Temperature fluctuation

After one of my Arduin MEGA boards died on me, I blamed the diode on the RAMPS board. So I cut it out to prevent future dead mega's.

But after I do that my temperature fluctuates enormously. Before it was lik max 2 degrees C, but now it is more than 20 degrees. This makes it hard for the reprap to correctly heat up and keep the heat.

So for now I have reattached my Diode, but i am afraid it will kill my MEGA again. I have also tried to power a thermistor circuit from a different power source, but it makes no difference. So my guess is that the stepper drivers make really much noise on the MEGA board, unless I hook the mega up to the PSU.

Any thoughs?

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Get rid of the diode. You only need it if you need to add an SD reader, or LCD.

Most likely, your temperature is fluctuating because you have 200k instead of 100k (or the other way around) set as your thermistor value in the configure.h. Try changing this. The thermistor should not be powered from anywhere else, other than the connector provided for it on the RAMPS, so put that back the way that it's supposed to be, and you will avoid confusion in troubleshooting.

There are more than one type of thermistor with 100k resistance at room temperature. So, you really need to use the correct temperature table for your thermistor. I also found that the default 20 pt table in the Five-D firmware is not fine enough at the temperature range of interest to us: 180-260C. With my 100k thermistor, I actually had large temperature oscillations at high temperature which went away when I increased the number of points in my table.

If it is the firmware, why would the diode fix my problem then?

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PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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Quote
brupje
So my guess is that the stepper drivers make really much noise on the MEGA board, unless I hook the mega up to the PSU.

This doesn't make sense. The diode is used to supply 12V from the RAMPS shield to the Arduino MEGA. Why would this affect the noise on the MEGA board? And why would it affect the temperature control on the extruder?

Without the diode, the Mega runs off the 5V from your USB port.

It could be that your USB port isn't providing enough power to drive the Mega, the thermistor AND the signal side of the Pololu's. Older machines and some laptops tend to not put out as much power on USB.

Most Mega's have a DC barrel jack on them, so you can feed them DC power (eg: plugpack/wallwart). This socket goes straight into a small rectifier that provides 5V to the Mega (and anything running off it - in the case of RAMPS, the thermistors and the signal side of the Pololu's). Note that the Mega cannot take more than 12V on that socket, and it needs to be at least 7.5V for the rectifier to work.

Note: If your RAMPS has a diode in it, then it's feeding power to that same rectifier already (same thing as the socket, just fed via the Mega headers). Without it, the Mega has to have power from its socket, or via USB.

Thanks, well the fun thing is: when I power the MEGA through the DC barrel without the diode, the temperature fluctuates again. So only if I power through the diode with the PSU, I get stable temperature readings. I am a bit lost at this point

@brnrd: I am not an electronics guy, I simply test it and these are my findings: stable temperature readings with diode, unstable without.

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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
Megatronics 3.0 €79.99 / Minitronics €37,19

Have you checked for some sort of short between the thermistor and the nozzle heater element?