Do output LEDs here need resistors?











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Consider:



Enter image description here



Here is the schematic I'm trying to work around. My major question is about the LED bar graph I want to use, on the right side; does it need resistors?



I saw three different versions of this microcontroller use ... sometimes with, sometimes without, and the datasheet did not really help. On this point, and it also displays an example of use without a resistor.



Bonus question: Is there something wrong in this?










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    The 3915 is not a microprcessor (UC.)
    – JRE
    Nov 24 at 15:56















up vote
1
down vote

favorite
2












Consider:



Enter image description here



Here is the schematic I'm trying to work around. My major question is about the LED bar graph I want to use, on the right side; does it need resistors?



I saw three different versions of this microcontroller use ... sometimes with, sometimes without, and the datasheet did not really help. On this point, and it also displays an example of use without a resistor.



Bonus question: Is there something wrong in this?










share|improve this question




















  • 3




    The 3915 is not a microprcessor (UC.)
    – JRE
    Nov 24 at 15:56













up vote
1
down vote

favorite
2









up vote
1
down vote

favorite
2






2





Consider:



Enter image description here



Here is the schematic I'm trying to work around. My major question is about the LED bar graph I want to use, on the right side; does it need resistors?



I saw three different versions of this microcontroller use ... sometimes with, sometimes without, and the datasheet did not really help. On this point, and it also displays an example of use without a resistor.



Bonus question: Is there something wrong in this?










share|improve this question















Consider:



Enter image description here



Here is the schematic I'm trying to work around. My major question is about the LED bar graph I want to use, on the right side; does it need resistors?



I saw three different versions of this microcontroller use ... sometimes with, sometimes without, and the datasheet did not really help. On this point, and it also displays an example of use without a resistor.



Bonus question: Is there something wrong in this?







led-driver lm3915






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Nov 25 at 11:51









Peter Mortensen

1,58031422




1,58031422










asked Nov 24 at 15:25









francois P

13817




13817








  • 3




    The 3915 is not a microprcessor (UC.)
    – JRE
    Nov 24 at 15:56














  • 3




    The 3915 is not a microprcessor (UC.)
    – JRE
    Nov 24 at 15:56








3




3




The 3915 is not a microprcessor (UC.)
– JRE
Nov 24 at 15:56




The 3915 is not a microprcessor (UC.)
– JRE
Nov 24 at 15:56










3 Answers
3






active

oldest

votes

















up vote
12
down vote



accepted










As always, READ THE DATASHEET!



The chip you use has programmable constant current outputs, so you don't need external resistors to determine the current, the chip does this for you.



The current sunk by each output is ~ 10 x the current out of the Vref.



enter image description here



In your case 10 * ( 1.2 V / (1k2 + 680 ) Ohm ) ~= 6 mA, which is OK for a normal (20 mA max) LED.



**strong text**






share|improve this answer






























    up vote
    5
    down vote













    The LM3915 regulates the current to the LEDs. You do not need a current limiting resistor for each LED.



    In your circuit, R9 and R10 set the LED current.



    Vref is 1.25V. The current drawn through that pin controls the LED current.



    According to the datasheet, the LED current is 10* the current drawn through Vref.



    In your circuit, that'd be 10*(1.25V/(680 ohms + 1200 ohms))



    So, about 6.5 mA.



    To make the LEDs brighter, you reduce R9 and R10 (but keep the ratio the same.) So, if you changed to R9=340 ohms and R10= 600 ohms, you'd get about 13 mA for the LEDs.



    The given current is per LED.



    I've simplified the calculation. The datasheet says you are supposed to take into account the current through the internal divider chain. That amounts to about 0.5 mA, so I just ignored it for simplicity's sake.






    share|improve this answer




























      up vote
      2
      down vote













      No, you don't need resistors with LM3915.



      The LEDs need to be driven with the correct current to achieve the desired brightness. The resistors are used with LEDs to adjust the current when they are driven with constant voltage. LM3915 drives the LEDs with a constant current, which you can program, so you don't need the resistors.






      share|improve this answer























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        3 Answers
        3






        active

        oldest

        votes








        3 Answers
        3






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes








        up vote
        12
        down vote



        accepted










        As always, READ THE DATASHEET!



        The chip you use has programmable constant current outputs, so you don't need external resistors to determine the current, the chip does this for you.



        The current sunk by each output is ~ 10 x the current out of the Vref.



        enter image description here



        In your case 10 * ( 1.2 V / (1k2 + 680 ) Ohm ) ~= 6 mA, which is OK for a normal (20 mA max) LED.



        **strong text**






        share|improve this answer



























          up vote
          12
          down vote



          accepted










          As always, READ THE DATASHEET!



          The chip you use has programmable constant current outputs, so you don't need external resistors to determine the current, the chip does this for you.



          The current sunk by each output is ~ 10 x the current out of the Vref.



          enter image description here



          In your case 10 * ( 1.2 V / (1k2 + 680 ) Ohm ) ~= 6 mA, which is OK for a normal (20 mA max) LED.



          **strong text**






          share|improve this answer

























            up vote
            12
            down vote



            accepted







            up vote
            12
            down vote



            accepted






            As always, READ THE DATASHEET!



            The chip you use has programmable constant current outputs, so you don't need external resistors to determine the current, the chip does this for you.



            The current sunk by each output is ~ 10 x the current out of the Vref.



            enter image description here



            In your case 10 * ( 1.2 V / (1k2 + 680 ) Ohm ) ~= 6 mA, which is OK for a normal (20 mA max) LED.



            **strong text**






            share|improve this answer














            As always, READ THE DATASHEET!



            The chip you use has programmable constant current outputs, so you don't need external resistors to determine the current, the chip does this for you.



