Need to shed som eliught on transistor switches

Discussion in 'General Electronics Chat' started by FrenchConnection67, Mar 7, 2014.

  1. FrenchConnection67

    Thread Starter Member

    Sep 9, 2013
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    hi,

    I was wondering why one of the motion sensors (HR100 hotron) that we use on auto doors which is fitted with a transistor switch NPN can handle both switching the negative and positive side of a circuit. The theory says that a npn can only switch to ground and pnp to positive but why when we wire that sensor's npn transistor to switch to positive, it works too.

    I even emailed hotron about this and they confirmed that it is a npn transistor (so as the manual) and it could interface with door controllers with 24vdc signals.

    ????? now I am confused. why would we have two types of transistors with two specifics functions if one can actually perform both.

    is there anything I am missing here? is there any specific wiring to make those transistors more flexible
    perhaps?
    thanks for your expertise
     
    Last edited: Mar 7, 2014
  2. inwo

    Well-Known Member

    Nov 7, 2013
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  3. tindel

    Well-Known Member

    Sep 16, 2012
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    I suspect the NPN is just completing the circular current flow of just the high-side or low-side of the circuit. Probably the low-side if there is only a single transistor. This is being used as a single pole - single throw switch.

    I'm not aware of a single transistor being able to do a double pole - single throw switch.

    Any answer is speculation without a schematic.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    21,529
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    It is all about direction of current, nothing more. PNP and NPN bipolar transistors are just the inverse of the other.
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    If the high side output using PNP then the Vout = Vcc-0.2V
    If the high side output using one stage of NPN then the Vout = Vcc-0.9V
    If the high side output using two stages of NPN then the Vout = Vcc-1.5V
    The output voltage similar as above, if using two stages of NPN and want to increasing the voltage close to Vcc, then it can be adding two bjts as npn and pnp and then the Vout will be as the first line.
     
  6. FrenchConnection67

    Thread Starter Member

    Sep 9, 2013
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  7. FrenchConnection67

    Thread Starter Member

    Sep 9, 2013
    11
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    thanks for your answer, please see below the explanation and diagram that hotron sent me:

    https://onedrive.live.com/?cid=89277b39817fdfa2#cid=89277B39817FDFA2&id=89277B39817FDFA2!2291&v=3


    let me know what you think
     
  8. bertus

    Administrator

    Apr 5, 2008
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  9. FrenchConnection67

    Thread Starter Member

    Sep 9, 2013
    11
    0
    so if I understood, we can wire a NPN transitor as switch on the high side of a circuit but the Vout will be decreased compared to a PNP transistor switch wired the same way. am I right???or I am still missing something
     
  10. crutschow

    Expert

    Mar 14, 2008
    21,422
    6,137
    If the output transistor does not have its common connected to the load circuit but just has two wires connected to the load, then it can switch either side of the load with equal efficiency. It effect it would be rather operating as an opto coupler would.
     
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