Why do we use a darlington sink driver?

Discussion in 'General Electronics Chat' started by blue6x, Apr 18, 2005.

  1. blue6x

    Thread Starter Active Member

    Apr 6, 2005
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    does anyone here knows why we have to pass positive rectangular pulse to a darlington sink driver??? I'm just wondering why because the output in an oscilloscope is quite different in that of the rectangular pulse I sent to that darlington sink driver. Anyone please share your thoughts. Thanks....
     
  2. David Bridgen

    Senior Member

    Feb 10, 2005
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    The shape of the pulse surely depends upon the application.

    Where are you measuring the input and output pulse? And what is the load?
     
  3. blue6x

    Thread Starter Active Member

    Apr 6, 2005
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    the input to this darlington sink driver came from a microprocessor, i used an oscilloscope to check it out and the input is a rectangular wave. The output of the darlington is then sent to a seven segment display. The output signal as viewed from an oscilloscope is somewhat similar to decaying signal just like in capacitor discharging, with some spikes though.. How is it explained.... Thanks by the way.
     
  4. David Bridgen

    Senior Member

    Feb 10, 2005
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    Can't say from your description.

    Can you post the circuit?
     
  5. blue6x

    Thread Starter Active Member

    Apr 6, 2005
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    I'm sorry but i don't have a soft copy of the circuit, i'll be attaching the data sheet of the microprocessor though nd the darlington sink driver. The darlington's pins 7 down to 1 except pin 2 are connected to the pin segments 60 to 65.The darlington's output is then connected to a seven segment display. Hope you could still help me with this. Thanks

    PS. input to the microprocessor is at pin 6, which is actually a rectangular wave based on my analysis using an oscilloscope, also, the output of the microprocessor being fed to the darlington sink driver is a rectangular pulse also, but as what i have said the output of the darlington sink driver has changed, but still it is a positive pulse. Thanks for your time taking this.
     
  6. blue6x

    Thread Starter Active Member

    Apr 6, 2005
    38
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    By the way, the application of this one is for a tachometer, that part i mentioned is for the display segment. thanks
     
  7. David Bridgen

    Senior Member

    Feb 10, 2005
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    The darlington stages are inverting so with a positive-going input pulse you should be getting a negative-going output, i.e. the collector should be going from some positive voltage down to 0V.
     
  8. blue6x

    Thread Starter Active Member

    Apr 6, 2005
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    I'm currently looking on the output pulse on the oscilloscope of the darlington, you were right, a rectangular pulse input turne to an almost zero output, and the zero input has gone high, then decreasing until the next rectangular pulse (i mean high input) is received.

    but wait, this could be my last question, but why is it necessary to use this darlington sink driver, i'm thinking of this because why we just don't accept that rectangular pulse as our input to the seven segment directly.

    Important: I've said that because there are also sink drivers that are non inverting. What are their purpose. Thanks for your time.... You were really of great help.
     
  9. David Bridgen

    Senior Member

    Feb 10, 2005
    278
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    The outputs from the uP can neither source (provide) nor sink (switch to ground) much current. The drivers can sink much more.

    The display will have its positive supply line permanently connected and this is common to all segments.

    For a segment to be lit it must have its other end connected to 0V ("ground".)

    As the outputs from the uP are positive-going you need an inverting driver.

    A non-inverting driver would be used where the driven device requires a voltage applied across it rather than connecting the device to ground as is done in your application.
     
  10. blue6x

    Thread Starter Active Member

    Apr 6, 2005
    38
    0
    thank you. now i think so much have been said and it helps me lot. i hope you could answer my further questions in the near future. Thanks.....
     
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