LM3914 help

Discussion in 'The Projects Forum' started by garrysam, Apr 14, 2008.

  1. garrysam

    Thread Starter New Member

    Apr 12, 2008
    2
    0
    HI
    I am setting up a circuit using LM3914 to drive 2 x 10 segment led bar graphs

    I need to measure from 1 volt to 10 v with each led being .5 volt
    I have attached a cuircuit. could some experts look at this circuit & tell me if this looks ok

    I have put 2 pots in to adjust the lowest & the highest led turn on states
    These are VR1 & Vr2, I have also added VR3 to adjust led brightness.

    I have done this circuit like this so I can use it fo many different applications, eg but adjusting the low voltage to start at 4 & top out at 8 or 1 to 10 as i require this time.
    As you will see I am not to electrics wise, but trying to learn more

    Thanks in advance on any coments or sugestions.

    Sam
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The LM3914 has an accurate regulated voltage reference at OUT that you are not using. It can be programmed with 2 resistors for any voltage from 1.25V to 1.5V less than the supply voltage. The pots can be driven from it.
    Then your supply voltage can be lower so that the LM3914 does not get so hot then the LED current can be increased for more brightness. Your LED current is max only 7mA now which is not bright.
     
  3. garrysam

    Thread Starter New Member

    Apr 12, 2008
    2
    0
    Hi
    Thanks for the tips
    I guess you mean to do this like this one attached

    With regard to the led brightness, i uderstand that led current=12.5/(r2+r3)
    so if the are 20millamp i would want a minimum of 625ohms is that correct
    then i could increase this value to dim the leds

    Thanks again for any asistance
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your voltage reference is only 1.25V. Then the lowest LED on the first LM3914 detects an input voltage of only 0.0125V which is where the input offset voltage could cause a 100% error. If another resistor is added to increase the reference voltage then the error will be reduced.

    You should not short together the outputs of two voltage references. Use only one of them but load the other the same so the LEDs will have the same brightnesses.

    You have lost the ability to change the max detected voltage.

    You can make it very complicated with an opamp or two and many pots as shown in the datasheet but maybe your first circuit will be fine if it has a regulated supply and its LEDs are set for more current.
     
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