Relay Problem

Discussion in 'General Electronics Chat' started by faisaldj, May 21, 2013.

  1. faisaldj

    Thread Starter New Member

    May 16, 2008
    6
    1
    Hi All,

    I have a relay circuit which has fourteen relay output (i.e 14 devices can be switched on or off from this) , I have attached the circuit for a single output.

    I am sending command to switch on the related output through the GPIO of a
    microcontroller board (2.9v), But whever I solder the IRLML2402 transisotr (Q24 in the attached picture) and power on the transistor then suddenly all the pins of the transistor (Q24) get grounded, Can anybody identify the error?

    with regards,
    Muhammad Faisal,
     
  2. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    That is how a depletion mode n-channel MOSFET works. When there is a voltage on the gate, the drain and source are connected.
    Q23 is a depletion mode p-channel MOSFET and it's works in the opposite fashion. When there is a voltage on the gate, the drain and source are disconnected.
    The way this is wired, the gate on the relay goes from 5 V to 0.5 V when the gate on Q24 goes from 0 to 2.9 V. This allows some current to flow through the relay.
     
    PackratKing likes this.
  3. timescope

    Member

    Dec 14, 2011
    298
    44
    Q23, TPC8124, is also enhancement mode according to the datasheet

    Timescope
     
  4. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    Ooooh what a mistake. :eek: They are both enhancement mode MOSFETs.
    Fortunately, I did describe how they would work correctly. :)
     
  5. faisaldj

    Thread Starter New Member

    May 16, 2008
    6
    1
    first thanks for the reply.

    As you identified that both of the transistors are enhancement mode so according to my understanding to correct the circuit Diagram I removed the R67(100k) between gate and source but Still I am getting the output which in ideal case should not be there.

    with regards,
    Muhammad Faisal.
     
  6. Gibson486

    Member

    Jul 20, 2012
    199
    12
    You drain is only getting something like 0.5V. Is that really what you want?

    edit: oh wait...nvm, I see what you did...carry on....
     
    Last edited: May 22, 2013
  7. timescope

    Member

    Dec 14, 2011
    298
    44
    R67 (100k) is required to turn off Q23.

    The GND of the attached circuit should be connected to the microcontroller circuit ground.

    Test Q24 with a meter for a short circuit between source and drain.
    If Q24 is ok, connect it and do the following:

    1. Disconnect the PE12 line from the microcontroller.
    2. Connect a 10k resistor between gate and source of Q24 (IRLML2402).
    3. Check the output voltage of Q23 (TPC8124) : it should be zero volts.
    4. Check the PE12 voltage level when the microcontroller output is LOW.

    Timescope
     
    Last edited: May 22, 2013
    faisaldj likes this.
  8. faisaldj

    Thread Starter New Member

    May 16, 2008
    6
    1
    @timescope:

    Thanks alot yes I got it right.
    connecting the resistor (10k ) between the gate and source of Q24 and I got it working
    perfectly with my my input from Microcontroller.

    But I thought that microcontroller should be capable of sinking current even without this 10 k resistor...because my GPIO pin was defined as push pull and even we enabled the internal pull down resistor too. But nevertheless its working with external one that you recommended.
     
    timescope likes this.
  9. crutschow

    Expert

    Mar 14, 2008
    13,050
    3,244
    The microcontroller should pull that pin to ground when it is powered, unless it's being set to a high impedance state.
     
    faisaldj likes this.
  10. faisaldj

    Thread Starter New Member

    May 16, 2008
    6
    1
    Hi ,

    I had got the circuit working with desired result but I am getting strange behaviour ,
    Initially the circuit is working perfectly OK but after sometimes It has no effect of the switching i.e it constantly gives the o/p irrespective the state of the input (from microcontroller).



    with regards,
    Muhammad Faisal.
     
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