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Relay switch addition to prevent heat bed cutting out

Discussion in 'Mods and Upgrades' started by Jerry RoBo 3D, Nov 23, 2013.

  1. Jerry RoBo 3D

    Jerry RoBo 3D Administrator
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    If you are having trouble with your heat bed cutting out, your polyswitch fuse on the arduino is oversensitive.

    You can buy a $5 part at radio shack to prevent your heat bed from cutting out ever again.
    http://www.radioshack.com/product/index.jsp?productId=20875286

    The heat bed will still be control in the exact same way. Only the wiring changes.

    I soldered the connections to the relay switch but you can also use Butt Terminals (double sided crimps).

    You don't have to take your bed off for this fix. It's shown in the picture that way for clarity.
     

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  2. tesseract

    tesseract Moderator
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    Hey Jerry can you gives us a little info on what this does to make the bed work such as: "it controls the powering cycle or something" and maybe how it does it.

    Some people may want to know what the issue turned out to be and how this fixes it.
     
  3. Jerry RoBo 3D

    Jerry RoBo 3D Administrator
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    Yeah no problem. This fix reroutes the power to the bed. Now the current that powers the bed will be directly from the power supply, will not have to pass through the arduino control board first (bypasses the polyswitch), but the on/off and temperature regulation is still being controlled by the arduino control board.

    The issue this corrects is the issue of your bed heating up and then turning off and cooling down when you don't want it to. The bed does this because the polyswitch on the arduino isn't behaving as it should. The polyswitch should allow 11amps to pass through it and heat up the bed with no problems. Our bed only draws about 9.5amps. However, some peoples polyswitchs are tripping and shutting down the heat bed prematurely.

    A fan blowing on the polyswitch helps to eliminate the problem but isn't a guarantee.

    To read up on how polyswitches work you can check out this wiki page: http://en.wikipedia.org/wiki/Resettable_fuse
     
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  4. tesseract

    tesseract Moderator
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    Thanks Jerry< I am half way through my mod as of right now and I understood the issue but others may not have. Thanks for the additional info I am sure others will understand what is going on now
     
  5. CAMBO3D

    CAMBO3D New Member

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    its not a fuse that controls power to the heat bed. Mosfets controls power to the heatbed. Just to clear that up. http://reprap.org/wiki/RAMPS_1.4.

    If your fuse is tripping, its because the heat bed is out of spec (ie ..less than 1.1 ohm) that could cause an over current condition which causes your poly fuse to trip.

    when you introduce an electromechanical relay into the circuit which constantly switches on and off, what you have done is add a device that will introduce noise into your sensitive arduino electronics. the constant switching of the pid routine will wear the relay prematurely..

    I would fix the fuse issue rather than add another problem. (ie.. change to a different fuse or control board)
     
  6. Jerry RoBo 3D

    Jerry RoBo 3D Administrator
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    Hi Cambo. I'm not sure what you are referring to about a fuse controlling the power to the heat bed. I don't see that mentioned anywhere.
     
  7. tesseract

    tesseract Moderator
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    More information Jerry I am not sure if this is the total fix yet.
    I installed the relay as you pointed out in the thread but what seems to be the problem now is that simply dalays the issue.

    I set the bed temp to 100
    have only the printer powered on a in addition to this a very low power consuming led 3 pack.
    I turn the bed on and it starts warming up nicely and I can hear the click click click of the relay as it is firing with the pulses it is receiving from the ramps board the temp begins to rise nicely.

    I have been checking the location of the circuit for warm from the top side of the base as I have in the past to see if the ramps board was heating. It is fairly cool as well

    Then at about 92C I hear the fan for the power supply turn on and the bed temp immediately begins to drop
    having never reached the 100 set temp and while the bed is still tuned I still hear the click click click of the relay only the bed temp is still dropping very slowly down to 84C and dropping it looks like it is trying top stay hot but just not able to.

    Back to the drawing board I guess
     
  8. pclabtech

    pclabtech Active Member

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    Awwww.amd had jumped for joy up and dwn hoping this one annoying problem can be fixed.
     
