Fan control opamp issues

Discussion in 'General Electronics Chat' started by anotheruser1, Jun 3, 2012.

  1. anotheruser1

    Thread Starter Member

    Dec 6, 2011
    30
    4
    OK what i am trying to accomplish with a LM741 for temperature control, and a LM393 for fan on/off is basically the LM741 will increase output to a transistor as temperature rises. The output is also tied to the LM393 so when the fan voltage reaches a mere 6.2 volts it will fire the transistor and turn on a relay which breaks the positive output of the transistor for the fan.

    My whole objective is to keep the fan off or turn it off if the voltage is below 6.2 volts to prevent stall and burning up. I consider myself a beginer so my circuit is probably all wrong, however, i got this idea to work on a breadboard and tried to transfer to proto board with out making notes and now i cant get it to work for anything.

    The drawing attached is what i think i had, but any help to make it work correctly would be greatly appreciated.
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    A couple of quick notes...

    The base voltage of Q3 must be able to get higher than the emitter voltage, however, you have the emitter connected to you positive supply through the coil.

    Also, you should have some current limiting resistors between the amplifier/comparator and the transistors they drive. This will help keep the transistors from being damaged. Remember the base-emitter voltage can be approximated as 0.7 volts when the transistor is on.

    A little cleanup of the schematic would also make it much easier to read.
     
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  3. anotheruser1

    Thread Starter Member

    Dec 6, 2011
    30
    4
    Thanks for your information. The drawing is my first try on the software, I hope I can get better at it.
     
  4. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    PLEASE work on the software! That drawing is soo difficult to read!
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Here you have a redrawn diagram

    [​IMG]

    And for sure this circuit won't work in real life as you expect.

    Q2 need base current limiting resistor.

    LM393 is a open collector output.

    Q3 is connected upside down.

    And more but I don't have time more today.
     
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    Last edited: Jun 4, 2012
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  6. anotheruser1

    Thread Starter Member

    Dec 6, 2011
    30
    4
    Parts and idea thrown in the trash. Didn't realize I was that dumb. Thanks to those who replied.
     
  7. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
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    Sorry to hear you say that. I have been around electronics since the late 1980's and still make mistakes and learn new things all the time. Don't think it is supposed to be easy. Anything worthwhile takes effort.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Pull your parts out of the trash - you can always use them for this or another project; you just need a different approach.

    There's a problem with the way the LM741 opamp is connected to Q1 that is not immediately apparent. This opamp has a common mode imput range that is about -V + 2.5v to +V - 2.5v ... so the output of the opamp won't go much above +V - 2.5v. This means that the transistor Q1 can't be turned off by the LM741, as the base would have to be raised to within ~500mV of the emitter in order to turn off the transistor.

    That's a mistake that plenty of people make; even those with a good bit of experience - it's something that's easy to forget.

    Drawing schematics isn't exactly intuitive, either - it takes practice to get really good at it.
     
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