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Resurrecting burnt out servos! — Parallax Forums

Resurrecting burnt out servos!

bulkheadbulkhead Posts: 405
edited 2006-01-14 16:12 in Robotics
Ok, I have several burnt out servos, and I believe I know the part that has failed (the same on each). I had the privilege (more like stroke of bad luck) to have one burn out right in my hand, with PCB exposed. I traced the burning smell (and heat) to a small thing that looks like a transistor (I don't know if it is or not). It has 3 pins going in, and 1 pin going out, which looks like its connected to the servo motor's red wire. On it, it says "D J R R" each letter on a corner of the face of the burnt part (transistor). Here are some pictures (sorry they are blurry, anyone have tips on getting cleaner shots of small things?)



Anyways, any insight on how to repair these servos would greatly be appreciated. I would imagine, the first step would be to find a wiring diagram for the servo? Then I'd probably find some replacement transistors (or whatever they are) and then pop 'em in.

Comments

  • Tronic (Greece)Tronic (Greece) Posts: 130
    edited 2006-01-10 07:29
    One thing you can do is to discart all the burned electronics and connect two wires directly on the motor so afterward you have a great DC motor with its own gears for turn reduction. You can then use some H-bridge to drive it around in a way simular to the parallax's continuous rotation servos.

    Unless they were expensive enouph to affort the repair...

    Thanos
  • Paul BakerPaul Baker Posts: 6,351
    edited 2006-01-10 13:41
    bulkhead said...
    ·(sorry they are blurry, anyone have tips on getting cleaner shots of small things?)

    Stick it on a flatbed scanner, whats the brand name of the servo you are using?

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    ·1+1=10
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2006-01-10 16:00
    You could always "wait" for another servo to strip out its gears and use the electronics from that.· ...OR... Most RC hobby shops sell just the electronic guts, so that you could replace the bad circuitry yourself. (See attached)
    ·
    Better yet, design your own controller.
    ·
    ·
    My first concern would be to try to figure out why you are going through so many servos’.· Are you supplying too much voltage to them?
    ·


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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
    SERVO.JPG
    800 x 1180 - 202K
  • bulkheadbulkhead Posts: 405
    edited 2006-01-11 02:28
    Well, the servos I have are Blue Bird BMS-620's. I modify these "high torque" servos for continous rotation, since they are more powerful than the parallax ones. They probably burn out since they aren't designed to run for long periods of time under a load. I'm looking for an easy fix to these servos. What I have in mind is simply replacing the transistor with a larger one, that can handle more current. Not only would it work again, but it would be able to handle larger loads without failing. If a compatible transistor could be found at RadioShack for a few bucks, it would be a much simpler (and cheaper?) solution to an H-bridge.
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2006-01-11 02:39
    In that case, check this link out....

    http://forums.parallax.com/showthread.php?p=525689

    http://forums.parallax.com/attachment.php?attachmentid=37088

    ...I have made this sort of modification several times to cordless screwdrivers.


    Ztek (<-I think) makes a nice 2A H-Bridge, I wil try to find a part number

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2006-01-11 03:15
    For some reason, I could not find anything on Zetek or Zitek .... I was sure they made SMT H-bridges.

    Ohh well..... Here is a 1A H-Bridge that I have used with great success...

    www.vishay.com/docs/70007/70007.pdf

    company's main page
    www.vishay.com/power-ics/

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • bulkheadbulkhead Posts: 405
    edited 2006-01-11 04:45
    Hmm, that's an interesting modification. I don't really understand how it works (I haven't learned much about electronics), but it looks like a good idea. Where exactly can I get the parts needed to make that modification? Where is a good place to get an h-bridge like the one in that link?
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2006-01-11 05:09
    pick your location....

    www.vishay.com/power-ics/motor-drive/sales


    ...based on the link above, Mouser or Allied Electronics look like they carry vishay components.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • kelvin jameskelvin james Posts: 531
    edited 2006-01-11 06:23
    I saw this a while ago, and when i saw this post, it reminded me of it. Have a gander at the link below, this could be what you need. Unfortunately, it only comes in kit form, but looks pretty simple to solder, and the assembly is very well described. Best thing is, it's designed to fit in a regular servo .

    kelvin

    www.hobbyengineering.com/H1194.html
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-01-12 15:52
    Solarbotics sells an H-Bridge that fits into the case and replaces the PWM control.
    In many instances, this frees up your microcomputer to do other tasks as the PWM generation is more complex than a simple binary left and right control.

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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • bulkheadbulkhead Posts: 405
    edited 2006-01-13 04:22
    Well, how much does an H-bridge generally cost? What is the max current load? I'm looking for a low cost way of controlling small motors. For around $30, I could buy an electronic speed control that can handle a 540 type RC car motor.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-01-14 16:12
    Is $12.50 per servo too much. That is what Hobbyengineering is pricing the H-bridge from Solorbotics for.

    You can simply gut the boards and run two wires out of the servo and you have a simple geared motor.
    From there, you can do anything you want to control power and direction. You can use two DPDT relays [noparse][[/noparse]take a look at the Sumobot in the Junkbotics book], build your own freeform BEAM H-bridge, or buy a well engineered device.

    The RC speed control may be sending you back into PWM servo boards. There may be some RC hobby suppliers that provide replacement boards as there are replacement gears and replacement cases available. But it is up to you to figure out it that is really saving money.

    Personally, I think that if the boards are burnt out, you have a chance to switch to a digitally switched H-bridge as a superior robotic setup.

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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
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