Why is my voltage drooping so much when driving a servo (also ripple problem)?

Discussion in 'The Projects Forum' started by Involute, May 17, 2016.

  1. Involute

    Thread Starter Active Member

    Mar 23, 2008
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    I'm controlling a servo from a 5V Arduino (Pro Trinket). I'm supplying 5V to the entire system from a 3A bench supply. scope_0.jpg shows the supply output when the motor's idle. Scope_1.jpg shows the output when the motor is not quite stalled, at which point it's drawing about 800 mA. I have the current limited to 1.5A at the supply. Why is the voltage drooping so much, and why is the ripple so high? I would think a 3A supply should be able to handle this without breaking a sweat, but it's a Korad, not a Keysight, so maybe not. Other explanations/suggestions? Thanks for your help.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,500
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    Include RC Servo in your title! There are many other kinds of servo's in the world!
    Max.
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Try turning the current limit up and see what happens.
     
  4. Involute

    Thread Starter Active Member

    Mar 23, 2008
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    Oops. Unable to edit it.
     
  5. Involute

    Thread Starter Active Member

    Mar 23, 2008
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    Bumped it to 3A; no change.
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    One more idea.
    Try adding a large cap to the output of the supply. Maybe 1000 ufd or whatever you have handy.
     
  7. hp1729

    Well-Known Member

    Nov 23, 2015
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    Sounds like it isn't a very good power supply. Do you have an alternative power source?
     
  8. Involute

    Thread Starter Active Member

    Mar 23, 2008
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    Good idea, thanks. I'd tried .1 uF and 4.7 uF without any effect before posting here. Still, the biggest I have on hand is 2200 uF and, while it helps in the unloaded case (scope_2.jpg), it doesn't help with the voltage droop in the loaded case (scope_3.jpg), though it has a big effect on the ripple
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Hmm, to bad.:(
    Sounds like something is bad wrong with your supply. Do you have a schematic for it?
     
  10. Involute

    Thread Starter Active Member

    Mar 23, 2008
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    No, it's a commercial bench supply (Korad KA3005D-3S), but I substituted a power supply harvested from somewhere that I had lying around and that solved the problem. Sounds like I need to ditch the Korad. Too bad. I really liked it until this.
     
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