current could be up to 10 times that high,
    gradually decreasing to the running current as the motor comes up to
    speed. Which could be a problem if you don't let the motor run/rest
    long enough per ON-OFF cycle to get rid of the extra heat generated
    during startup.

    In any case, if that turns out not to be a problem, I suggest you
    use a slow-blow fuse:

    http://www.littelfuse.com/data/en/Data_Sheets/313_315.pdf

    Take a look at the chart on the bottom right of the page and you can
    see that a 2 amp fuse will carry 5 amps for 7 seconds before it
    blows, but then it also has to cool down, so you may need an
    electronic solution.

    Before you get into that though, I suggest that you run the thing
    with various duty cycles for long enough to determine whether the
    motor will overheat.
     
  2. ectoplasm

    ectoplasm Guest

    The result: (direct link to 127kB jpeg picture)
    <http://img512.imageshack.us/img512/5526/img1236spw3.jpg>

    I made it using the double 555 solution on
    <http://casemods.pointofnoreturn.org/pwm/circuit2.html>

    And the MOC3041 triac driver to switch on zero crossings, controlling
    a BT138X triac (12A, so it could be used for a number of purposes).
    The red LED is on when the triac is on. Both the period and pulse
    width can be independently controlled by the potentiometers from about
    0.25 to 4 seconds (pulse width should not exceed period of course).
    For any purpose, there's a switch that allows switching to a larger
    time base of about 3 to 100 seconds (pulse width up to 45 seconds).
    The red switch is to bypass the thing so the appliance is turned on as
    normal.

    Anyway, it does exactly what I needed it to do, and it might also be
    useful in other applications.
     
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