help!!! temperature controlled fan

Discussion in 'The Projects Forum' started by raybies999, Jan 30, 2013.

  1. raybies999

    Thread Starter New Member

    Jan 30, 2013
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    0
    Hi, I really hope that someone can help me. I am a teacher and I am having to help a group of pupils make electronics projects. I have little experience with this subject and I need a bit of help please.

    I have one student who wants to make a fan that automatically turns on at approximately 25 degrees. I have attached a photo of the circuit we have been working on but one of the transistors (Q1) keeps blowing up. I'm not sure how to go about fixing this issue or if it even works properly. If anyone has any ideas, I would be truly grateful. Also alternatively if anyone has a different simple circuit that we could try, that would be brilliant. Please help me. Thanks, Rachaelle
     
  2. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    Add a diode across the motor with the + side connected to the +voltage. That will allow the inductance in the motor to dissipate its energy when the transistor turns off suddenly. 1N4001 should work.
     
  3. raybies999

    Thread Starter New Member

    Jan 30, 2013
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    0
    Thanks so much for replying. I can't seem to get that to work. It does seem to protect the transistor but the motor doesn't seem to move any more. I'm finding this so tricky. Any other thoughts? Thanks.
     
  4. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    What are the specs of your motor? Stall/start current?
     
  5. #12

    Expert

    Nov 30, 2010
    16,261
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    Adding the diode should have no effect whatsoever except at the millisecond when the transistor turns off.

    Perhaps you had already blown up this transistor and didn't think about that when you added the diode? Perhaps the motor uses so much current that the transistor fries just from the current needed and the inductive reaction isn't really what kills it? Maybe you misunderstood and put the diode in series with the motor, but it should be in parallel?

    Many things to check. Do a few measurements and report back here when you have more information.
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Did you install the diode with the stripe facing the + Supply? If you put it in "normal" (stripe toward negative), it will short circuit the transistor and send the power straight to ground.

    Also add a 0.1uF ceramic or film cap across motor + and motor -, as well as another at the 555 Vcc and Vee/GND.
     
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