I'm looking into upgrading my heated bed and replacing it with a silicone bed. I know nearly nothing about electronics, so I would like someone more knowledgeable to confirm if what I have theorized is correct or I'm going to burn my house down trying it.

Basically, I'm considering setting up a 200W@12V heater, and I am using this premium ramps, which reportedly has Toshiba TK72E12N1, S1X MOSFETs.

I did some basic math following Tom's guide, if that is correct, I would be pulling 16 A current. Assuming I plug the heatbed directly to the ramps, that would mean the MOSFETs are losing approximately (considering 20 % more resistance than the datasheet says as per Tom's advice) about 1.2 W, which means the MOSFET would heat up about 100 °C above room temperature, so the total temp would be around 125 °C, which is below the graded 150 °C in the sheet.

According to this math, I'm thinking I can just plug that heater to my RAMPS board without any issues. Should this cause any issues? Will it set a fire or melt down something? Anything else I should consider?

  • $\begingroup$ Welcome to 3D Printing! The song by Burning Heads comes to mind here :o) $\endgroup$ Sep 23 '18 at 12:30
  • $\begingroup$ haha, yeah it does! hopefully, I won't end up singing that lol $\endgroup$
    – uorbe001
    Sep 23 '18 at 15:21
  • $\begingroup$ A fire can only occur if specific components are fed far more power (proportional to current in general) than they are designed for. So, don't try to make a component, or a board, do things outside its ratings. $\endgroup$ Sep 24 '18 at 14:22
  • $\begingroup$ @CarlWitthoft I had a stepper driver catch fire for a second once (quickly pulled the plug so it didn't cause any damage other than to the driver itself). I think it was a broken driver which was shorted, but since that experience I'm never confident with any changes I do to the electronics lol $\endgroup$
    – uorbe001
    Sep 25 '18 at 15:08

The heated bed port on your board has a 11A fuse. It will not work for a heated bed requiring 16A of current, no matter how good the MOSFETs might be. Note that the terminal block might also not be rated for that much current. You'd have to check, because often it is not the MOSFET itself that catches fire but the wiring or terminal blocks.

Also, keep in mind RAMPS only drives the gate with 5V. The value specified in the datasheet (for 10V) plus 20% is probably a bit optimistic. The datasheet doesn't specify the resistance at 5V, so you're guessing at what the resistance might be. Since you're already running the part quite close to its limits, and considering ambient temperature might rise above 25C, I'd be cautious. The designers of that board only intended it for use at 11A, probably with good reason...

  • 1
    $\begingroup$ Addendum: One might get away with more load if one uses a larger heatsink and a board cooling fan, but even that won't stop an 11 A fuse from blowing under 16 A load. Replacing fuses with higher rated ones or shorting them is a fire that just waits to happen. $\endgroup$
    – Trish
    Sep 23 '18 at 10:21
  • $\begingroup$ Hadn't taken that into account, thank you for saving me from burning my place down! I will use a separate mosfet + heatsink board, and probably stick a fan in there for extra safety, thanks! $\endgroup$
    – uorbe001
    Sep 23 '18 at 14:39

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