Relay Transistor connection

Discussion in 'The Projects Forum' started by Gary94, Sep 24, 2013.

  1. Gary94

    Thread Starter New Member

    Sep 24, 2013
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    Hi guys! I had spend s couple of days trying to connect an npn transistor to a 12V relay. I am quite a newbie at this but I heard it is possible to use an npn transistor to translate a 5v source from a microcontroller to a 12V relay drive signal. If so is there a correct connection for this? I had tried to connect them with my limited knowledge but the relay contactor will not close. Any help is very much appreciated. :)

    [​IMG]
    Links for the pic layout: http://i41.tinypic.com/302309j.jpg
     
  2. Efron

    Member

    Oct 10, 2010
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    15
    To drive a 12V relay you need a 12Vdc power somewhere. the transistor will be there to activate the relay when receiving a 5V input from your microcontroller but cannot translate into 12V from a single 5V power supply.

    So, you must split your circuit in two. One low power input circuit (driven by a 0-5V signal from microcontroller as shown in your schematic) and one 12V power circuit to power up the relay when the transistor is ON.

    To properly power up the relay, your transistor must reach saturation when ON. For that, you need to know the current needed to power the relay, as well as the beta factor of the transistor to correctly compute R1.

    In your schematic, what is the circuit on the right side?
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  4. ScottWang

    Moderator

    Aug 23, 2012
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    I always like to buy the 5V relay, becuase I can use it at 5V,6V....24V, if the power source is great than 5V, then it just needs a current limited resistor.

    Why you want to using a 12V relay?
    What voltage that you usually used? (5V,9V,12V?)
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    For TTL and 5v Micro's I tend to use a Mosfet, depending on the power required, e.g. for up to 200ma you can use a the 'Fetlington' 2N7000.
    R1 not needed.
    Max.
     
  6. #12

    Expert

    Nov 30, 2010
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    I just did this on the Chat page.
     
  7. ScottWang

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    Aug 23, 2012
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    That's what mcgyvr mentioned on 3#.
     
  8. #12

    Expert

    Nov 30, 2010
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    True. I just made a drawing of it.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I can never understand the preference for bi-polar in this and TTL applications?
    As well as not loading the source, one component is eliminated.
    Max.
     
  10. ScottWang

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    Aug 23, 2012
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    Until now the load still didn't shows up, must be the load ran away and play where.
     
  11. #12

    Expert

    Nov 30, 2010
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    Most beginners have some BJT transistors. They are cheap, easy to find, and don't have static electricity issues.
     
  12. Gary94

    Thread Starter New Member

    Sep 24, 2013
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    [​IMG]

    Hi guys, this is the updated circuit. Does anyone have any idea why is the ammeter U5 having 320.288MA? And is there anything wrong with my circuit? Thanks.
     
  13. ScottWang

    Moderator

    Aug 23, 2012
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    You missing the current limited resistor on Base, why?
     
  14. Gary94

    Thread Starter New Member

    Sep 24, 2013
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    But once a put a resistor (33k Ohm), the led will not light up.
     
  15. ScottWang

    Moderator

    Aug 23, 2012
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    You have to using the resistor less than 4.7K for Base, when the Vcc is 5V, the resistor will be less than 9.1K when the Vcc is 10V.
     
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