need help about the relay..!

Discussion in 'Embedded Systems and Microcontrollers' started by sheth_ka, Sep 28, 2009.

  1. sheth_ka

    Thread Starter New Member

    Sep 4, 2009
    3
    0
    hi guys,
    i hav a project that needs to control and activate a 12 Volts dc buzzer(mostly use in cars).
    this buzzer will be used as an alarm when the countdown ends.
    to control the buzzer i used pic16f84a,the program embedded to this pic enables the buzzer to turn ON automatically.
    i hav successfully done this using 6V dc buzzer, and now i hav to use a more bigger buzzer which eventually needs a 12V dc supply.
    and the first thing that comes to my mind is to use a relay switch or something.
    unfortunately, i dont hav any idea about this relay. also, i dont know what kind of relay to be used.
    i need help with this simple problem, if you hav any idea or solution, pls post it ASAP.
    i would be glad if you include some schematics of the relay. :D
     
  2. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    Last edited: Sep 28, 2009
  3. Tahmid

    Active Member

    Jul 2, 2008
    344
    25
    Hi sheth_ka,
    I attached the different circuits for activation of 12v buzzer with 5v control logic from microcontroller. You can use any one of the methods. If you plan to use MOSFET, you must make sure they are logic level MOSFETs so that they can be directly driven from microcontroller signal. Instead, if you use regular power MOSFETs like the mentioned IRF540, you have to use an additional driver to drive the MOSFET as they can not be directly driven by the microcontroller signal.
    Thanks.
    Tahmid.
     
  4. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    Actually, if you connect the 100K and 10K resistors like shown, the microcontroller will be able to switch it just fine, I have it connected this way in a few circuits involving a Pic16F84A and it works fine. But then again, I could be wrong, six of my projects work with it, I don't see why it wont work for you.
     
  5. Tahmid

    Active Member

    Jul 2, 2008
    344
    25
    Hi,
    I never tried to use MOSFET to drive buzzer from microcontroller as I always use MOSFET in high power applications where I always use above 6v to drive the MOSFETfor getting optimum output and according to datasheet, the output capacity of IRF540 is 100V and 28A and when used in smps, it can withstand upto 110A current.

    According to the datasheet of IRF540, the threshold voltage is around 4v but condition stated there is that this is for 250uA(microAmpere) current. Maybe your circuit is working since load is not very high but maybe when you use a big relay or something like that your configuration may not work properly. For proper operation of power MOSFET like IRF450, gate voltage required is greater than 6v and running MOSFET with only the threshold voltage will not provide maximum result which MOSFETs are meant for.

    My advice is based on the Principal of optimum use of Power Mosfets.
    Thanks.
     
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