Sinking mode.

Discussion in 'General Electronics Chat' started by Voltboy, Nov 26, 2007.

  1. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    Hello.
    I was reading about how to power LED's to microcontroller and I saw Sink and Source mode, I undestood the source mode very well. But the sink mode didn't made much sense to me.
    How if a pin is low, the led will be on, doesn't the pin when its low is like an unconnected pin, or does it connects to ground?

    Thanks :rolleyes:
     
  2. Distort10n

    Active Member

    Dec 25, 2006
    429
    1
    If the output is similar to a NPN inverter, and the LED is connected to the collector then this is an example of sinking current. The transistor is turned on, the cathode of the LED is brought close to GND, and pulls current from whatever supply it is connected to.
     
  3. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    In a PIC microcontroller, if the output of a pin is low it means it is like an unconnected pin, or it means that it is closed to ground?
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    When a pin is low, normally it presents 0V, relative to ground, which means that the voltage difference between that pin and Vcc will be the same of Vcc. Therefore, a LED connected to Vcc (making the IC sinking current) will light it the pin is low, but not when the pin is high.

    TTL is often used to light LEDs this way, because it can sink up to 16mA, but can only source 2mA. Therefore, sourcing a LED with TTL is not a good idea.
     
  5. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    What makes sinking current in TTL up to 16mA and sourcing about 2mA?
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    If the TTL output is not open collector, then it will usually be some variation of a totem-pole. The upper transistor will have a collector resistor, but the lower transistor will not have an emitter resistor.
     
  7. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    what exactly is a totem pole?
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Totem pole:
    totem.jpg
     
  9. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    Ohh thanks.. now I get sinking and sourcing..
    Sinking occurs when the lower pin of 'other stuff' is high and sourcing occurs when the higher pin of 'other stuff' is high?
     
  10. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Yes. The lower transistor sinks and the upper one sources. The upper one is in series with a resistor which will limit current.

    Here's another look at sinking and sourcing:

    sinksource.jpg
     
  11. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
    197
    0
    Thanks, you really helped me a lot :D.
     
Loading...