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Connecting a heated bed

Discussion in 'Mods and Upgrades' started by colton81, Dec 14, 2017.

  1. colton81

    colton81 Active Member

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    It’s been awhile since I’ve been on here but I got a quick question. On the C2 I’ve been converting it to a bigger printer I was able to successfully install the R2 firmware along with the R2 Pi os System. The problem I’m having is the heated bed I’m using a separate power supply for the heated bed and a mossfett to control it. The issue is the mainboard isn’t triggering the mossfett when I connect it to the heated bed pins. It sees the thermistor connected but just not triggering it. I tried connecting the bed directly as well to the main board and it’s not working. Any ideas where the connectors are for the controller to trigger the mossfett?


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  2. WheresWaldo

    WheresWaldo Volunteer ( ͠° ͟ʖ ͡°)
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    We need to see how you wired it. But the Mainboard for the C2 is the exact board used in the R2. So there is already a mosfet controlled by hardware PWM. You can't hook a mosfet to a mosfet, it doesn't work that way. That is probably the issue. Again without a diagram of how you wired everything it is impossible to tell. Look at all the posts about using an SSR to add a bigger heater, because adding a bigger heater is exactly the same procedure as you want to do.

    Also the firmware for the C2 and R2 is identical with the exception of bed size and not enabling the non-existent bed heater. There would have been no need to reflash it all. If your printer thinks its an R2 how do you avoid the different bed size issues?
     
  3. colton81

    colton81 Active Member

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    I’m not home right now but the attached picture is the exact same mossfett I was using and I connected the mks pins to the positive and negative bed heater green pins on the board the thermistor was connected to the S- pin on the board. So since it already has a mossfett then that’s why it wasn’t working with that. As for connecting the bed directly I connected the two wires coming from the bed and ganged the positive together on the board and the negative together and connected each wire to positive and negative. I double checked my bed by connecting directly to the PSU and it works but obviously no way to control it. As for the bed size I changed that in the marlin firmware to fit the bigger bed I have. Still have issues with the autobed leveling sensor which I need to sort out in the firmware I believe.


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  4. colton81

    colton81 Active Member

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    And my multimeter died on me but when I checked a few weeks ago on the bed heater pins on the board they were putting out very weird voltage. Another thing I noticed and I’m not sure it’s supposed to but when my hotend heats up I get a blue flashing led light but when I tried heating the bed up no blue light would light up. Leading me to believe that maybe it’s not trying to actually heat up[​IMG]


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  5. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    The bed (on a C2) connectors from the main board likely put out 19v (as a wild guess).
    On the R2 that would be 24v...(another guess)
     
  6. colton81

    colton81 Active Member

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    That’s what I thought but maybe my meter was just acting up. I am powering the board with a 24v PSU and connected the power through the 2 green pins that say PWR IN. Does anyone happen to have a link to a post where someone shows how they wired the heated bed. All I’m seeing is everyone posting that either they are going to do it or did it but didn’t post pictures of how the wiring goes?


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  7. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    An R2 owner could take a picture for you :)
     
  8. WheresWaldo

    WheresWaldo Volunteer ( ͠° ͟ʖ ͡°)
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    The Bed outputs on the Robo Mainboard put out an equivalent to the power supply output voltage.That is 24V and 19V respectively on the R2 and C2.
     
    #8 WheresWaldo, Dec 14, 2017
    Last edited: Dec 14, 2017
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  9. WheresWaldo

    WheresWaldo Volunteer ( ͠° ͟ʖ ͡°)
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    [​IMG]

    The connector marked Bed is the thermistor connection for the bed

    [​IMG]

    The 4-pin connector on the end here is the connector for the bed heater. It is two pairs of the exact same voltage rail, two plus and two minus. If you care trying to add a heater there is really no need to add a second power supply and/or additional mosfet. The bed heater is a purely resistive load and 19V will power it just fine. It might be a bit slower than having 24V but it would still work with no additional hardware.
     
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  10. colton81

    colton81 Active Member

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    Thanks for the pictures that helps. I will try again connecting. I tried connecting one wire to both positives and one wire to both negatives but that didn’t work. Any idea why there is two wires going into one pin on each end of the connector?


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  11. Kilrah

    Kilrah Well-Known Member

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    Just make sure the PSU can handle the additional load.
     
  12. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    Probably to spread the load across multiple wires. They have been a bit paranoid about that after the bed smokage incident with the R1
     
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  13. WheresWaldo

    WheresWaldo Volunteer ( ͠° ͟ʖ ͡°)
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    Maximum draw across the heated bed MOSFET is usually 11A on a 100% compatible RAMPS 1.4 board. 11A @ 24V is a maximum of 264W. If you use the standard PSU (19V) and because of the size of the C2 I would think 100W bed heater is more than enough, that only draws just over 5A @ 19V, well within the Mainboard capacity. The power supply is the only question.
     
  14. Kilrah

    Kilrah Well-Known Member

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    And rigidity. The wires have to flex with the movement of the bed, and 3 thin wires is a lot more flexible than 2 or 1 larger one(s) of comparable total cross section.
     
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  15. colton81

    colton81 Active Member

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    It didn’t snap to me until last night when I was going to connect the bed to the board that my heated bed is a 12v bed and this is the printer I was building a year ago converting the C2 to the FT-5. I don’t know if the 12v bed can handle the output from the board. I dialed down the psu to 20v but that’s as far as I can go.


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  16. colton81

    colton81 Active Member

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    The board is supposedly 19amps at 12v so if I’m correct wouldnt the main board give out or the bed?


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  17. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    If you run a 12v heater at 19v (or 24v) what happens depends on the heater. Some heater pads are rated at 24v and sold as 12v or 24v with the difference being the 24v use will have higher wattage. Not all are... if yours is a hard 12v rated one then you can smoke it or the controller. Look at see what you bought.
     
  18. colton81

    colton81 Active Member

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    [​IMG]

    This is the bed any idea how I could tell on the bed it’s self they don’t really give much information on their website


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  19. Kilrah

    Kilrah Well-Known Member

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    Specs are pretty clear, you could verify by measuring the resistance of the bed which should be about 0.65 Ohm given the specs.
    But if it's any close to the spec you can completely forget. At 20V it would draw more than 30A and dissipate a whopping 600+W...
     
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  20. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    Some of the ones in the bottom of your screen-shot are examples of 12v/24v rated :) Yours ... not so much
     

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