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Understanding the MK2A PCB HeatBed, R1 swaparoo

Discussion in 'Mods and Upgrades' started by jim3Dbot, Aug 30, 2016.

  1. jim3Dbot

    jim3Dbot Active Member

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    There's many Heater PCB videos out there.......but, had some questions, wanted to see first hand how they compare, in this series .........so here we go.....in an attempt to "understanding the PCB HeatBed"

     
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  2. jim3Dbot

    jim3Dbot Active Member

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    Here's some pics of the MK2a purchased.......and Here's some actual measurements from the PCB, not manufacturers specs.


    DSC08853.JPG DSC08854.JPG
    • 31.5mm x 21.5mm x 2mm (12-3/8" x 8-7/16" x 0.080")
    • Five mounting holes - 0.125" dia.
    • Thermistor center slot - 0.320" x 0.092"
    • Two PCB pads with wire mounting holes of - 0.082" (perfect for 14awg stranded cable)
    At this time, I'm thinking of Silicon 400 strand #14awg cable...........extremely flexible & has a diameter of 0.070".......note 10-12 amps isn't really a lot of current, especially for #14awg, residence wire 15amps........rating is not for the wire, but the insulation....Silicon wire would carry a whole lot more current & would not fry.........the silicon insulation, (200c) will protect the cable from the HeatBed. The 14awg is required simply for the voltage drop being much less than say 16awg or even 18awg.

    The resistive tracks are 0.055" wide. The PCB is 1oz copper, translates to 1.37mils thick. Many online calculators are available for calculating current rise in a PCB track as well as track resistance with X amount of current, considering track width, length, ambient and thickness..........

    I ordered this silicon pad, will be here by the end of the week.
    Silicone Thermal Pad 400mm x 205mm x 0.5mm ebay item # 121756225287
    The thermal width of the MK2a is 205mm, exactly as this listing.
    The thermal length is 300mm.

    Take Care..........
     
  3. danzca6

    danzca6 Well-Known Member

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    Sorry Jim but where did you buy the PCB? Was it straight from the reprap site?
     
  4. jim3Dbot

    jim3Dbot Active Member

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    Purchased on ebay, came very fast, recommend seller, PCB came in excellent..........hold off, until I run some tests.
    I just completed a video, I will be releasing early this evening.......along with install in another week.....need to see, how it holds together...warping, etc.........

    eBay item number:
    262095214593
     
  5. danzca6

    danzca6 Well-Known Member

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    Oh I'm not running out to buy just yet :) Just curious.
     
  6. jim3Dbot

    jim3Dbot Active Member

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    Here's a pic of the 'optional' LED & Resistor SMT mounting pads.......They will accommodate the larger 1206 style components, 3.2mm pitch. You only need one LED & one resistor. Designer placed two LED footprints side x side.........this allows the power supply to be polarity insensitive.....Solder one in the correct way & forget adding the second one........At 12v may install a 1K 1206 current limiting resistor. You got to be creative with the long side viewing perspective...really don't see a need for the LED, personal opinion......

    DSC08860.JPG .
     
    #6 jim3Dbot, Aug 30, 2016
    Last edited: Aug 30, 2016
  7. jim3Dbot

    jim3Dbot Active Member

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    A close-up view of the 'resistive' Traces.....Again, 1oz cu & 0.055" width.........

    DSC08859.JPG
     
  8. jim3Dbot

    jim3Dbot Active Member

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    Okay, Here is where it begins to get fun.......follow the maze.....

    DSC08853.JPG


    Starting at the bottom...... the pad on the right.....go right, start to go north, hang an immediate tight left, snake back & forth 29 times, and you will exit to the main bus highway on the west side, about a quarter way north of the lower left corner. Now go south on the busway........hang a left......exit on the left lower pad.

    Repeat this three more times, but go further north along the East busway...before making the left.

    What we have here is four parallel traces, each consisting of 29 traces...or four parallel resistors each carrying 2.5 to 3 amps of the 10 to 12 amps our power supply is producing.

    Now these 29 traces are approx. 11.8" in length, which equals 348" each. When I go to the PCB design tables & plug in the numbers of: length 348", thickness 1.37mils, not mm, and a width of 0.055", I am returned a number of 3+ ohms.

    Why did I measure 1 to 1.1 ohm? Since the four traces are in parallel, I would expect to see about 4 ohms if I isolate a section.......Lets do it.........for the sake of science..........
     
