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After a series of semi catastrophic breakdowns (fried heated bed controller and polulu driver etc), aswell as less than perfect reliability (almost half of my print attempts failed one way or the other) and generally not so good print quality , i set to hunt down all the failure sources one by one !

***Nov 29 2010 01 pm : sorry about the pictures not showing up , there was a slight glitch while updating certain wordpress elements***

Here is the breakdown of all problems and how they got solved:

  • The Tip120 transistor controlling the heated bed power supply finally fried:
    • which is no surprise, as i completely miscalculated the power consumption (TIP 120s are rated at 5 A max and i was using …9 A !!), i will post a detailed article on the heated bed design and power consumption calculations later on. So i replaced it with a TIP142, rated at 10 A, and voila ! heated bed working again, and a lot less electrical glitches.
  • There were a TON of strange glitches, weird variations and serial connection cut-offs:
    • Uppon further analysis this came from at least 4 different problems on the machine :
      • The heated bed controller issue mentionned above
      • The power connector linking the repstrap to the PC’s psu (machine side) , was very “touchy” , a slight movement and power would be cut off , and it kept getting worse.. So i opened the connector to find …it had melted on the inside ! This is what i get from pushing 150w through filmsy cables and cheap , bad quality connectors…

        Flickr Tag Error: Call to display photo '5214173949' failed.

      • It turned out that both sides of the power cables were firmsy, so i also swapped out the molex connector (PC side) with a better quality version aswell
      • The cabling, both of the heated bed and the machine’s power supply was of a too small diameter aswell , and led to overheating, so all of them were swapped out

        Flickr Tag Error: Call to display photo '5211738017' failed.

        (upgraded version)

All these improvements, changes and adjustements, plus the fact that i kept fine tuning the skeeinforge settings , led to this :

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left: before the changes / right : after

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Flickr Tag Error: Call to display photo '5211726049' failed.

Quite satisfying overall ! Next up : speeding up  the machine!

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2 thoughts on “Doboz repstrap: increasing build quality and reliability

  • November 30, 2010 at 05:22

    mosfets are much better than BJTs for those sort of currents. I use IRL3803 which can switch 16A with no heatsink, and can connect directly to my arduino’s output.

    • December 13, 2010 at 00:22

      Thanks , Triffid_Hunter, that’s something i didn’t really know much about when i started working on the electronics, i will most likely upgrade it later down the line: 16A with NO heat-sink would definitely be an improvement !!


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