LM3914 HELP - Connecting multiple units

Discussion in 'General Electronics Chat' started by FADM Stern GNSF, Mar 25, 2011.

  1. FADM Stern GNSF

    Thread Starter New Member

    Mar 17, 2011
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    0
    Can anyone help me with connecting multiple LM3914's together. I cant find documentation anywhere for using more than two, and I need 6 tied together (yep, 60 led's). I have breadboarded my 6 and it does work, but the resistor values for Rhi / REF OUT are driving me nuts.

    From the data sheets, projects, etc, the values seem to increase (looks like double) as you increase chips, but the LED's brightness changes with each group of 10.

    Is there a formula I can use to set these resistors so I can keep the LED current the same with each chip, or do I need to make 6 seperate external references for the chips .

    It would also be nice to have an adjustment so I can vary the LED current of ALL 60 together (I need aprox between dim and 8mA as a full 10ma get the chip too warm for my liking)

    Any help would be greatly appreciated
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    How do you want to drive the LM3914?
    In bar or dot mode?

    On pin 7 of the LM3914 there must be a resistor connected to ground.
    This resistor determines the led current.
    See also the LM3914 internals diagram.

    [​IMG]

    For connecting the LM3914 together, connect the pins 6 to pin 4 of the next one.
    This way you are making a long reference line over all LM3914's.
    The reference voltage should be connected at the last LM3914 (pin 6).
    The first LM3914 must have the lower reference (pin 4).
    All input pins must be connected together (pin 5).

    Bertus
     
  3. FADM Stern GNSF

    Thread Starter New Member

    Mar 17, 2011
    5
    0
    Thanks for the response. I am driving it in bar mode (another reason for keeping the LED current low)

    My problem is basically that all info including the data sheets show examples with a max of 2 LM3914's. I need 6 connected and no where can I find any info on what values to set the resistors for subsequent IC's

    I have all inputs tied together (pin 5) with pins 4 and 8 set to V- on the first one. Pins 6 and 7 tied together feeding the next IC's pins 8 and 4, with a resistor to GND. This go's on in a chain all the way down the line of 6 IC's. I have it breadboarded and it does work, but with a few issues;

    a) Not sure what to make the subsequesnt resistors for pins 6 and 7 (data sheet shows an example of a 20 segment meter, page 12 with a 1.1K for the first IC and 2.4K for the second IC. What would the 3rd IC be .... 3.3K accumulative, or 4.8K factoring??).

    b) Mine works but each IC in the string seems to be driving the 10 LED's with different current levels (brightness varies for IC block of 10 LED's to the next)

    I am sure there must be a formula somewhere to calculate each successive resistor value based on the internal divider and previous ones, but for the life of me cant figure it out short of trial and error resistor swaping lol.

    Hope I explained this so you can understand as its hard for me to describe it without rambling on and driving you nuts lol
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Let us assume that LM3914 for the lowest values is nr 1 AND LM3914 for the highest values is nr 6.
    Make the following connections on ALL LM3914's:
    Connect pin 9 to pin 3 (for bar mode).
    Connect a resistor of (1.25 / (Iled/10)) between pin 7 and pin 2 connected to pin 8.

    Connect pin 4 nr1 to lowest reference voltage.
    Connect pin 6 nr1 to pin 4 nr2.
    Connect pin 6 nr2 to pin 4 nr3.
    Connect pin 6 nr3 to pin 4 nr4.
    Connect pin 6 nr4 to pin 4 nr5.
    Connect pin 6 nr5 to pin 4 nr6.
    Connect pin 6 nr6 to the high reference voltage.

    I hope this helps.

    Bertus
     
  5. FADM Stern GNSF

    Thread Starter New Member

    Mar 17, 2011
    5
    0
    Thanks so much Bertus, I got so wrapped up in the ref pins I didnt see the forest for the trees :) Works great now, just need to tinker with it a bit to try to get the LED's to pop on and off instead of dimming in and out. Going to look at the hysteresis example and see if I can adapt it for my circuit without having to duplicate it 6 times.
     
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