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I recently switched to a RAMPS 1.4 on an Arduino Mega 2560. Ever since I have extruder temperature swings a couple of minutes into the print, but it looks like a problem reading the temp rather than actual fluctuations in the temperature (as can be seen in the attached pic). Also, I've noticed that the MOSFET is getting really hot when I heat the heated bed.

What is the problem and how can I fix it?

enter image description here

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  • $\begingroup$ You seem to be asking about two separate issues (temperature swings and MOSFET overheating). Please ask only a single question per question. $\endgroup$ – Tom van der Zanden Oct 25 '17 at 16:58
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For the overheating, you may need to turn the juice down a little on your Power Supply. Typically, there is a potentiometer inside that trims the voltage level coming from there. Some things can heat up if it's over-volting and needing to drop more voltage for the board.

For the temperature reading stuff, make sure the thermistor is plugged in all way and not possibly dangling near the hot-end block. I have had that happen before and it will report improper temperatures as it goes along, and often results in the hot-end getting too hot and messing with the filament. If it's properly secured it might be just crappy or broken in some other way and you would need to replace it. It's reasonable practice to have a few extra thermistors lying around as having them go out at lousy times will bum you out.

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  • $\begingroup$ Thanks for accepting, but which of these things did it turn out to be? $\endgroup$ – Steve-O-Rama Oct 25 '17 at 18:29
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    $\begingroup$ It turns up that the two problems came from over-volting... Apparently connecting the two sets of power supply of the RAMPS in a series (connecting the power supply to the 11A and connecting in a series the 11A to the 5A) was a bad idea and creating an overload on the board. $\endgroup$ – AsafB Oct 28 '17 at 10:23
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For the really hot mosfet I would say it might be a good idea to get one of those external mosfet module boards. It just seems like a good idea to me, to not have the huge current of the bed-heater flowing through the ramps board. And for the thermistor wires, do you have them twisted together? If not, try tightly twisting the pair (of + & -) together to ensure there is no interference from other signals. Careful not to put stress on where the wires are attached to the actual thermistor-head. Honestly, all of your wiring should be in twisted-pair configuration. https://en.m.wikipedia.org/wiki/Twisted_pair

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  • $\begingroup$ Are you sure that having the thermistor wires twisted will do anything besides look neat? According to the page you linked, twisted pair helps eliminate noise in differential mode transmission. I don't think thermistors use differential mode transmission. $\endgroup$ – Tom van der Zanden Oct 22 '17 at 6:39
  • $\begingroup$ Im so sorry Tom, I never knew that I have a comment (I never got a notification) $\endgroup$ – nabzim Oct 30 '18 at 4:29
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I saw that you said you switched to a RAMPS 1.4 . I'll warn you that the componetry for the cheap $20 RAMPS 1.4's have significant issues, including under-speccing parts, using wrong part, not including enough copper for traces on the boards, and other quality concerns that can equal "FIRE" (not a euphemism).

You've noticed temperature fluctuations as well, as seen in the graph. This could be the result of a few different things.

  1. You din't PID tune the Hotend. Read M303 for the full way to do this. Then either using EEPROM or by compiling the Kp, Ki, Kd variables, and store them in your configuration.h (assuming you're using Marlin).

  2. Your thermistor is loose. When it heats, it loosens and creates a 'not great' contact against the hotend, giving spotty results.

HOWEVER the really concerning thing is that the power MOSFET isn't specced high enough for you board. If you've noticed it very hot to the touch, and also collaborated by a really 'squirrely' heated bed graph, then you have some significant problems with this part.

There is a solution. But you ever hear the adage "If you buy cheap, prepare to buy thrice." ? It applies in your situation. Because in order to route around the crappy MOSFET, you need an external MOSFET board.

THIS is a good place to see what part I'm talking about, common places to buy them (Amazon and eBay are good for quicker, and Aliexpress is cheaper and longer). The aforementioned link also includes wiring diagrams and setup.

So, it looks like a set of problems, with only 1 actually costing money. And ideally, those MOSFET boards are around $10 from Amazon(Prime), or up to $20 for the really nice TriGorilla ones.

IF/WHEN you buy a Power MOSFET board, make sure to EXCEED the amount of watts when compared to your heating solution. If you're using 12v, then you need an ohmmeter to get the resistance of the board.

Solve for amps:

 Amperage = volts/resistance

Then once you have amps, take:

 amps*volts = watts  

This is how much watts your heated bed uses. MAKE SURE TO EXCEED THIS BY 10%

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