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DC Motor Stall Current and Power Supply Questions

Discussion in 'Power Electronics' started by rbevis333, Apr 2, 2013.

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  1. rbevis333


    Apr 2, 2013
    I’m new, please forgive my excessive questions.

    I’m trying to build a coil winder and I need to power a Pololu 37D 19:1 gear motor with that draws 300 mA free running and has a stall current of 5 A.

    I tested on Arduino, but now I am protyping on a breadboard using a PIC18 microcontroller. Using a 12v power supply right now, but planning on setting up as laid out below when I’m done prototyping.

    I plan to use a 15v wall wart, with an LDO to step down the motor voltage to 12v, a switch mode regulator to step down to 5v for the PIC, and a STMicroelectronics L298N dual H-Bridge to control the motor with the PIC.

    I figured I could get all this done with just a few programming hiccups, but I greatly underestimated power supply design.

    My problem is that my wall wart has an output current of 1 A, and my H-Bridge has an output current of 2 A, 3.5 A if I tie both sides of the H-bridge together, and my stall current for the motor is 5 A.

    My question is how do I make this work? What would I need to do to protect my circuit? Neither my power supply, nor my H-bridge output enough current.

    I don’t expect the motor to stall considering its torque (84 oz-in) and its application winding 42-43 AWG magnetic wire, but I want to be safe.

    From my research, these are the only things I could think of.

    1) Put a current sensor and a shunt resistor in series with the motor. Not exactly sure how this works, couldn’t find a schematic or detailed explanation. What does this do? Would it reset everything, including my PIC and LCD which is tracking rotations?
    2) Use a beefier power supply with a higher output current. I would prefer not to do considering the ones I found that output 5 A look more like my laptop power supply and cost a good deal more. Would this even be enough considering I heard that reversing the motor can draw twice the stall current? I plan on have a reverse feature, but I wouldn’t make it where you could go directly CCW from CW without some sort of long pause. Also, would a power supply with this much output current available mess with my PIC considering it and all of the components powered by it will draw nowhere near 5 A?
    3) Use an H-Bridge with a higher output current. I couldn’t really find any with any kind of output current for my needs at a reasonable price, but I can keep researching.
    4) Just make an AC current rectifier and an H-Bridge to my specs. More work and research, but maybe cheaper than buying beefier parts.

    Am I over thinking this? I just need to know what a simple solution would be, or a point in the right direction.

    Any help would be greatly appreciated, and again, sorry for the excessive questions.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The simple solution is to have a power supply capable of 6A or more and an H bridge capable of 10A or more.

    Doing this means you won't stress anything (except maybe the motor if you stall it)

    Measuring the current and cutting power if the current is exceeded is great, but have a fuse too so that if your software fails, something will fail before the power supply (you probably won't be able to beat the H-Bridge to failure if it comes to that)
  3. rbevis333


    Apr 2, 2013
    Thanks for the advice. After more research I've learned a little more about switch-mode power supplies, and I'm either going to just buy one, or try and build one.

    I may have given up on the current limiter, probably because I found a few higher amp power supplies that are more in my price range, but will still do a little research on it.

    As for the H-Bridge, I'm having trouble finding one that is rated that high. Is that something you would just build yourself?
  4. Laplace


    Apr 4, 2010
    Have you thought of using a switched current limiter? Since the machine loading is expected to be much less than the available motor torque, the only time that the power supply will be asked to supply the full stall current is for a fraction of a second during startup. So add a resistor to the motor circuit to limit stall current to the transient maximum available from the wall wart (somewhat greater than 1 A). Once the motor is up to a good speed the resistor can be switched out of the circuit but there should be a slow-blow fuse to protect the power supply if the motor should unexpectedly become stalled during normal operation of the machine. The PIC will control the turn-on and turn-off of the motor; it can also control the timing of inserting the series resistor in the motor circuit. Of course, this approach only works if the machine load is always less than 20% of the available motor torque.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    What is simple at low currents can rapidly get hard at high currents.

    Here is a 10A H bridge driver. I don't particularly recommend it, but it is illustrative that complexity increases with current, so it's more than just bigger mosfets.

    The same is true of power supplies. It's unlikely that you can build a power supply for the price you can buy one, unless you have some very special requirements (yours don't sound very special right now).
  6. BobK


    Jan 5, 2010
    Using a linear regulator for the the 12 supply for the motor and a switching regulator for the 5V supply for the PIC is backwards. The micro will use only a mA or 2, unless there are other things powered at 5V, a 78L05 regulator would be fine.

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