Does position of loads on a circuit matter?

Discussion in 'The Projects Forum' started by jellytot, Feb 9, 2016.

  1. jellytot

    Thread Starter Member

    May 20, 2014
    Let's say I have a 5V output on a voltage regulator, and I have a 5V motor that draws 1A (rapid and short on-off cycles), and a CPU that takes 5V and draws 20ma. Does it matter which load comes "first" (closest to the regulator output), so that there is less chance of the CPU resetting itself?
  2. Sensacell


    Jun 19, 2012
    I think you know what is going to happen.

    DC motor and CPU on the same power bus- you will need to take great care to prevent the CPU from crashing from noise and / or dropouts.
    The motor doesn't care.
  3. jellytot

    Thread Starter Member

    May 20, 2014
    If I knew, I wouldn't be asking. ;)

    Motor has flyback diode, and grounds are separate. Random resets of the CPU still occur, so I'm wondering if the load order on the power bus has an effect.
  4. crutschow


    Mar 14, 2008
    But the correct answer is that you don't want them on the same bus at all.
    They should each have their own bus wire going directly to the power supply (star connection).
    And the CPU should be well decoupled with both a 47μF capacitor across the CPU power bus and a 0.1μF ceramic capacitor directly across the CPU power pins.
  5. jellytot

    Thread Starter Member

    May 20, 2014
    I have a 330μF aluminum cap and a 0.1μF ceramic capacitor directly across the CPU power pins. I'm afraid I don't know what you mean by a "capacitor across the CPU power bus". May I trouble you (or anyone willing) with a schematic or diagram of this?
  6. Sensacell


    Jun 19, 2012
    Scope the power bus, turn the motor on and off- see what the voltage does.

    DC motors look almost like a dead short at 0 RPM, chances are your regulator is dropping out.
    In this case, the only solution would be to limit the motor inrush current with some means, maybe a resistor that gets shunted once the motor revs up?
  7. Wendy


    Mar 24, 2008
    You can also isolate them to some extent, with lots of heavy duty LC coupling.
  8. Picbuster

    Active Member

    Dec 2, 2013
    A quick explanation why you have to be careful.
    This should give to an idea where and why to put what.
  9. hp1729

    Well-Known Member

    Nov 23, 2015
    the heavier load should be closer just to keep the wires shorter.
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    You could also try diode isolated outputs, like this :-
    Roderick Young likes this.
  11. atferrari

    AAC Fanatic!

    Jan 6, 2004
    But how the diodes would mask the voltage drop, Alec? If it goes down for one it goes down for the other? Am I wrong?
  12. Lestraveled

    Well-Known Member

    May 19, 2014

    We are approaching this problem from the wrong direction. Lets turn this question into a statement. A CPU, randomly resetting while controlling a heavy load, such as a motor, is a clear indication that power management and distribution has been done incorrectly. @Sensacell was right on about the causes of this. Electromechanical devices like motors are hell on power supplies. When the rotor is not moving and full voltage is applied, the motor will appear more as a short than an inductor. Can your power supply and distribution handle a dead short for a half a second?? Of course not.

    You need to rethink how your power supply system works. Redesign your power supply so the voltage feeding the CPU is unaffected by a stalled rotor/short on the motor. The approach suggested by @Alec_t might work well for this application.

    The diodes isolate a dead short on one leg, from drawing the voltage down on the other leg. While the short is occurring, the other leg is literally running on energy stored in its capacitor. So, the capacitor must be sized to operate the CPU for a half second by itself.

    I suggest that the diode isolation be used before the regulator of the CPU.
    Last edited: Feb 10, 2016