Sinking vs sourcing currents

Discussion in 'General Electronics Chat' started by Dritech, Jun 11, 2015.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi all,

    When connecting an LED to a microcontroller IO pin, what is actually the difference between connecting it in current sinking mode or current sourcing mode (apart from the face that in current sinking you have to output logic 1 for the LED to turn on and vice versa)?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Some CMOS chips can sink about 3X what they can source. With TTL it is even worse, more like 15X...

    Consult the data sheet for the source/sink capability of any chip you use to drive a LED
     
  3. #12

    Expert

    Nov 30, 2010
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    Arrgh! Terminology and semantics!
    It is my habit to think of the microcontroller to be sourcing or sinking current, so your last statement sounds backwards to me. The only difference I see is that you must look up the capability of the chip in both modes because some can conduct more current in one direction than the other.

    Sometimes you might be doing a waste power budget and choose for best saturation voltage inside the chip.

    and...MikeML got there first.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    e.g an input device connected to +ve sources, the input device will sink the device.
    The pdf shows how it pertains to PLC but the principle is the same.
    Max.
     
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  5. #12

    Expert

    Nov 30, 2010
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    Thanks for the reference, @MaxHeadRoom . It resembles my model of the universe, but I've learned a hundred times that my model is only one of the acceptable models.
     
  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Thanks a lot for the replies.
    When it comes to calculation, is there a different formula for connecting an LED in sinking or sourcing?
     
  7. #12

    Expert

    Nov 30, 2010
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    The only difference is in the saturation voltage of the sourcer/sinker. All else is EIR and PIE, as usual.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    The data sheet should state the source and sink current capability - if neither is enough for the LED you'll need a driver transistor anyway.

    It should also state how close the output can get to Vss or Vdd at the specified current load - this indicates what voltage you need to subtract from the value of Vdd to calculate the current limiting resistor - the LED of course has a Vf volt drop which you have to add to the balance of Vo when you calculate the resistor.
     
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