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A transistor as a switch

  1. Sep 2, 2012 #1
    Am doing a project that has to use a transistor, as a switch, actually switching a 12V relay switch, that is actually responsible for switching another circuit, but as a result the relay doesn't switch ON, but makes noise continuously, in this clearly certain that the voltage across the relay is around 5V, can this be the cause of the noise, and by the way what can i do to solve it, am not aware of transistors with large large current gain, since the base current can't be easily altered and its very small, guys can you help me out here....
  2. jcsd
  3. Sep 2, 2012 #2


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    Science Advisor
    Gold Member
    2017 Award

    hi godwin

    there's a number of answers I could give you, all are valid but not necessarily specific to your situation.
    Can you post a pic of the schematic of your circuit so that it can be looked at to see where you may have gone wrong.

    some of the specific things we need to see are...
    1)... what is driving the transistor ... ie... what is connected to its base
    2)... voltage on the base and is there a current limiting resistor
    3) .... type of transistor
    4) ... where and how the relay is connected to the transistor
    5) ... type of relay.

  4. Sep 8, 2012 #3
    There it is man... Than you!!
  5. Sep 8, 2012 #4
    How can i insert a picture.. I have just uploaded the schematic diagram as attachment.. Is it going to work??
  6. Sep 8, 2012 #5
    sorry the battery is -9V..
  7. Sep 8, 2012 #6


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    The diagram attachment process did not work, so maybe you could try again.

    Some relays take a lot of current and are very difficult to drive with a transistor.

    Also if you are powering a relay with a 9V battery, these have very poor current capability and my not be able to power the relay.

    You can get enhancement mode Mosfets which can drive a relay but require very little current from the driver circuit as long as you have 4 volts or so.
  8. Oct 15, 2012 #7
    How am i supposed to do it.. Just attaching??
  9. Oct 15, 2012 #8


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    Staff: Mentor

    To attach a schematic you must first have it as a picture file on the computer you are using to post these messages. Then while typing your next message scroll down below the message composition box to where it says Attach Files, click on Manage Attachments.

    An alternative to the above is to first store a copy of the file on any file sharing site (there are dozens of free ones), make sure it has permissions that allow the public to access it there (without needing a password, obviously), and then include the URL of that file in your message here.

    Good job that your project isn't urgent. :wink:
  10. Oct 16, 2012 #9
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