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[on:]

This may be more of a "servo" question than an Arduino question so if I'm in the wrong place let me know.

I have been working on a simple robotic arm as a learning project.  I have some standard HiTec servos (HS-311 salvaged from an RC plane I don't fly much any more) controlling the lower joints and two HS-55 servos controlling the hand.  My sketch reads the value from a potentiometer on analog 2, maps it, and writes to digital pin 10 using servo.write.  Similar to the "knob" example sketch.  I'm also using an external power source to run the servos and I have a common ground between the servo power and the Arduino board.The servo power is a 600mA NiCad pack and the voltage from the pack according to my multi-meter is 5.4v.  I've checked and double checked the connections.

I was testing the joints two at a time (because I only have two potentiometers at this point) and it works perfect with the standard servos.  When I hooked it up to the HS-55 they BOTH actually glowed in the back, smoked, and got hot - within seconds of powering them up.  I went back to the standard servos and they worked.  What could be going on here...any thoughts or suggestions?

The only thing different is my battery pack.  I was running a larger 6v pack (1400mA) but it died so I switched to the smaller pack and the problem seemed to emerge...but it still works with the standard servos so that might be a red herring. 

[Reply #1 on:]

I would guess a wiring problem, letting current flow where it wasn't supposed to.

http://www.servocity.com/html/hs-55_sub-micro.html

[Reply #2 on:]

I'd agree that a wiring problem was likely the cause...but I checked the connection like 100 times after things went badly and I'm about 99% certain that I didn't do something majorly stupid like reversing the power connection to the servos.  What else could have been the likely candidate for a problem?

[Reply #3 on:]

What else could have been the likely candidate for a problem?

Bad wiring. In my experience failure to properly ground small servos back to their power supply can result in them getting hot and operating erratically. With a high current power supply, they may smoke.

[Reply #4 on:]

What else could have been the likely candidate for a problem?

Bad wiring. In my experience failure to properly ground small servos back to their power supply can result in them getting hot and operating erratically. With a high current power supply, they may smoke.

Thanks for the help.  I'm guessing (but I ripped everything out last night so I can't double check) that the most likely problem is that I may have had my servos grounded back to just the Arduino and then the Arduino grounded back to the power but not all linked together.  Too bad...$20 worth of shot servos.  Live and learn!

[Reply #5 on:]

Too bad...$20 worth of shot servos

The motor part of the servo probably is still good.

[Reply #6 on:]

Too bad...$20 worth of shot servos

The motor part of the servo probably is still good.

Can you get replacement boards for these little servos?  I haven't seen any - or should I just get new servos and use the gears and motors from the burned out ones as replacements?

[Reply #7 on:]

It's very possibile to replace servo control boards and their are some available to hobbiests.  A good place to start looking would be http://www.openservo.com/.

[Reply #8 on:]

I did a bunch of searching, the electronics does not appear to be a purchasable part.
http://www.amainhobbies.com/index.php/cPath/61_120/n/Radios-Accessories-Servo-Parts/page/1

Seems to be a pretty inexpensive servo, $9.99 and under

http://www.servocity.com/html/hs-55_sub-micro.html