Reason for choosing NPN transistors for Relays

Discussion in 'General Electronics Chat' started by muruga86, Mar 24, 2005.

  1. muruga86

    Thread Starter New Member

    Mar 24, 2005
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    Can any one give me technical details why we use NPN transitors mostly over PNP for switching high voltage relays using Microcntrollers.
     
  2. pebe

    AAC Fanatic!

    Oct 11, 2004
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    It's easier to interface an NPN transistor. 'High voltage' relays will probably require a minimum of 12v to operate them. It is likely that the 5v micro supply may be derived from the 12v supply by means of a 7805 regulator, so that the 5v and 12v supplies share a common -ve rail.

    An NPN can easily be provided with base drive from within the 5v rails, but a PNP with its emitter connected to +12v would need a base voltage of about +11.4v and so would need an NPN to drive it.
     
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  3. muruga86

    Thread Starter New Member

    Mar 24, 2005
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    thanks for ur replay,

    but transitors are current controlled devices hence amount of base current determines the switching on state current conduction, can any one explain more technically...


    murugan
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    You are correct in stating that transistors are current controlled devices.

    That is what pebe is saying.

    When you are using 5 volt logic signals from your microcontroller to control relays that frequently require +12V and sometimes higher voltages to energize them, you need to translate from the 5V logic signal to a 12V relay drive signal. In this case, it is more convenient to develop the controlling current relative to ground. Thus you use a single NPN transistor.

    As pebe pointed out, if you choose to use a PNP transistor to drive the 12V relay then you will require an NPN device to level shift between the 5v control signal and the 12v drive signal used to switch the PNP driving the relay. This is because the emitter of the PNP must be connected to the +12V relay power rail and so in order to shut the PNP off, since transistors are current controlled devices, you must deprive the base of current which you cannot accomplish with a base drive that does not swing very close to the +12V power rail.

    Right? pebe.
     
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  5. pebe

    AAC Fanatic!

    Oct 11, 2004
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    Absolutely, hgmjr :)
     
  6. buddyengineers

    Member

    Mar 19, 2005
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    i wuld like to raise a query here..,after reading ur discussion on PNP Transistors.., i feel that using using a PNP Transistor will be more effective. When using Micrcontroller (uC) if the o/p goes logic low then i can use PNP to drive the relay .., Cant I???? i seriously didnt understand ur reply abt the transistor not being able to turn off.., do u mean tht the o/p of the uC is logic high (abt 5v) and so u using a PNP transistor is waste or wat??? Reading ur discussion has actually confused me a lot..,

    I am really confused now hang on: I want to control a 12v relay thru a microcontroller (uC). So i use a PNP transistor with base at uC pin and emmiter connected to relay, now when the uC pin goes logic low, the PNP Transistor turns ON and the relay is activated and when the pin goes logic high the transistor is OFF. So here my aim is achieved. Now wat i want to ask u is am i wrong anywhere..

    Please do reply
     
  7. pebe

    AAC Fanatic!

    Oct 11, 2004
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    You are connecting the PNP base to uC pin and the relay to the emitter. So you must be connecting the other end of the relay to +12v and the collector to 0v. Right? That is using the transistor in common collector mode.

    Yes, with a low on the uC pin the transistor will turn on. The emitter and so the bottom end of the relay would go down from +12v to +0.6v (the emitter must be 0.6v above the base). The relay will have 11.4v across it so would pull in.

    But how would you turn the PNP off? You need to take its base to +12v. But even with uC pin high, the base can only go to +5v.

    With +5v on the base its emitter can only rise to +5.6v. With the voltage across the relay now reduced from 12v to 6.4v (12 - 5.6) the relay will almost certainly stay pulled in. To turn off the PNP you need +12v on the base. and you cannot get it from a 5v supply.

    I hope that makes sense.
     
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  8. hgmjr

    Moderator

    Jan 28, 2005
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    buddyengineers,

    Sorry if we confused you. Pebe and I are both interested in assisting you and any other curious viewer of this post in getting a clearer understanding of the issue raised by muruga86's question.

    Maybe you can sketch up the circuit diagram of the PNP driver arrangement you have in mind. It is possible that you are thinking of some circuit arrangement that I have not encountered. I feel sure a picture would clear up a number of points.
     
  9. buddyengineers

    Member

    Mar 19, 2005
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    hey man ur grt .., i was thinking stupid.., certainly the PNP transistor base will need to pulled to +12v to turn it off.., +5v will not turn it off.., yes ur right..,Thnx for the reply.., have a nice time
    Cya
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    muruga86,

    Did we clear up this issue for you?

    hgmjr
     
  11. HallMark

    Member

    Apr 3, 2011
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    This thread is really helpful. I was messing up with PNP to make Relay working out.

    Still I want to know if I can turn Relay ON and OFF using single PNP transistor with GPIO swing upto 0-3.3V ?
     
  12. Meixner

    Member

    Sep 26, 2011
    116
    21
    Unless the relay is powered with 3.3 V. the answer is no.
     
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