Two Transistor

Discussion in 'General Electronics Chat' started by Saheli, Oct 26, 2013.

  1. Saheli

    Thread Starter New Member

    Jul 9, 2013
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    While going through 555timer circuits i came across this one. in this circuit 555timer is used as an astable multivibrator which is well understood, but i want to know the role of using two transistor pnp and npn here. what is its use? Can we not use only one transistor?
     
  2. ErnieM

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    Apr 24, 2011
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    Schematic?

    Link?

    Any clue to what you are looking at?
     
  3. wayneh

    Expert

    Sep 9, 2010
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    That arrangement retains (does not invert) the output.
     
  4. MikeML

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    Oct 2, 2009
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    The two transistors form a High-Side switch to drive a high current grounded load. If pin 3 of the 555 is high, current will flow through R4 into the base of Q6, turning it on hard. That pulls down its collector, causing current to flow out of the base of Q5, turning it on hard.

    Think of the two transistors as a hybrid NPN-PNP Darlington. The 555 alone is capable of sourcing ~200mA. With the added transistors, depending on how it is set up, this driver could source up to 10A.
     
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  5. shreeram

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    Nov 29, 2012
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    What is the problem in using two transistors???
     
  6. MikeML

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    No problem. Using none, one or two depends on how much current the load requires (exceeds the intrinsic current sourcing/sinking rating of the 555) and if one end of the load is to be connected to ground or Vcc.
     
  7. #12

    Expert

    Nov 30, 2010
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    Changing to one transistor would make this a different circuit. If you want to use only one transistor, you have to change the circuit, and the ability to deliver current to the load will change.
     
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  8. Saheli

    Thread Starter New Member

    Jul 9, 2013
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    Does that mean by using two transistors we are actually increasing the load current? i am thinking to use this in triggering of Marx circuit.
     
  9. Saheli

    Thread Starter New Member

    Jul 9, 2013
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    Does that mean by using two transistors we are actually increasing the load current? i am thinking to use this in triggering of Marx circuit.
     
  10. Saheli

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    Jul 9, 2013
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    I am thinking to use this in triggering of Marx circuit.
     
  11. MikeML

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    What is a "Marx Circuit"?
     
  12. #12

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    Nov 30, 2010
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    No. The load controls the load current. The transistors only control the maximum current available.
     
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