Motors not stopping anytime

Discussion in 'The Projects Forum' started by maliha tamkeen, Mar 28, 2016.

  1. maliha tamkeen

    Thread Starter New Member

    Jan 6, 2016
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    Hi
    We are working on
    http://www.sciencebuddies.org/science-f ... #procedure
    When we tried first everything worked fine.We were able to turn off the motors in normal light at night.The motors turned on only with the torch light.We also set the potentiometer so that the motors are off on normal lights.
    When we try now,the motors do not turn off ,even in dark.Please help.We have to get it resolve fast.
    Please let us know.
    Thanks
    Maliha
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Sounds like something shorted, do you have the circuit in schematic form?
    Max.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Is this 'homework help'?
     
  4. maliha tamkeen

    Thread Starter New Member

    Jan 6, 2016
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    this is an individual project.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Did you try taking the gates to common and see if the mosfet turns off?
    Max.
     
  6. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    The back-emf from the motors may have killed the mosfets.
     
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  7. TheButtonThief

    Active Member

    Feb 26, 2011
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    +1

    Where coils and semi's are concerned, you should always place a flyback diode across the coil to prevent the back-EMF from causing damage to the rest of your circuit. Here, your MOSFET is removing voltage from the motor but it's still connected to the circuit and so current is still allowed to flow. When voltage over a coil is removed, the coil is left with an inductive charge (not unlike the charge in a capacitor), the coil wants to dump this charge very quickly and when it does, the current can spike enormously which will damage whatever components are in series with the coil if they're unable to handle such a volume. A diode over the coil in the reverse polarity to your conventional current flow will supress this current spike.
     
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  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Also I would have tended to control the gate digitally instead of in a linear fashion, use a comparitor LM311 for e.g. it would also allow for a sharper on/off setting.
    Max.
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    It's a pity the linked Bristlebot article didn't specify use of flyback diodes. Lots of poor innocent FETs will have died unnecessarily :(.
     
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  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    With the circuit shown, if using LDR's I would have thought the average latency of these type of photo detectors would result in a slow turn on/off of the Mosfet limiting the effect of opposing BEMF.
    Max.
     
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