Why are these LED current limiting resistors so big?












1












$begingroup$


I'm looking at the charging circuit for a lithium ion battery that was made by Adafruit. Here's an excerpt:



enter image description here



If there's a 5V rail from the USB port and a 470 Ohms resistor, is that limiting the current through the LED to 6 mA just to keep the brightness low? The datasheet for the charger has 470 Ohms resistors in the reference design too. I'm just not sure why.






led battery-charging current-limiting







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$endgroup$








  • 1




    $begingroup$
    Modern LEDs are quite efficient, and the application here is just a status indicator, not trying to light up a room. Why would you waste more power than you need?
    $endgroup$
    – Dave Tweed♦
    3 hours ago






  • 1




    $begingroup$
    If it's just an indicator, you only need it to be visibly on, and for most LEDs 5mA or so is more than enough for that.
    $endgroup$
    – Hearth
    3 hours ago
















1












$begingroup$


I'm looking at the charging circuit for a lithium ion battery that was made by Adafruit. Here's an excerpt:



enter image description here



If there's a 5V rail from the USB port and a 470 Ohms resistor, is that limiting the current through the LED to 6 mA just to keep the brightness low? The datasheet for the charger has 470 Ohms resistors in the reference design too. I'm just not sure why.






led battery-charging current-limiting







share|improve this question









$endgroup$








  • 1




    $begingroup$
    Modern LEDs are quite efficient, and the application here is just a status indicator, not trying to light up a room. Why would you waste more power than you need?
    $endgroup$
    – Dave Tweed♦
    3 hours ago






  • 1




    $begingroup$
    If it's just an indicator, you only need it to be visibly on, and for most LEDs 5mA or so is more than enough for that.
    $endgroup$
    – Hearth
    3 hours ago














1












1








1





$begingroup$


I'm looking at the charging circuit for a lithium ion battery that was made by Adafruit. Here's an excerpt:



enter image description here



If there's a 5V rail from the USB port and a 470 Ohms resistor, is that limiting the current through the LED to 6 mA just to keep the brightness low? The datasheet for the charger has 470 Ohms resistors in the reference design too. I'm just not sure why.






led battery-charging current-limiting







share|improve this question









$endgroup$




I'm looking at the charging circuit for a lithium ion battery that was made by Adafruit. Here's an excerpt:



enter image description here



If there's a 5V rail from the USB port and a 470 Ohms resistor, is that limiting the current through the LED to 6 mA just to keep the brightness low? The datasheet for the charger has 470 Ohms resistors in the reference design too. I'm just not sure why.






led battery-charging current-limiting




led battery-charging current-limiting






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 3 hours ago









D. PatrickD. Patrick

237310




237310








  • 1




    $begingroup$
    Modern LEDs are quite efficient, and the application here is just a status indicator, not trying to light up a room. Why would you waste more power than you need?
    $endgroup$
    – Dave Tweed♦
    3 hours ago






  • 1




    $begingroup$
    If it's just an indicator, you only need it to be visibly on, and for most LEDs 5mA or so is more than enough for that.
    $endgroup$
    – Hearth
    3 hours ago














  • 1




    $begingroup$
    Modern LEDs are quite efficient, and the application here is just a status indicator, not trying to light up a room. Why would you waste more power than you need?
    $endgroup$
    – Dave Tweed♦
    3 hours ago






  • 1




    $begingroup$
    If it's just an indicator, you only need it to be visibly on, and for most LEDs 5mA or so is more than enough for that.
    $endgroup$
    – Hearth
    3 hours ago








1




1




$begingroup$
Modern LEDs are quite efficient, and the application here is just a status indicator, not trying to light up a room. Why would you waste more power than you need?
$endgroup$
– Dave Tweed♦
3 hours ago




$begingroup$
Modern LEDs are quite efficient, and the application here is just a status indicator, not trying to light up a room. Why would you waste more power than you need?
$endgroup$
– Dave Tweed♦
3 hours ago




1




1




$begingroup$
If it's just an indicator, you only need it to be visibly on, and for most LEDs 5mA or so is more than enough for that.
$endgroup$
– Hearth
3 hours ago




$begingroup$
If it's just an indicator, you only need it to be visibly on, and for most LEDs 5mA or so is more than enough for that.
$endgroup$
– Hearth
3 hours ago










1 Answer
1






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$begingroup$

The MCP73831 datasheet specifies a maximum source/sink current on the STAT pin of 35 mA and 25 mA respectively - so if the designer wanted brighter LEDs she could use lower-value resistors without worrying about overloading the output drivers on the charge controller IC. Higher currents would eat into the current available for charging, but not really to a significant extent.



So yes, I think the reason is simply that 6 mA is plenty bright enough for a status LED. Some would even say it's too bright, especially with modern high-efficiency LEDs.






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    1 Answer
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    $begingroup$

    The MCP73831 datasheet specifies a maximum source/sink current on the STAT pin of 35 mA and 25 mA respectively - so if the designer wanted brighter LEDs she could use lower-value resistors without worrying about overloading the output drivers on the charge controller IC. Higher currents would eat into the current available for charging, but not really to a significant extent.



    So yes, I think the reason is simply that 6 mA is plenty bright enough for a status LED. Some would even say it's too bright, especially with modern high-efficiency LEDs.






    share|improve this answer









    $endgroup$


















      3












      $begingroup$

      The MCP73831 datasheet specifies a maximum source/sink current on the STAT pin of 35 mA and 25 mA respectively - so if the designer wanted brighter LEDs she could use lower-value resistors without worrying about overloading the output drivers on the charge controller IC. Higher currents would eat into the current available for charging, but not really to a significant extent.



      So yes, I think the reason is simply that 6 mA is plenty bright enough for a status LED. Some would even say it's too bright, especially with modern high-efficiency LEDs.






      share|improve this answer









      $endgroup$
















        3












        3








        3





        $begingroup$

        The MCP73831 datasheet specifies a maximum source/sink current on the STAT pin of 35 mA and 25 mA respectively - so if the designer wanted brighter LEDs she could use lower-value resistors without worrying about overloading the output drivers on the charge controller IC. Higher currents would eat into the current available for charging, but not really to a significant extent.



        So yes, I think the reason is simply that 6 mA is plenty bright enough for a status LED. Some would even say it's too bright, especially with modern high-efficiency LEDs.






        share|improve this answer









        $endgroup$



        The MCP73831 datasheet specifies a maximum source/sink current on the STAT pin of 35 mA and 25 mA respectively - so if the designer wanted brighter LEDs she could use lower-value resistors without worrying about overloading the output drivers on the charge controller IC. Higher currents would eat into the current available for charging, but not really to a significant extent.



        So yes, I think the reason is simply that 6 mA is plenty bright enough for a status LED. Some would even say it's too bright, especially with modern high-efficiency LEDs.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 3 hours ago









        pericynthionpericynthion

        4,128929




        4,128929






























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