Need opinion on DC motor effects in circuit

Discussion in 'The Projects Forum' started by matthew798, Dec 4, 2014.

  1. matthew798

    Thread Starter Member

    Jan 16, 2013
    38
    2
    Hello everyone.

    I was hoping I could get an opinion on how I have set up a CNC breakout board for the Stepstick series of drivers. In the top left corner, there is a mosfet which will be used to control motor speed via PWM. J1 is a 36V power input, J2 is the connector for the motor and JP10 is the PWM signal input. What I am worried about is stray high voltage spikes from the motor feeding back into the rest of the circuit and exceeding the max voltage of the motor drivers. Diode 1N4004, IMO, doesn`t seem like it would be enough to protect the rest of the circuit from the motor. I was thinking that I should perhaps add a diode between the J1+ and J2+ to allow current to flow ONLY towards the motor. Another problem I am worried about would be the 3 capacitors (C1, C2 and C3) smoothing out the voltage supplied to the motor and negating the whole purpose of the PWM. Short of buying a completely separate power supply, I'm not sure what I could do. What do you guys think?

    circuit.png
     
  2. matthew798

    Thread Starter Member

    Jan 16, 2013
    38
    2
    Bump. Anyone?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,524
    2,369
    The schematic is not really complete and is a generalization.
    Are the drives a proven product?
    Who came up with the schematic layout?
    Max.
     
  4. matthew798

    Thread Starter Member

    Jan 16, 2013
    38
    2
    I made the schematic. The drivers are commercially available and widely used. I assure you that the schematic is complete. I already have a working prototype board based on a slightly earlier version of this schematic All you need to know is that there is a DC motor plugged into J2. My question is simply: will the diode placed across the leads of J2 be enough to GUARANTEE the rest of the circuit (black box it if you must) from voltage spikes? Would it be wise to place a diode between J1+ and J2+, or is there another, better way of isolating a DC motor?
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    If I've understood you correctly, that would prevent back-emf induced current from flowing, thus making the J2 1N4004 ineffective and leading to excessive voltage across the FET.
    Is the 1N4004 rating adequate for handling the peak motor current?
    Perhaps add an RC snubber across the motor for extra protection?
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,524
    2,369
    Keep in mind that removing power from the motor results in a generated voltage the same polarity as the supply when freewheeling or in overhauling mode.
    This is why either either regeneration or dynamic braking is used.
    Max.
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    760
    It's better to get atleast 3 to 5 amp diode

    All I need is one diode across the motor in reverse direction.
    The diode will clamp the spikes and protect the Mosfet too.
    The rest you do not need to worry. The caps won't effect the PWM but you might need to add a Capacitor depending on ur PSU supply rating
     
    Last edited: Dec 6, 2014
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