I recently attached a servo to my Arduino for the first time. I ended up needing a 470uF capacitor wired like this tutorial shows because my servo was freaking out and causing my laptop to throw "power surge" warnings (I have my Arduino connected to my laptop via USB).

Although I'm glad I got this working, this leaves me with several concerns.

  • Why was a capacitor necessary to "stabilize" my Arduino? In other words, why was it the solution to my Arduino being able to properly power my servo?
  • In that tutorial I linked above, there is a Fritzing diagram of the circuit with the capacitor. Can I assume this is a capacitor wired in parallel with the servo? Why parallel and not series?
  • What's so special about a 470uF capacitor (vs., say, 100uF)? I just used a 470uF because thats what the tutorial said to use, but what math could this number have been based on? In other words, how might I have arrived at this number myself?
  • I've heard capacitors can be extremely dangerous to work with, after all they store power. I'm now afraid to even touch the capacitor on my breadboard! How can I tell if its safe to remove the capacitor (even after I've disconnected the Arduino from its power source)? I have a multimeter but I'm not sure what setting/range I could set it to.
  • Big capacitors can be dangerous, small ones not so much. You might get a tiny spark. – immibis Aug 27 at 1:17
up vote 7 down vote accepted
  • "Why was a capacitor necessary to "stabilize" my Arduino?"

A servo motor draws a substantial current in short peaks. And USB power is not designed to provide such current peaks (and the cable makes it worse), hence the current peaks cause the voltage to vary, which the Arduino is not designed for. A capacitor acts as a buffer for current, so as long as the USB power can deliver the average current, the capacitor will help by smooth out the current peaks.

  • "Why parallel and not series?"

Think of a capacitor as a reservoir. But beware of overinterpreting anlogies, better read a decent electronics beginners book.

  • "What's so special about a 470uF capacitor (vs., say, 100uF)?"

The uF figure is the amount of buffering. Enough is enough, but better be on the safe side (for this use, more is OK). The value of 470 uF is probably based on experience. Just by accident, it is the same value I let my students use :)

  • "I'm now afraid to even touch the capacitor on my breadboard!"

Capacitors can be dangerous if they store either a high voltage or a large amount of energy that can be released in a very short time. In your case neither applies, so don't worry. But you might see a small spark when you short the leads of your capacitor, even after you have removed the USB power. A resistor (for instance 1k) in parallel to the capacitor solves this.

  • Was just typing an identical answer :-) So I just +1-ed this one. – Bimpelrekkie Jun 14 '15 at 17:48
  • Yeah, it can feel like a speed-typing match. But my worst experience was typing a long answer, which then could not be saved because the question had been deleted! – Wouter van Ooijen Jun 14 '15 at 17:51
  • @WoutervanOoijen I've had several "answer deleted" experiences. In the most extreme case (maybe two?) I made a new question with suitable improvements and answered it :-). – Russell McMahon Jun 14 '15 at 20:42
  • Same here. Had questions put on hold or closed at least dozen times over the last couple of years. Can be frustrating!! I think once I transposed what I had written into a series of comments applied under the question. – Michael Karas Jun 14 '15 at 23:19

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