Sinking and sourcing?

Discussion in 'Homework Help' started by carnage123, Feb 25, 2006.

  1. carnage123

    Thread Starter New Member

    Feb 18, 2006
    7
    0
    Right now, I'm a little bit confused about the sinking and sourcing configuration. I know that the sinking has something to do with ground and sourcing has to do with applied power.

    Can anyone give me an example of a simple sinking and sourcing situation. I remember seeing an example using pull-up resistors. That reminds me, I also need to know exactly how this pull-up resistors work.

    Any responses are appreciated, thanks alot!
     
  2. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    Hi. You understanding is not quite right.

    Sinking is the ability to have multiple inputs into the input of a logic gate and sourcing is the ability to drive multiple gate from one output. Eg, if you had the output of a NAND gate split (or fanning out) to feed the inputs to four other NAND gates then the first gate is sourcing 4 gates. Data sheets will specify the maximum sourcing (and sinking) that a gate will be capable of without buffering. Iy you wanted to drive or source 20 gates then you may need to use say 5 buffers so each buffer drives only 4 gates. Sinking is similar but on the other side. If you consider the input of a gate being driven by 4 other gates this would be ok but if you wanted to have 20 gate feeding into the 1 then again you would need buffers or probably OR gates to share the load.

    All this is do to the I/P and O/P impedence of the gates. As you add gates to the O/P of another gate then the overall load impedence will be reduced. As impedence or resistance reduces current must increase and hence there will be a finite amount you can add before you overload the driving or sourcing gate.

    Hope that helps a bit.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Windoze killa, I think you're leading carnage123 down the garden path. I have never seen that definition of sourcing and sinking.
    Here is a simple description of sourcing and sinking. The actual devices doing the sourcing and sinking may be different from those shown, but the important thing is the direction of the current.
     
  4. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    Ron, although I agree with what that page says I guess it is a bit of what school you went to. I was taught as I mentioned and hence the way I then taught it. Admitedly that was 15 years ago I still think it is correct. Maybe if carnage123 could tell us which one he was think along it would be easier to explain.
     
  5. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    RonH is right. You may have been taught its application to logic chips, but the concept covers just about any circuit where a load has to be switched to take current from a power source.
     
  6. carnage123

    Thread Starter New Member

    Feb 18, 2006
    7
    0
    Since my question was'nt too clear and I'm sorry for that, I have posted a basic sinking and sourcing cicuits on the attachment. These are the ones I was shown at school, I don't know if they're good examples or not, I have somewhat figured out what they are and how they work, if someone could give me a good explanation of how they work in relation to sinking and sourcing configuration that would be awesome. Thanks!
     
  7. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    The IC supply rails Vdd and Vss connected to Vcc and ground have been left out of the circuit, but these need to be considered as they are part of the sourcing or sinking operation.

    In the top circuit, when J1 is closed the input of the invertor is taken high. That puts the output low and current will flow from Vcc through D2, R1, U3A output and Vss to ground. So U3A 'sinks' current from Vcc to ground.

    In the bottom circuit, with J2 open circuit, U3B output is high so it is sourcing current through R2, D1 and ground. When J2 is closed U3B output is low so U3B is neither sourcing nor sinking.

    I hope I've explained that well enough.
     
  8. Dan123

    New Member

    Feb 28, 2006
    1
    0
    Hi,

    Just my understanding. A sinking configuration is a circuit with part of its output load (eg pull-up resistor) connected to Vcc. The circuit will take in currrent (sink) from Vcc through the pull-up load.

    A sourcing configuration is a circuit with its output load connected to ground. The circuit usually supply current (source) to the load.
     
  9. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,787
    Even if you learned about sinking and sourcing in the context of logic gates, things have changed my friends. In the original TTL(Transistor-Transistor-Logic), Logic inputs were the emitters of multiple emitter transistors. These inputs can and do SOURCE current, about 1.6mA. LS-TTL was a considerable improvement to 0.4mA. Today almost all logic is CMOS and the inputs are very high-impedance. They do not sink or source very much current at all.

    TTL Totem-pole outputs were very good curent sinks, and lousy current sources. The reason for this is that as soon as they tried to source appreciable current the voltage would drop. Open collector outputs are similarly great current sinks and even lousier current sources than totem-pole outputs. It is the pullup resistor on an open collector output that provides what carrent it can.

    This brings us to CMOS output stages, which because of their complementary output stages can source or sink current in approximately equal amounts.
     
  10. carnage123

    Thread Starter New Member

    Feb 18, 2006
    7
    0
    Thanks for the response pebe, I understand exactly whats going on now. Thanks to everyone who replied



    "Even if you learned about sinking and sourcing in the context of logic gates, things have changed my friends. In the original TTL(Transistor-Transistor-Logic), Logic inputs were the emitters of multiple emitter transistors. These inputs can and do SOURCE current, about 1.6mA. LS-TTL was a considerable improvement to 0.4mA. Today almost all logic is CMOS and the inputs are very high-impedance. They do not sink or source very much current at all."

    This leads me to another question. If things have changed such as you say, why in the hell am I learning this in college. I'm not arguing against you or anything like that, all I'm saying is, what is the point then.

    From what I read, Japanese vehicles use a sinking configuration in their circuits vs. sourcing configuration here in North America or Europe. So its all history by now I guess.
     
  11. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,787
    Patience young jedi. About circuits first learn you must. In the real world to have a bag of tricks useful it will be.
     
Loading...