Help with DC motor circuit

Discussion in 'The Projects Forum' started by jmullo, Jul 7, 2016.

  1. jmullo

    Thread Starter New Member

    May 10, 2016
    3
    1
    I am looking for some advice in regards to a DC motor control circuit i am trying to build out of some electronic parts I have lying around. See attached control circuit diagram.

    The idea is that a timer will trigger the relay to either run the motor in reverse at full voltage or it will run forward stopping and starting as per the waveform from the 555 timer. So on for about 2 seconds and off for about 2 seconds. This waveform can be changed using R1 and R2 which are trim pots.

    The circuit works as expected until I hook the motor up. When the motor is meant to be following the waveform from the 555 it will actually stop and start very erratically. Any help on the reason for this would be great.

    Also this is one of my first electronics projects and any feedback on my design would be appreciated.
     
  2. Marcus2012

    Member

    Feb 22, 2015
    349
    26
    Hi

    What supply voltages are you using for the 555 and the motor? (and what is controlling the coil on the relay out of curiosity?). Try using a BJT instead of a FET and see if you have better luck.

    EDIT never mind I see it says 12 next to it so I'm assuming 12V, try a NPN BJT, it's not voltage driven like a FET.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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  4. wayneh

    Expert

    Sep 9, 2010
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    3,026
    What do you mean that it works as expected? Do you mean the simulation or a real build? The timer is working until the motor is connected?

    I'd be suspicious that the power to the timer is seeing interference from the current draw of the motor and this is disrupting the timing.
     
  5. jmullo

    Thread Starter New Member

    May 10, 2016
    3
    1
    wayneh, I checked the voltage at the trigger pin of the 555 timer and you're right I am seeing inference when the motor starts drawing current which is then throwing out the 555 timer.

    Would a solution be to use a larger rated power supply? Or maybe a separate supply for the motor?
     
  6. wayneh

    Expert

    Sep 9, 2010
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    3,026
    Either of those could help a lot. Putting a voltage regulator ahead of your 555 is similar to giving it a separate supply. You have lots of choices now that you know what the problem is.

    A simple thing to try would be adding a diode and "large" electrolytic on the power supply to the 555. This might be enough juice to power it through the transients caused when the motor starts. The motor may not draw nearly as much once it's running, so maybe you just need to get through a few milliseconds.
     
  7. jmullo

    Thread Starter New Member

    May 10, 2016
    3
    1
    I am now running the motor on a separate supply and the motor and 555 are working well.

    Thank you all for your help.
     
    wayneh likes this.
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