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Sink source current input and outputs

  1. Dec 22, 2011 #1

    1. could somebody explain me in very simple terms what are sourcing / sinking I/O.
    2. In which cases i have to think about these things while interfacing.
    3. i did lot of browsing and studied but every time i get confused, suggest me a simple lab
    experiment so that i can do each of them separately and observe and also how if i connect
    wrongly the set up fails?

    Thanks and regards,
  2. jcsd
  3. Dec 22, 2011 #2

    jim hardy

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    usually it means that the output pin can either supply (source) current to its load,
    or accept (sink) current from the load.

    Simple lab: hook up a 555 timer IC as inverter, by tying TRIG and THRESH together and using as input. Now high on input drives output low and vice versa.
    Next connect a small lamp to output pin.
    Observe that lamp can be lit by tying its other side to +supply and driving output LOW , 555 is sinking current from lamp.
    Now tie lamp's farside instead to common and observe it can be lit by driving output high - 555 is sourcing current to lamp.

    555 has "Totem Pole" output, so it can source or sink. Not all logic families are so equipped.

    analog amps output drive capability is specified usually by a graph, read datasheet carefully. LM324 is an interesting one, observe its weakening sink capability at very low output voltage.

    that should give you the concept - play around with it until it seems natural.
  4. Dec 23, 2011 #3
    thank you very much for the reply it was very simple to understand. one more small doubt is generally output pin should be used for sourcing only as you would want to source current to load why you would want to sink current? i do not know whether my question is correct or not? You have mentioned sourcing and sinking current for output does the same logic applies to input port also?

    thanks in advance,

  5. Dec 24, 2011 #4
    Output need to both pull up or pull down. Output source current with trying to pull up, output sink current when trying to pull down.
  6. Dec 24, 2011 #5


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    The input to the next stage may appear equivalent to a resistor connected to ground. But with a different circuit, it may be equivalent to a resistor connected to +Vcc meaning that the driving stage must sink current when the driving stage wants its output to be held LOW.

    Yes. For example, the multi-emitter transistor designed at the input to a TTL gate requires that you sink current from its input pin if you want to hold it at logic LOW.
    Last edited: Dec 24, 2011
  7. Dec 24, 2011 #6

    jim hardy

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    aha = now you're getting somewhere.. one needs to be aware what's just inside the pins of his IC else he'll make mistakes interfacing to it.

    as Nas-O2 observed, TTL input requires that its driver sink current to ground to pull low. To get best speed out of TTL your driver should yank input high by sourcing a little current.

    See if you can find an old appnote on logic families of late 60's-early 70's
    there's RTL, DTL,TTL, ECL, and so on. They got more sophisticated as desire for speed increased. An understanding of the concepts will strengthen your foundations....
    the families were designed so if you stayed in same family you didn't have to pay much attention to interface, just wire 'em up.

    In NE555 datasheet is an understated line ".. will interface to any logic family.."
    that's because 555's input is high impedance and output has active pull-up and pull-down transistors (called Totem Pole)
    i use them for interface in hobby projects.. built in hysteresis makes a great schmitt-trigger.

    old jim
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