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Over voltage and under voltage protection

Discussion in 'Electronic Design' started by Jack// ani, Nov 16, 2004.

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  1. Jack// ani

    Jack// ani Guest

    Hi, I need a schematic or idea, which would protect my machine against
    any overvoltage/undervoltage fault. Idea is to switch off the relay
    when uC reads high output. So I need an arrangement that could produce
    high output (say 5volts), when voltage exceeds 240volts or falls below
    180volts ac.

    Thanks for any suggestion
     
  2. Ian Stirling

    Ian Stirling Guest

    A/D on the micro?
     
  3. Dingus

    Dingus Guest

    Use two comparators (LM311 etc). One monitoring UC the other OC.
    Outputs of both to a common transistor driving a relay. Place the
    relay contacts in series with whatever you want to control.
    This configuration is called a window comparator. You can
    purchase a ready made device from Omron and so on.
     
  4. Jack// ani

    Jack// ani Guest

    No.
     
  5. Ken Smith

    Ken Smith Guest


    I assume that you have a transformer, full wave rectifier and voltage
    regulator making the Vcc for the micro to run on and to power the relay.
    The AC voltage before the regulator makes a nice safe place to see the
    line voltage reduced by the transformer. The 5V you run the micro on
    should be good enough to serve as the reference for the comparison.

    Lets say you have this:

    ----- Vx
    ! Vur
    -+-->!-----+------ to caps etc
    ( !
    ( !
    ( !
    +----GND !
    ( !
    ( !
    ( !
    ---->!-----


    I suggest this:
    +5V
    !
    \
    Vur /
    D1 R1 ! \ 10K
    Vx---->!----/\/\/--+-----------!-\ ! Logic
    ! ! >---------+------ To Micro
    / +5V---!+/ LM311
    R2 \ !
    / GND
    !
    GND


    D1 = 1N914 or the like


    R1 and R2 are about 10K such that:


    180 = K * (0.7 + 5V * (R1 + R2)/R2)

    K = the transformer's turns ratio


    The voltage is too low, if the micro doesn't see the Logic go low for at
    least:


    0.707 = cos(Angle/2)

    Angle = 90 degrees

    The voltage is too high if the micro sees the Logic go low for:


    0.707 * 180/240 = cos(Angle/2)

    Angle = 116 degrees

    The micro can time the low pulses.

    I avoided measuring at the peak because the rectifier draws a pulse of
    current at the peak to charge the caps. This would make the peak
    measurement lower than the true value because of the resistance of the
    transformer.
     
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