transistor as a relay help

Discussion in 'The Projects Forum' started by robkiller, May 21, 2014.

  1. robkiller

    Thread Starter New Member

    May 21, 2014
    6
    1
    So I feel that this is so simple yet I am not getting it done. I want to take a transistor and place it in a normally closed state to run a normally open relay on a 12vdc system. I want to turn the transistor off using positive 5v ( I am guessing by adding positive voltage to the base) This will turn off the relay back to its normally open state. When power is taken away from the Base of the transistor the relay kicks back on.

    Please any help would be great. I am new to posting to the forum but I am watching it all the time. I am new at this and am learning circuit basics still.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    3,024
    Your title suggests you want to replace the relay with a transistor, but it sounds like you want to control the relay with the transistor?

    And you want +5V to open the relay, and 0V to close (activate) the relay?

    Any chance you could accept reverse logic, so that +5V activates the relay?
     
    Last edited: May 21, 2014
  3. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Can you post what you have.

    You may need a pnp with coil in collector circuit.
    Emitter to +V
    Base resistor to grd (neg).
    Pos. turns off.
     
  4. robkiller

    Thread Starter New Member

    May 21, 2014
    6
    1
    I want to use a transistor that is normally closed to keep on a relay. I want 5v to make the transistor become open to click off the relay. 5v needs to control the transistor. I also have 12v needing to run through the transistor to run the relay.
     
  5. robkiller

    Thread Starter New Member

    May 21, 2014
    6
    1
    I will try to post what I have I know how to get the pnp transistor to be normally closed and run the 12v relay but I cant figure out how to get 5v to turn it off.
     
  6. robkiller

    Thread Starter New Member

    May 21, 2014
    6
    1
    This is what I have. The parts with green dots is the npn transistor witch works. The led light is in place represents the relay. Its the 5v to turn it off that is not working. You can see what I have on the pic that is not working in none green dots.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,498
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    Why not use the transistor biased on and then turn in it off with a simple switch or take the input to common.
    I assume your set up has two supplies 5v & 12v?
    Using a 2n7000 you could take the source to the common connection of the 5v/12v, place a 10k resistor from gate to +5v, connect relay from drain to +12.
    When you power up the relay will be on.
    Turn the relay off by taking the gate to common, either by switch or other circuit output.
    Max.
     
  8. robkiller

    Thread Starter New Member

    May 21, 2014
    6
    1
    My device can only be coded to control the 5v. I want the code to turn it off. I dont have control of the common side of the board. The relay will be controlling 120v. Will your idea work with the 5v turning it off and on?
     
  9. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    Much schematic. No words.
     
  10. robkiller

    Thread Starter New Member

    May 21, 2014
    6
    1
    That looks simple enough. I will try it out thanks so much.
     
    Artbuc likes this.
  11. wayneh

    Expert

    Sep 9, 2010
    12,086
    3,024
    Will the MOSFET get turned on enough with only ≤2.5V on the gate? Might need the lower 4.7K to be more like 10K.

    I'll remind the OP once again: If you can reverse the logic, you can eliminate the PNP transistor, which is serving only to invert your control signal.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    2,364
    Yes, whether you switch to +5 or 0v (common) doesn't matter as long as you achieve the same thing with the right configuration.
    Max.
     
  13. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    Max has been recommending these so many times that I finally read the datasheet. Try it. You'll like it.
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,498
    2,364
    I believe they were originated by Siliconix back in the 80's which coined the description of them as a FETlington, due to them being equivalent to a bi-polar darlington.
    I have used them from the start in so many applications and never experience a failure yet.
    I think Siliconix are now Vishay.
    This is the original promotional Lit.
    Max.
     
    Last edited: May 22, 2014
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