  9. jim3Dbot

    jim3Dbot Active Member

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  10. jim3Dbot

    jim3Dbot Active Member

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    Using an exacto knife, cut trace under the red meter probe....there it is about 4 ohms per branch..........then 12v/4ohms = 3 amps......now if the resistance stays the same.....and it will not............the heatbed will dissipate...........12v x 3a = 36 watts times 4 circuits or 144 watts.

    There will be a video showing the resistance change as heat is applied to the heater, coming up..........hope this will help with the PCB vs Silicon heater choice........right now I'm not sure...........Take Care

    DSC08863.JPG
     
    #10 jim3Dbot, Aug 30, 2016
    Last edited: Aug 30, 2016
  11. danzca6

    danzca6 Well-Known Member

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    When I was looking at PCB boards a week ago I saw a warning about what to look for in a board that I found on the reprapwiki. Do you know if the one you have falls into this warning or is it OK? Btw, like reading through this thread. You have a great delivery of this material.


    WARNING
    "WARNING!!! Be careful where you buy PCB heatbeds from!! There is one very important requirement - the heatbed must be etched directly from 35um copper clad! ASK YOUR SELLER!!! If the board is plated, as it's normally done, the manufacturer cannot guarantee the final thickness of copper or the evenness of the copper around the board. If it is uneven, the heat won't be evenly distributed around the board or the board won't have a high enough heat output overall.

    Your biggest warning sign is plated holes! Plating of holes requires copper plating.
     
  12. jim3Dbot

    jim3Dbot Active Member

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    Thanks Danza........PCB is of good quality...........yep, I did read that..........There are holes, only two for the 14awg wire...but they are for mechanical support, not current carry...........place the wire thru the hole & solder to the pads........you will be fine.........Good point, glad you commented...........My concern is warppin'........seen a lot of that in PCBs in my time..........mounting the PCB may be best on aluminum..........Don't think edge mounting with bulldog clips will work but I will chk it out first..........time to relax now.......I'll youtube that video tomorrow in the AM.........Later
     
  13. danzca6

    danzca6 Well-Known Member

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    In the world of mesh leveling.. a bit of warp... Fuhgeddaboudit! :)

    Maybe glass on top and cork on the bottom mounted to an aluminum plate with thumb screws and springs for manual adjustments?

    Looking forward to see more Jim
     
  14. jim3Dbot

    jim3Dbot Active Member

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    Adding an External Heat Source to the MK2a HeatBed

     
  15. danzca6

    danzca6 Well-Known Member

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    Jim great video. So is your plan to redesign the y axis on the Robo or just try and attach this with the existing rails? Maybe you are going to do something totally different.
     
  16. jim3Dbot

    jim3Dbot Active Member

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    Ha Ha Ha...........Plan???..........Sorta like the IR............winging it............Just finished an interesting video of operating the PCB from an auto-transformer, Results surprised me..........I'll post tomorrow............Danz, thanks for the feedback......Take Care Bro
     
  17. Gregg Teaby

    Gregg Teaby New Member

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    This can be fixed. Remove the build platform then remove the hex head bolt for the y axis idler pulley. Cut as much off the head as possible to make it low profile. Replace the bolt. Buy a roll of 1/8 cork at Lowes or Home Depot, about $10. Cut out a piece to fit over the current 1/16 cork. Make sure it is not to wide, it will interfere with the y axis bearing blocks. Use aluminum duct tape to hold it place. Reassemble and now check your temp. You will be surprised.


    Sent from my iPad using Tapatalk
     
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  18. jim3Dbot

    jim3Dbot Active Member

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  19. jim3Dbot

    jim3Dbot Active Member

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    Hello Greg.........Thanks for the comment...........Adding another 1/8" of insulation will definitely help, but if you check with an IR thermometer.....will the temperature drop off as it does with the stock set-up.........I'm thinking it will.......glass isn't a good conductor of thermal energy. That's what started it all. My first print was a 9" spool holder...........the ends warped, realized after a couple of tries, it became apparent the bed was not hot away from center...........If you have an IR would like to hear your results...Thanks guy
     
  20. danzca6

    danzca6 Well-Known Member

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    Heat dispersion is interesting. I know that E3D came out with a new PCB with their BigBox that is designed to disperse the hot zones more evenly across the whole build volume. Neat technology really. Also Josef Prusa has made a big leap with the MK2 I3 he recently released with his new MK42 PCB bed. I think this is why I love watching Jim's research into how PCBs are working. It's something I don't have a 100% grasp on but Jim is really helping fill in those gaps. Cool thread and looking forward to reading/watching more.

    heatbeds-1024x468.jpg
     

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