some help with LM3914

Discussion in 'The Projects Forum' started by david1234, Nov 27, 2013.

  1. david1234

    Thread Starter Member

    Nov 27, 2013
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    Hello ,
    I;m trying to make a battery tester , the voltage is 12<Vb<9
    I want to understand how to do this
    I have read what is writing here
    http://forum.allaboutcircuits.com/showthread.php?t=35567

    very helpful,
    but I didn't understand how to calculate the R3 for the lower voltage
    what is the formula ?

    I understand that for the High end this is what I need
    R2=10K , R1=1.21K

    according to this:
    Vmax=1.25(1+R2/R1) -
    where R1 connected between PIN7 and PIN8
    R2 connected between GND and R1
    like shown in the datasheet Figure 1

    any help will do

    Thanks ,
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    First, a few questions:-
    1) Do you want the tester to be powered by the battery under test?
    2) Will the 3914 be operated in dot or bar mode?
    3) What sort of LEDs will you use?
    4) How much current can the battery provide? Just a few mA, 1A, ....?
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Also, what kind of battery? Determining State of Charge of most batteries by measuring voltage is not very useful..., and then mVs count.

    A 2-1/2 digit 15Vdc LED/LCD voltmeter is < $5 on FleaBay. It has a resolution of a few mV. An 3914 will have a resolution approaching 1/2V per step, which is not useful as a battery fuel gauge.
     
  4. david1234

    Thread Starter Member

    Nov 27, 2013
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    1. I want the tester to be powered by the battery itself
    2. will work in bar mode (if I understand correct - this way when it's full I see 5 leds,right?)
    3. simple leds - If=80mA .
    4. the battery is something like 2Ah or even more.
    the check will be momentarily-not all the time' 1 sec from time to time

    do you need anything else in order to help me?
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    There is a circuit on page 2 of the datasheet, they have set the upper voltage to 5v, by making R1 1.2K,R2 10.5K the max voltage will be 12.5v, and each led will be on at 1.25v increments ,

    show us your circuit ,so we can see what your doing.
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I think you could put a resistor on the bottom of the internal resistor ladder between the RLO (pin 4) and ground to scale the active range of the LED's to the range you want. Then you would have smaller volts/LED step.
     
  7. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    Dodgydave - you are right this is what I have done
    took the circuit on page 2 and then use the formula to know what will be the Vmax

    but I want to know how can I change the Vmin - that will be not 0V but 9V or even 10V
    how do I do this? I know I need to add a resistor to PIN4 but which? and how to calculate it?

    tubeguy - how do I calculate it? to know what to put?

    Thanks .
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I would say a 30K resistor will give you 9v on pin 4 when 12v is on ref hi.


    inside the ic are 10 X 1K resistors in series, so you need to drop 3V across these, thats 300uAmp, so your resistor has to drop 9v at 300uAmp, thats 30K
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
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    To run this circuit from the same battery, you should use a voltage regulator to power the circuit at a voltage below the battery minimum voltage minus the regulator drop-out voltage. Need to double check the minimum operating voltage for the LM3914.

    Many simple LED's typically are 20ma max and only need 10ma's or less for reasonable brightness.
     
  10. david1234

    Thread Starter Member

    Nov 27, 2013
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    Dodgydave - can you shoe me how did you calculate this ?
    I don't understand the logic

    Thanks ,
     
    Last edited: Nov 28, 2013
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You want to set the voltages of 12V and 9V at the ref hi and lo pins, thats a difference of 3V across the internal resistors of 10K, so 3V/ 10K = 300uA current,

    the resistor on pin 4 has to drop 9V at a current of 300uA, 9V/300uA = 30K ohms.
     
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You could use the circuit on page 13 of the datasheet to vary the voltages on refhi and reflo separately.
     
    Last edited: Nov 28, 2013
  13. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You can't drive LEDs directly at 80mA from an LM3194. Why such high current type for a tester? Use ordinary LEDs, driven at just a few mA each.
    If you are testing lead-acid batteries be aware their voltage can be much higher (e.g. 13.8V) than 12V.
    Using only 5 LEDs to span the voltage range of interest will affect the choice of resistor values.
     
  14. david1234

    Thread Starter Member

    Nov 27, 2013
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    so is this schematic is good , and this is what I need to built?
     
  15. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Battery voltage more than 12 and less than 9?! I'm sure you meant 9<Vb<12. :)
     
  16. Dodgydave

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    Jun 22, 2012
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    Yep get it made....
     
  17. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Your reference voltage will change as the battery voltage drops if you power the tester directly from the battery under test, correct ?
     
  18. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    To the OP:

    You blew me off the first time I said this, but I'll say it again.

    A 12V lead acid battery is ruined if it gets to 11V! Why set the range from 9 to 12v?

    To be useful as a battery state-of-charge fuel gauge, a voltmeter should have an expanded resolution in the range from 11.5V (maximally discharged) to 14.5V (under charge).
     
  19. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Negligible change. The internal reference voltage changes by a maximum of 0.03%/V over a supply range of 3-18V according to the spec.
     
  20. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I wasn't clear enough, I was talking about the voltage to the comparator resistor string.
     
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