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Solid State Relay or MOSFet for heater?

Discussion in 'Mods and Upgrades' started by joea, Mar 10, 2022.

  1. joea

    joea Active Member

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    A lot of chatter about SSR and MOSFet to switch power for the bed heater, as some RAMPS cannot handle the current.

    So, what is the current hot setup (pun intended)?
     
  2. fred3d

    fred3d Member

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    Anything to make the heater current stop cooking the connector on the board and on that end of the cable would be a big improvement.
     
  3. joea

    joea Active Member

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    I was kinda hoping for a specific brand or type know to work well and last a long time.
     
  4. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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  5. joea

    joea Active Member

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    mark tomlinson likes this.
  6. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    Yes, I just did the bed heater -- that is the one that RAMPS boards have fail most often. Your similar one looks much the same. You want at least 15 amps rating and that one you linked is 30 -- should be fine.

    I used the Anderson powerpoles for ALL of my printers as the removable connectors for the bed heat :)
    We also used them on the CNC machine we built (an XCarve clone more or less).

    They are what we used (the 45amp ones) in our Robots we built (way back in the day) since they are better (IMHO) than Deans connectors.
     
  7. joea

    joea Active Member

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    Anyone know if the two +12v supply connectors on the RAMPS board are common or isolated from each other? That is, do they both need to be supplied for the board to work properly?
     
  8. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    Everything including schematics are here: https://reprap.org/wiki/RAMPS_1.4
    It is their board after all :)
    If memory serves I think they both need to be supplied, but they can share a ground and come from one DC soiurce.

    One rail is capped at 5amps? the bed heater one at 13a? I would need to go look myself, but those are close.
     
  9. joea

    joea Active Member

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    Yeah, that's what I was thinking. the upper pair is marked 11a or 13a in some pictures. Seems odd as 15a seems to be the going quoted current for the heater bed.

    Been a while since I tried to decipher board layouts from just images like that. In any event, seems I will need both connected to allow using the D8 connector as the "switching signal" for the external MOSfet. I used the third pair of +12v for my LED strip (0.5A maybe), so see no problem sharing that with the external board. Can always move that if needs be. The anderson won't be in for a few more days so I will wait till then to do anything.
     
  10. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    I suspect the original design of the RAMPS expected the current to be 11a on that output but the cheaper heaters all pull 11a minimum which meant that even if it did not pull more it was going to eventually get to the point where it would fail. The external relay/mostfet will pul almost nothing in comparison so it is the best solution. I was disappointed that the RepRap folks never officially redid the design to add something similar on the board.
     
  11. joea

    joea Active Member

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    Project complete, more or less. Mounting the MOSfet is a bit problematic, but it works well. Heat sink does not seem to even get warm.

    The Power pole connectors were a bit of a problem as I had a hard time getting the inserted, fearing to break something. When the bed did not heat at first I was perplexed but soon realized I did not seat the PP contacts in the housing properly. Seat of the Pants is not always the best way to learn.
     
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