Grounds and bridge rectifiers

Discussion in 'General Electronics Chat' started by svdsinner, Nov 17, 2013.

  1. svdsinner

    Thread Starter Member

    May 17, 2011
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    2
    I decided to build my own power supply for some stepper motors from an old transformer I had that puts out 28V AC. The AC goes through a bridge rectifier and comes out a bit shy of 40V DC, gets smoothed with a big capacitor and an LM317 and is producing voltage just fine in isolation. The GND is determined by the negative DC port of the bridge rectifier. (Its currently a floating GND)

    However, I am going to be using an Arduino and a Stepper motor controller to handle the motion of the steppers. They both use +5V and their GND is set by their voltage regulators.

    My motor controller allows separate power and signal grounds.

    1) Is it safe to have two separate grounds? (One relative to the Arduino and one relative to the motor power supply, with no guarantee that they would actually both be equal) If it is safe, is it wise to have two separate grounds?
    2) Can I ground the negative terminal of the bridge rectifier without changing the relative output? (IOW, will I still get the same voltage out if I ground the negative port?)
    3) If I were to ground the negative port, should I use the ground wire from the mains, or the ground from the Arduino? (I'm guessing grounding a 40V power supply through a micro-controller would be bad, but is it?)
    4) If I grounded the negative port to the ground-wire from the mains, would that keep the GND tame enough to share it with the Arduino?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    ooh, so many questions.

    1) No, you should not have separate grounds. You need to make a distinction between earth ground and COMMON. Before we talk about GROUND, let us talk about COMMON.

    2) All of your circuits, Arduino, motor controller, power supply, oscilloscope must share a COMMON 0V reference (which is usually labelled as GND).

    Yes, you should connect the -ve terminal of your bridge rectifier to COMMON.

    3) Your Arduino is powered by a power supply whose -ve terminal is also connected to COMMON.

    4) The ground wire from the AC mains outlet is a safety ground. At some point in your house wiring it is connected to EARTH GROUND.

    No current must flow in this GROUND.
    Note the difference. The COMMON wire between your power supply and the Arduino or motor controller is the return to the power supply and therefore carries current.

    Hence, your first objective is to make sure all of your circuits share a COMMON reference point.

    Finally, in your power supply, connect the AC mains GROUND to the chassis of your power supply. Connect the COMMON to the chassis GROUND.

    Sounds convoluted but I just want you to know the difference between COMMON and GROUND.
     
  3. gootee

    Senior Member

    Apr 24, 2007
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    Very good explanation.

    Svdsinner, remember that earth ground does nothing for you except keep you and others SAFE, from electrocution. And, it is absolutely REQUIRED, if your box connects to the AC mains. Also, it must not be soldered. It should be bolted, or welded. Otherwise a fault current could possibly melt the solder and disconnect the safety earth ground.

    You should also research STAR GROUNDING. The main idea there is that you don't want the ground-return currents from other parts of a circuit to induce voltages across the ground-return conductors themselves that will then bounce the ground point voltages for sensitive parts of a circuit, such as low-level signal inputs' ground-reference points. So to avoid that, you run SEPARATE ground-return conductors all the way back to the single star ground point, from different parts of a circuit.

    Another thing to avoid, during design, layout, and build, is LOOPS. Don't make any that enclose any geometric area. Otherwise you've made some good antennas, especially for time-varying magnetic fields, which will generate corresponding currents in every loop, proportional to the area enclosed by the loop. Conversely, a time-varying current in a loop with enclosed area will propagate a time-varying magnetic field. See Faraday's Law. Shielding won't do much to attenuate magnetic fields. So you have to not make any gaps between the conductors in any of your circuit loops.

    Loops could be a big problem, in your current project. (Motors...!) Pay special attention to the currents going to and from your motors, and within their controllers. The flow and return paths must never be separated. Same goes for any AC and transformer pairs. Also pay special attention to low-level signal-type pairs. Every one of them needs its ground reference/return RIGHT up against it, everywhere.

    If you have a wire or PCB trace going anywhere BY ITSELF, you're doing it wrong. Using a ground plane makes that much less likely, and much easier to lay out.

