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I got a new servo a few days back (RC Servo, Futaba FP-S148). I first tested it out with the Sweep sketch on Arduino, powering it with the Arduino 5v and GND pins only. It was working, just fine.

Today I was trying to use it in my robot and I tried powering it with 2 LiPo batteries (Samsung ICR16850 2200mAh, from an old laptop battery) connected in series, giving 8.32v. As soon as I connected my servo, it started rotating randomly, I had not connected it to my Arduino yet. I quickly took it out.

Next, I used a L7805 to get 5.13v regulated supply out of my batteries that I used earlier. When I connected my batteries to the servo, and the servo to the Arduino, uploaded the sketch, the servo started behaving rather strangely, it first did a complete turn and then stopped. Only a humming sound came from the servo. Strange thing is, whenever I connect one of my Multimeter leads to the power cables, the servo immediately turned in the opposite direction only as long as only lead was in contact with either the positive or negative wire. Otherwise, the servo just gives a humming sound.

I stripped down the servo and checked the motor. It is working fine. Moreover now the servo doesn't even makes the humming sound. It just sits there, no movement, no sound.

Have I fried my servo? Or is it some other issue?

  • If it doesn't work and it used to, under the same conditions. It's broken. – Paul Apr 25 '16 at 18:08
  • @Paul broken in the sense burned out by using higher voltage than the limit? – YaddyVirus Apr 25 '16 at 18:18
  • @YaddaVirus I meant in general. In a device that does not have memory. If something did work, it should still work. And if it doesn't, it's of no use. (If you can't repair it) – Paul Apr 25 '16 at 19:43
  • 8.32v is greater than 6v. You can expect it to do horrible things. – Majenko Apr 25 '16 at 20:02
  • @Majenko which means a fried servo? – YaddyVirus Apr 25 '16 at 23:35
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As soon as I connected my servo, it started rotating randomly,

The random movement could be from a floating control line, i.e. random voltages signals are being interpreted as random positions, and the servo is responding.

I stripped down the servo and checked the motor. It is working fine. Moreover now the servo doesn't even makes the humming sound. It just sits there, no movement, no sound.

Assuming it was put back together properly, then it sounds like the control circuitry has been damaged.

I had not connected it to my Arduino yet. I quickly took it out.

Have you tried it on the sweep example again, or a servo tester? It could be working.

Strange thing is, whenever I connect one of my Multimeter leads to the power cables, the servo immediately turned in the opposite direction only as long as only lead was in contact with either the positive or negative wire.

Sounds like you are accidentally sending it control signals with your multimeter.


Your servo is dead if:

  • it is properly connected and doesn't respond to any control signals
  • it is properly connected and responds randomly to control signals
  • it is properly connected and responds randomly to a grounded control line

n.b. unconnected (floating control lines could cause random behavior in an otherwise good servo).

  • I did try it again with the sweep example, it didn't work. Most probably gonna have to get a new servo. – YaddyVirus Apr 26 '16 at 8:36
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Since a servo is basically the combination of this components:

  1. Motor
  2. Gearbox
  3. Position sensor (i.e a potentiometer)
  4. Motor control electronics (e.g some useful stuff to protect the system and to relate d.d.p. over it's node to position => angle[i.e could by just an operational ampifier])

servo disassembled

The unusual behaviour could have been raised by a single component of the system, most likely the variable resistor. There's no guarantee of sucess, but maybe we can use this event to discover soimething! My advce is try to discover the potentiometer used by the producer of the servo by looking at related datasheet. Then solder/attach temporarly an half-value or a quarter-value, resistor if the servo change it's orientation consider a replacing, if not just get rid f it.

Hope helps :)

Reference https://www.pololu.com/blog/15/servo-servo-motor-servomotor-definitely-not-server

  • Umm thing is, a replacement isn't very easy for me. Servos here cost a lot, both offline and online. I did open up my servo and I didn't find any signs of burns or something on the PCB under the motor.. And the motor is working just fine. – YaddyVirus Apr 25 '16 at 18:17
  • @YaddyVirus How you mean? I can order a 3$ servo off e-bay with free shipping to almost everywhere. It's logical that a better quality servo will cost a little more. But if you can't repair it, it'll be the only way to go. Test the potentiometer and motor individual. And/or contact the manufacturer for a replacement control-circuit. – Paul Apr 26 '16 at 8:22
  • @Paul the cheapest servo I found, (not talking about Mini servos) costed about 350 Rupees (roughly 5.25 $) + shipping. Both on Amazon and E-bay. While that may not sound expensive, for a teen who blasts away all his already little pocket-money on electronic components, that sure is expensive. – YaddyVirus Apr 26 '16 at 8:39
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    @YaddyVirus "blast away" indeed. If it's that expensive, why didn't you check the datasheet, a quick search told me it's 6V max. You've now "wasted" about 1 to 3 hours debugging a dead servo, you could've used that time to wash your parent's car for some money ;) – Paul Apr 26 '16 at 9:09
  • @YaddyVirus It's good to see that you're at least using your money for something good. Learning electronics can come in very handy! You should convince your parents of this matter, they might support you (financially). I have the feeling that you're quite sure the servo is broken, but just don't want to accept it. If you can get a little more money (with work or by funding) you'll be able to make mistakes and re-order stuff (or take on bigger/other projects). If you've learnt to first check the rating, you might say it was worth even more than 5$ (could've blown up a $30 part). – Paul Apr 26 '16 at 9:21

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