LM317 with battery charging indicator

Discussion in 'General Electronics Chat' started by tonir, Aug 17, 2012.

  1. tonir

    Thread Starter New Member

    Aug 9, 2012
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    Please somebody help me. I wanted to have an LED to indicate that my battery is being charged. I tried first by placing the LED on the Input side and then on the output side (with a 1K series resistor, so that less current flows through the LED) of the LM317. Both the times this resulted in less current at the output.

    Now my question is how to have an LED showing that the battery is being charged without reducing the output current for the battery.
     
  2. tonir

    Thread Starter New Member

    Aug 9, 2012
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    0
    I am using a 12v 7ah battery. The output of LM317 is set at 14.2v and current limit set to 400mA. The LED i'm using has a voltage drop of 2.5v and the current rating is 30mA. Though there is voltage drop at the output when LED is used, as i mentioned before, use of LED reduces the output current. Please somebody help me out.
     
  3. BillB3857

    Senior Member

    Feb 28, 2009
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    Schematic, schematic, schematic.............
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Do you want to detect when current is flowing to the battery or just that there's voltage available?

    Post a schematic.
     
  5. #12

    Expert

    Nov 30, 2010
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    Sometimes it just requires work.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You have to use a comparator to light the LED when the charging current drops to about .03C with the battery voltage at about 13.6V.

    Here is a design I did for a temperature compensated battery charger for lead acid battery with end of charge detect.
     
  7. tonir

    Thread Starter New Member

    Aug 9, 2012
    14
    0
    Hi #12,

    I tried your method. The current at the output didn't decrease. Thanks you saved my day.
     
  8. tonir

    Thread Starter New Member

    Aug 9, 2012
    14
    0
    Thank you everyone for your replies
     
  9. #12

    Expert

    Nov 30, 2010
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    Bountyhunter's circuit is also valid. Which one you choose depends on how accurate you need to be...and how good you are at assembling something with a couple of dozen parts in it.

    and whether you have enough extra voltage available to drive the current indicator in my circuit.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
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    If you put a current sense resistor in the ground wire (after the battery) then it won't affect the voltage regulation and can be used as a 0.6v sense signal to turn on a NPN, which can then light the LED.

    That only requires a headroom of 0.6v compared to >2v or so to operate a LED and its series resistor.
     
    #12 likes this.
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