LM3914 - V Meter (resistor calculations)

Discussion in 'General Electronics Chat' started by deanp, Feb 17, 2012.

  1. deanp

    Thread Starter New Member

    Feb 17, 2012
    1
    0
    Hi All

    Im totally new to electronics, and require some help in trying to setup a 0-5V voltage meter using the LM3914 IC (datasheet). The meter will be powered by, and monitor the same rechargeable battery pack (4 x 1.2V @2000mAh). I have read Electronics for Dummies and can still remember the common formulae for electricity from high school i.e V=IR, P=IV, resistors in parrallel.

    I have managed to find a Simple Voltage Monitor sheet from NS (http://www.national.com/ms/LB/LB-48.pdf) with the exact specification I need, but I really would like to know how to calculate the resistors that are required for this project.

    Im also confused over what pins 4-8 actually mean, but from I could piece together from various implementations of this project on the internet:

    • Pin 4 - Vrlo - Lower range of the voltage to be monitored (0.9V x 4 = 3.6V) 0.9V is the voltage at which each battery would be considered as depleted.
    • Pin 5 - Signal source
    • Pin 6 - Vrhi - High range of voltage to be monitored ( 1.425V x 4 = 5.75V) 1.425V voltage at which each battery is at max charge.
    • Pin 7 - Ref Out Vrhi - Vrlo = 2.1V, the voltage range to be monitored i.e. 0.21V per/LED.
    • Pin 8 - ?



    I appreciate any help thats provided, and I apologise if I've left any information out of the post.
     
    Last edited: Feb 17, 2012
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Please post ur diagram .
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    If you look at the internal schematic of the device it's self explanatory. I have one from a design I did I will attach it.

    You have a string of ten comparators with 1K resistors in a string. You can ground the bottom of the string or put a resistor there to raise the first trigger point up. The 1.25V reference is there to let you make a resistive divider to set the bottom and top voltage set on the resistor string to any value between 0 and 1.25V.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I hope you're not implying that 1.25V is the maximum value that pin 7 can be set to.:confused:
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    It is using the internal reference and simple resistor dividers. The simple way is to st the ends of the resistor string to some voltage in that range then scale the input to match it. That's what the part was designed for, but you can use higher voltage on the comparators.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Just to clarify: Pin 7 can be set to any voltage between ≈1.28V and Vcc-1.5V.
     
Loading...