Slowly stop small dc motor

Discussion in 'The Projects Forum' started by Matt D., Dec 30, 2015.

  1. Matt D.

    Thread Starter Member

    Nov 10, 2013
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    Hello I have a dc geared motor, about 6v and 500ma, and I want there to be a small delay in stopping it.

    When the motor is on it goes through a SPST normally closed switch. When I open the switch to turn off the motor I would like about a 1-2second delay before the motor cuts off completely.

    Could a large capacitor across the switch provide the necessary current to drive the motor for the brief amount of time? If so how large of a cap would it take?

    I tried doing this with a 470uf capacitor I had laying around and it didn't seem to do anything.

    Thanks for your help and (obviously) my electronic knowledge is limited.

    Matt
     
  2. ian field

    Distinguished Member

    Oct 27, 2012
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    Across the switch probably won't do much - except sputter the contacts each time you close the switch and short the charge in the capacitor.

    The capacitor might be more effective across the motor, but you'll still need a lot to keep the motor going more than a fraction of a second.

    If your 6V is well regulated - you can get a bunch of 2200uF 6.3V electrolytics from a scrap PC motherboard.

    With a big bank of capacitors; the switch has to handle the instantaneous charging current - total current is limited by source impedance and leads etc, so not quite as bad as dumping the capacitor directly into the switch contacts. But you should keep mindful of the switch on surge.
     
  3. Brownout

    Well-Known Member

    Jan 10, 2012
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    It would be much better to drive the motor with an active device, say a transistor. The you can use a much smaller capacitor to drive the transistor. Since the transistor amplifies current, a much smaller capactor can be used, as it will drain more slowly.
     
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  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    If you put a large inductor in series in addition to the shunt capacitance, this will prevent some of the relay sputtering (unless the inductor happens to resonate with the capacitor!) A protection diode across the inductor will fix this.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you want to retain full voltage for the 1-2 sec, then a 555 could be set up for a delay off timer.
    A mosfet or relay could control the motor.
    Max.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I'm slowly starting and slowly de-accelerating a DC gear motor using Pulse-Width-Modulation generated by a $4 Arduino mini....
     
  7. Colin55

    Member

    Aug 27, 2015
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    Here is the circuit you need:
    MotorOverRun.gif
     
  8. Matt D.

    Thread Starter Member

    Nov 10, 2013
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    Thanks guys for the help. I'm trying to keep it as simple and cheap as possible. And don't need to keep the full voltage. A lower voltage would be good as it would slow down the motor and give a delay.

    Thanks for the circuit Colin. I didn't think of this until now but since I'm using batteries do you think this circuit would use much quiescent current?
     
  9. Colin55

    Member

    Aug 27, 2015
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    Zero current
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    The TS has 6V to play with - easily enough for a logic level MOSFET (a regular one might work, but not quite make its headline RDSon).

    The MOSFETS on old PC motherboards are usually at least 50A and somewhere close to logic level. The voltage rating is usually 30V, and sometimes as low as 20V - so it'd have to be protected from motor back emf.
     
  11. BR-549

    Well-Known Member

    Sep 22, 2013
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    When you hit the stop switch........do you want the motor to wait 2 sec. before stopping?

    OR when you hit the stop switch....do you want the motor to take 2 sec. to go from running speed to a full stop?
     
  12. Matt D.

    Thread Starter Member

    Nov 10, 2013
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    BR-549 I think either would be fine. They both kind of seem the same to me.
    It doesn't have to be too precise - about 1 to 2 seconds.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Then the 1-2 sec 555 timer would do it. It would keep full voltage on the motor for a period.
    Max.
     
  14. Colin55

    Member

    Aug 27, 2015
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    "Then the 1-2 sec 555 timer would do it. It would keep full voltage on the motor for a period.
    Max."

    Didn't you read the original requirement ???????
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    YES I also read #11 &12 !!!
    Max.
     
  16. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The chip is a 555 timer. It is wired as a monostable (one-shot) timer. The resistor and capacitor to the left of chip set delay, currently set to 1.1 seconds.

    image.jpg
     
    Last edited: Jan 2, 2016
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  17. Colin55

    Member

    Aug 27, 2015
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    Where did you get this idiotic circuit from ???
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It would be far more constructive to offer your viable alternative suggestion rather than abrasive comments!:rolleyes:
    Max.
     
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  19. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You are right. There was power to the transistor for 1 second but no power to the motor. Error has been corrected.
     
  20. Colin55

    Member

    Aug 27, 2015
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    "It would be far more constructive to offer your viable alternative suggestion rather than abrasive comments!:rolleyes:
    Max."

    I have already provided a solution.
    If you look above you will see he is using batteries and a 555 circuit does not drop to zero current.
    A 555 is an absurd suggestion.
    This problem has already been solved. Get on with something else.
     
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