  9. Jerry RoBo 3D

    Jerry RoBo 3D Administrator
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    Tesseract, I haven't noticed that yet on our printers. Do you have a voltmeter? Can you please check what's happening with the voltage coming from the power supply? It sounds like your power supply could be having an issue.
     
  10. Jerry RoBo 3D

    Jerry RoBo 3D Administrator
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    Can anyone that has had this heat bed cut out issue please take a look at their arduino board and look closely at the orange square polyswitch at the F2 location and report what is written on that component?

    I would like to see if there are any variances in that component used on these arduino boards.

    Thanks!
     
  11. tesseract

    tesseract Moderator
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    here they are
    F2
    2013-11-27 20.56.05 HDR.jpg

    F1--just in case
    2013-11-27 20.56.20 HDR.jpg
     
  12. Jerry RoBo 3D

    Jerry RoBo 3D Administrator
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    Tesseract,

    Were you able to get a voltmeter onto the DC output of your power supply when the bed drops in temp?
     
  13. tesseract

    tesseract Moderator
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    yeah it was wierd the voltage before I turned teh bed on show 11.98V turning oon teh bed it dropped to 11.96V and stayed that way got up to 91 with it set to 100 when it started to fall the voltage temp increased to 12.18 then back to 12.06 then 11.98 never got warm again.

    Currenlty I my bed is completely dead now the when I installed teh realy per your instruction above and powered the bed on it began clicking about twice a 3-5 times a second and never stopped all the way up to the highest temp it made it to 91~92 and even then it did not stop clicking as the bed was dropping in temp it was still clicking away and only stopped when I turned the bed off.

    Since then the wire broke right at the connector and I tried to solder it back but it won' t power up at all now talked to Braydon and he said he would look into what could be done to fix it at this point.
     
  14. Jerry RoBo 3D

    Jerry RoBo 3D Administrator
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    Tesseract,

    From my experience with half a dozen printers I've put the relay on, the relay should not have a rapid clicking. Do you have the latest firmware installed on your arduino?
     
  15. tesseract

    tesseract Moderator
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    Yep now you see part of my problem
     
  16. Mike Kelly

    Mike Kelly Volunteer

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    I wouldn't use a 15A relay on a 30A power supply. As kapton heaters are resistive heating element it will pull max current to drive the heaters P=I^2*R. You'd need a relay rated at at least 30A to survive PWM control from the ramps.
     
  17. Jerry RoBo 3D

    Jerry RoBo 3D Administrator
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    Mike,

    P=IV. I believe you mean V = IR. From these two equations would could derive the following two equations:
    P=I^2 R, and P=IV^2/R

    But P doesn't equal IR.

    The kapton heater has a natural resistance of about 1.3 ohms. If we use our good friend Ohm's law V = IR
    we get I maxing out at about 9.25 amps. It's not possible for the kapton heater to pull any more current than 9.25 amps. So no, they will not pull the maximum amperage out of the power supply. This would only happen when the resistance drops to near zero, which would be a short.
     
  18. Mike Kelly

    Mike Kelly Volunteer

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    Sorry you're right, I meant I^2 not just I., vacation and what not

    In this instance voltage doesn't matter. Current draw is not limited by voltage, hence why a 12v car battery will shock a lot worse than a few button cells at 12v.

    Base Ohms law doesn't apply in this situation as the heater is doing work, which can be simplified to only a function of Current and resistance. If you know the max power of the heater you can determine it's current draw. If we assume that it can pull max current from the power supply, P=30A^2*1.6 or 1400 W max. Though we need to know the energy density of the kapton heater you chose to know the max power output.

    Voltage just allows for less current requirements to achieve the same power output.

    *Disclaimer, I'm a mechanical engineer and sometimes get confused by electrical, but I'm fairly certain heaters draw max power based on energy density if they're able to, regardless of voltage draw.
     
  19. tesseract

    tesseract Moderator
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    I wanted to "Like" the two previous comments as they are both pretty descriptive and information but what the Hell are they talking about.

    I just want it to work mann!!!
     
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  20. Casey

    Casey New Member

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    I just put a small heatsink on my bed's polyfuse. Hopefully that sinks enough heat to allow the bed to heat as it should.
     
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