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Simple LM3914 Question

Discussion in 'Electronic Basics' started by ahonda55, Oct 27, 2005.

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  1. ahonda55

    ahonda55 Guest

    Hi all , i have a circuit with two LM3914 ICs and 20 LED
    the input of the 3914 is 2v (not starting from zero) to 7 volts fo
    exampl

    how can i calculate the resistor values to put in (Ref. Low) and (Ref
    High) terminals of the IC to achive the desired range of volts

    I have the data sheet but somehow i could not get that informatio
    from it ;

    sorry for ascing many questions . i am beginner !

    Thanks a lot friends :
     
  2. On page 3 of the data sheet:
    http://cache.national.com/ds/LM/LM3914.pdf
    the divider resistance is pretty sloppy, 8k to 17k.

    What is your desired range of signal indication?

    You need to create a voltage for each end of the divider to define the
    indicating range, and it has to be between zero and 1.5 volts less
    than the positive supply rail.

    On page 8, they discuss how you use two resistors to program the
    reference regulator to produce an arbitrary voltage, and how the
    values of those resistors also set the LED current. This is a good
    way to create the desired voltage for the upper end of the divider
    string. You can use a resistor between the bottom end and ground to
    elevate the voltage of the lowest signal indicated, but you cannot
    calculate this resistor very accurately, because of the large
    tolerance of the divider resistance. You may need to try different
    values, or use a trim pot to set the voltage while measuring the
    voltage at pin 4 with a meter.
     
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