            The current sunk by each output is ~ 10 x the current out of the Vref.



            enter image description here



            In your case 10 * ( 1.2 V / (1k2 + 680 ) Ohm ) ~= 6 mA, which is OK for a normal (20 mA max) LED.



            **strong text**







            share|improve this answer














            share|improve this answer



            share|improve this answer








            edited Nov 24 at 21:32

























            answered Nov 24 at 15:51









            Wouter van Ooijen

            44.1k150117




            44.1k150117
























                up vote
                5
                down vote













                The LM3915 regulates the current to the LEDs. You do not need a current limiting resistor for each LED.



                In your circuit, R9 and R10 set the LED current.



                Vref is 1.25V. The current drawn through that pin controls the LED current.



                According to the datasheet, the LED current is 10* the current drawn through Vref.



                In your circuit, that'd be 10*(1.25V/(680 ohms + 1200 ohms))



                So, about 6.5 mA.



                To make the LEDs brighter, you reduce R9 and R10 (but keep the ratio the same.) So, if you changed to R9=340 ohms and R10= 600 ohms, you'd get about 13 mA for the LEDs.



                The given current is per LED.



                I've simplified the calculation. The datasheet says you are supposed to take into account the current through the internal divider chain. That amounts to about 0.5 mA, so I just ignored it for simplicity's sake.






                share|improve this answer

























                  up vote
                  5
                  down vote













                  The LM3915 regulates the current to the LEDs. You do not need a current limiting resistor for each LED.



                  In your circuit, R9 and R10 set the LED current.



                  Vref is 1.25V. The current drawn through that pin controls the LED current.



                  According to the datasheet, the LED current is 10* the current drawn through Vref.



                  In your circuit, that'd be 10*(1.25V/(680 ohms + 1200 ohms))



                  So, about 6.5 mA.



                  To make the LEDs brighter, you reduce R9 and R10 (but keep the ratio the same.) So, if you changed to R9=340 ohms and R10= 600 ohms, you'd get about 13 mA for the LEDs.



                  The given current is per LED.



                  I've simplified the calculation. The datasheet says you are supposed to take into account the current through the internal divider chain. That amounts to about 0.5 mA, so I just ignored it for simplicity's sake.






                  share|improve this answer























                    up vote
                    5
                    down vote










                    up vote
                    5
                    down vote









                    The LM3915 regulates the current to the LEDs. You do not need a current limiting resistor for each LED.



                    In your circuit, R9 and R10 set the LED current.



                    Vref is 1.25V. The current drawn through that pin controls the LED current.



                    According to the datasheet, the LED current is 10* the current drawn through Vref.



                    In your circuit, that'd be 10*(1.25V/(680 ohms + 1200 ohms))



                    So, about 6.5 mA.



                    To make the LEDs brighter, you reduce R9 and R10 (but keep the ratio the same.) So, if you changed to R9=340 ohms and R10= 600 ohms, you'd get about 13 mA for the LEDs.



                    The given current is per LED.



                    I've simplified the calculation. The datasheet says you are supposed to take into account the current through the internal divider chain. That amounts to about 0.5 mA, so I just ignored it for simplicity's sake.






                    share|improve this answer












                    The LM3915 regulates the current to the LEDs. You do not need a current limiting resistor for each LED.



                    In your circuit, R9 and R10 set the LED current.



                    Vref is 1.25V. The current drawn through that pin controls the LED current.



                    According to the datasheet, the LED current is 10* the current drawn through Vref.



                    In your circuit, that'd be 10*(1.25V/(680 ohms + 1200 ohms))



                    So, about 6.5 mA.



                    To make the LEDs brighter, you reduce R9 and R10 (but keep the ratio the same.) So, if you changed to R9=340 ohms and R10= 600 ohms, you'd get about 13 mA for the LEDs.



                    The given current is per LED.



                    I've simplified the calculation. The datasheet says you are supposed to take into account the current through the internal divider chain. That amounts to about 0.5 mA, so I just ignored it for simplicity's sake.







                    share|improve this answer












                    share|improve this answer



                    share|improve this answer










                    answered Nov 24 at 15:53









                    JRE

                    20.3k43767




                    20.3k43767






















                        up vote
                        2
                        down vote













                        No, you don't need resistors with LM3915.



                        The LEDs need to be driven with the correct current to achieve the desired brightness. The resistors are used with LEDs to adjust the current when they are driven with constant voltage. LM3915 drives the LEDs with a constant current, which you can program, so you don't need the resistors.






                        share|improve this answer



























                          up vote
                          2
                          down vote













                          No, you don't need resistors with LM3915.



                          The LEDs need to be driven with the correct current to achieve the desired brightness. The resistors are used with LEDs to adjust the current when they are driven with constant voltage. LM3915 drives the LEDs with a constant current, which you can program, so you don't need the resistors.






                          share|improve this answer

























                            up vote
                            2
                            down vote










                            up vote
                            2
                            down vote









                            No, you don't need resistors with LM3915.



                            The LEDs need to be driven with the correct current to achieve the desired brightness. The resistors are used with LEDs to adjust the current when they are driven with constant voltage. LM3915 drives the LEDs with a constant current, which you can program, so you don't need the resistors.






                            share|improve this answer














                            No, you don't need resistors with LM3915.



                            The LEDs need to be driven with the correct current to achieve the desired brightness. The resistors are used with LEDs to adjust the current when they are driven with constant voltage. LM3915 drives the LEDs with a constant current, which you can program, so you don't need the resistors.







                            share|improve this answer














                            share|improve this answer



                            share|improve this answer








                            edited Nov 25 at 11:50









                            Peter Mortensen

                            1,58031422




                            1,58031422










                            answered Nov 24 at 15:45









                            TemeV

                            665




                            665






























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