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I am a newbie to 3D printing and I am making a 3D printer from scratch (Cartesian). I've got a RAMPS 1.4 shield and an Arduino Mega 2560 board. I have a few questions and it would be very nice if I can get answers.

The onboard MOSFET (STP55NFO6L) for the heatbed is dead and I think that the reason is because of the high resistance of my heat bed (1.8 Ω).

I have searched a lot and I am fully confused about what to do.

  1. I am thinking of buying an external MOSFET and wire it up with the RAMPS fan MOSFET (STP55NF06L) and use the same heat bed.

  2. Buy an external MOSFET and replace the onboard MOSFET (dead one) with the fan MOSFET (because of the underlying 11 A circuit) and use same heatbed.

  3. Same as 2., but buying a new heatbed also.

  4. Buying a RAMPS 1.6 and use old heatbed.

I am totally confused because of lot of searching. Please anybody help me. What should I do?

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  • $\begingroup$ I don't have experience with that hardware, but this article on the RAMPS 1.4 shows that using an external power MOSFET module will solve 2 problems - the MOSFET and the polyfuse. $\endgroup$ – Andrew Morton Aug 21 at 17:18
  • $\begingroup$ The reason for the MOSFET getting (too) hot is the high current draw of the hotbed heater, not the low resistance of the hotbed heater, although the two are related. $\endgroup$ – Andrew Morton Aug 21 at 17:20
  • $\begingroup$ Thanks for replying! So basically I should buy a new MOSFET module with low RDSon and use it with same bed and wire it up with Fan MOSFET? $\endgroup$ – Jagraj Singh Aug 21 at 19:00
  • $\begingroup$ As I mentioned earlier, I am not familiar with that hardware, but I would think it would be better to replace the failed MOSFET. That way, you can wire up the MOSFET module to the on-board heated bed connection, otherwise you will have to make extra firmware changes. Also, you do not want to have failed components on the circuit board, because they could cause other failures. $\endgroup$ – Andrew Morton Aug 21 at 19:37
  • $\begingroup$ Sounds perfect to me, thank you. And one more thing, my heat bed is mk2b 214*214 and i have read that it's high resistance is cause it support 24v also. So any idea if it's wrong or not? $\endgroup$ – Jagraj Singh Aug 22 at 8:01
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The resistance of the heated bed being too high can not have caused the MOSFET to burn out. Only a too low resistance could cause that.

Keep in mind that measuring relatively low resistances (such as the one of your heated bed) is difficult, and if you just used a regular multimeter it might indicate a wrong value (e.g. due to the resistance of the test leads or a poor connection between the probe and heated bed). Therefore, the actual resistance might be (slightly) lower.

A 1.8 Ω heated bed at 12 V will draw around 7 A of current. This is well within the capabilities of the MOSFET on board of the RAMPS. So either the resistance of the heated bed is lower than you measured (if the resistance was 1.2 Ω or lower this could cause the MOSFET to burn out), or you simply got unlucky with the quality of your RAMPS board.

The (supposedly) high resistance definitely doesn't make the heated bed compatible with 24 V. Using a 1.8 Ω bed with 24 V would cause a 13 A current draw and 320 W of power. This is a lot more than is sensible for a 214 mm x 214 mm bed.

All of the alternatives 1 and 4 you have listed are reasonable options. There is no reason (option 3) to replace the heated bed as there is no indication it is faulty. I see no reason to take option 2 since it involves desoldering and resoldering the fan MOSFET for no reason (if you are using an external MOSFET the fuse is no longer a limitation). There is a chance you'll damage the MOSFET doing this and option 1 only requires a trivial firmware change.

Option 4 (upgrading to RAMPS 1.6) is purely a matter of personal preference.

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  • $\begingroup$ Thank you so much. Well about the resistance i actually minus the resistance of probes from the actual reading and it's 1.8 and one more thing, can you please suggest me a good mosfet (external). I have been looking but there are so much of them and I really don't have any experience with such stuff. Please! $\endgroup$ – Jagraj Singh Aug 23 at 20:41

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