NPN Transistor switching a relay help!

Discussion in 'General Electronics Chat' started by onejames, Jun 12, 2010.

  1. onejames

    Thread Starter New Member

    Jun 12, 2010
    I am trying to build a simple circuit where a NPN transistor switches a relay.


    my input is coming from an infrared photo-transistor.

    My problem is the NPN (2n2222), passes 9.4v and relay engages with out any V+ on the base. When i apply a voltage to the base i get the full 12V out.

    I have read for hours, including the tutorials on this site and think i understand this, but apparently i am missing something.

    Any help would be greatly appreciated.

    Thank you in advance for sharing your wisdom.

    - James
  2. SgtWookie


    Jul 17, 2007
    Hi James,
    It isn't rocket science. However, you need to give us a bit more in the way of specifics of what you have. Values and part numbers would be a big help.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    If you have placed the component parts in the relationship shown in the schematic, the only reason for the 2N2222's behavior is that the transistor is shorted C - E.

    Here is a link to our Ebook on how to use a meter to check a transistor -
  4. onejames

    Thread Starter New Member

    Jun 12, 2010
    Thanks for the quick reply! i will start with testing the transistor, and see where that leads me.
  5. Bychon


    Mar 12, 2010
    Putting a resistor from base to ground might help by draining off any leakage currents, but it's more likely the transistor is kaput or you've installed a wiring error.
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    My guess is that the collector and emitter leads are swapped. Base-emitter breakdown will explain the behavior you are seeing.
    Check it out, onejames.