    Same goes for DC power and ground.

    Your loose wiring must ALL be paired (flow and return), with all pairs tightly twisted, ALL the way to each end.

    Also, use DECOUPLING capacitors, right AT every point of load (everyplace power and ground connect to any active device, or motor, or enter a board), to supply the fast-transient currents (confining them to a very small loop), and also to act as BYPASS caps, to short any high frequencies to ground. You will usually use at least two caps for each power pin or connection: a very-physically-small one, probably X7R ceramic, within a couple of mm or less of the device body and a larger one, an electrolytic, within a few centimeters, but closer would be better.

    Good luck!

    Cheers,

    Tom
     
    Last edited: Nov 17, 2013
    GopherT likes this.
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Tom, when you give an explanation, you really give a good one!
     
  5. svdsinner

    Thread Starter Member

    May 17, 2011
    39
    2
    Two minor questions. As mentioned
    Wouldn't the bridge rectifier's negative terminal be constantly moving current between itself and the Arduino GND? (At the same frequency as the mains)

    Do the voltage regulators on the Arduino just handle it?

    Secondly, if grounds always ground together, why would my controller (a TB6560AHQ) have separate grounds not only for the signal ground, but for both phases of the motor coil outputs?

    Btw, I feel honored to get such great responses so far. Top notch.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    MrChips points out the confusion when a power supply common is not differentiated from earth ground, this comes unfortunately from the fact that in N.A. the term GND is used for both earth grounded systems and non-earth grounded system to indicate a particular P.S. common.
    If you use terms such as 5v common, 24v common etc, this usually removes any confusion.
    Besides the symbols for each are quite different.
    Max.
     
    Last edited: Nov 18, 2013
  7. svdsinner

    Thread Starter Member

    May 17, 2011
    39
    2
    Thanks MaxHeadRoom!

    That PDF was excellent reading. Covered a lot of stuff but still digestible.

    Did I read it correctly to summaries as follows:
    Yes, the ground will not be perfect, however the imperfections will be small and won't cause problems. (The PDF was showing GND noise in the general range of +/-.1V) which shouldn't cause any problems with the Arduino, especially with plenty of decoupling capacitors designed in.

    Also, the ground issues will increase with frequency, so if the steppers run smoothly slowly but ugly fast, I may want to investigate further. (Max frequency of pulses from controller is set to 130Khz)
     
  8. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    True.

    I can't read calculus, but I can applaud that PDF for illustrating that you are going to be chasing "ground" for the rest of your life. Ground faults, ground loops, ground not actually being ground...these are realities and solving them is almost an art. When you can see in your head what is really going on at the "ground" level, you will be admired by your peers. For now, keep it as simple as you can. YOU are still in the process of understanding, "common". You can make a quiet island for your Arduino but you already know the first pulse to a motor will change the game. It's good practice. You will need it.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    When designing a power supply and control board for stepping motors, don't just think of ground as GND.

    Think power supply RETURN.

    High current loads need a low impedance path for the current to get to the load. They also need a low impedance path for the current to return to the power supply.

    This is not your signal GND.

    Create a separate 0V reference for your control signal GND.
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The whole idea of keeping common as close to 0 volts as possible becomes more and more important with new logic chips running as low as 0.8 volts and considered HIGH at only 0.35 volts.

    http://www.nxp.com/documents/data_sheet/74AXP1G57.pdf

    - just launched multifunction logic gate by NXP. Power consumption less than one microAmp and drive current of 2.5 mA at 0.8 volts.
     
  11. gootee

    Senior Member

    Apr 24, 2007
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    Remember that the "frequency" is just the repetition rate. The edges of the pulses will have frequency content that is FAR above the repetition rate.

    If possible, always use edge times (rise and fall times) that are no faster than you need them to be.

    Unfortunately, if there are mosfets involved in creating the pulses, slower edge times would increase the power dissipation in the mosfets.
     
    Last edited: Nov 19, 2013
  12. gootee

    Senior Member

    Apr 24, 2007
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    Thank you. It helps when it's something that I've already had to think about and explain to others.
     
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