CR-10 Thermistor Signal Noise

I’m running a small lab of CR-10s and was having trouble with one unit getting the hot end temperature to stabilize. I went through the whole dance of auto-tuning and then manually tuning the heater PID, but I just couldn’t get the temperature to stabilize. The reading was jumping up and down in a way that shouldn’t have been possible given the thermal mass of the heater block, so I took the control unit to my electronics bench and hooked up the oscilloscope.

What I saw was a lot of high-frequency noise on the supply voltage to the thermistor. I was able to isolate it to two sources: the USB connection to my laptop and the LCD interface. The interface noise was particularly interesting in that it only showed up once a second when the LCD updated.

So I guess my question is, has anybody else experienced problems with signal noise on these boards? Also, is there a schematic anywhere? I’m thinking I could just add a capacitor in parallel with the thermistor to low-pass filter the signal as a quick fix.

Any thoughts are appreciated.

Don’t you hate it when somebody 3 years ago asked a question on forum, about a problem you have now, and they never got a response? Me too.

Here’s a quick recap :wink: I was having problems with unstable hotend temperature control, mostly at high temperatures (>240C). I traced it down to electrical noise in the 5V power supply on the Creality main board.

TL;DR If you have trouble keeping steady at higher temperatures you might need to repair or replace your main board.

A bit of background: the thermistors used to measure temperature on these units are negative temperature coefficient (NTC). That means that as they get hotter they go down in resistance. They are also non-linear, meaning that the change in resistance is not proportional to the change in temperature. At high temperatures small changes in temperature make much more significant changes in resistance.

The microcontroller on the main board reads temperature as a voltage. This voltage ranges somewhere between 0 and 5V. The thermistor is part of a voltage divider which divides 5V by ratio of two resistors. As it’s resistance gets lower it lowers the voltage read by the microcontoller. Electrical noise was making the 5V supply ripple with a swing of about 0.5V, or 10% of the supply voltage. This was a very fast ripple, so you’d need an oscilloscope to see it.

10% sounds like a lot, right? Well remember how the problem only occurred at higher temperatures? Remember that non-linearity thing I mentioned earlier? That change in voltage had much more effect on the temperature reading at those temperatures.

Okay, cool story, but what was the solution? I noticed one of the capacitors on the board seemed just a little bit domed on top. This was the main supply smoothing capacitor for the 5V supply. It’s a 220uF 6.3V electrolytic. Replacing it with a new one cut the noise about in half and made temperature control stable enough to print again.

Now, some of you electronics geeks in the audience might be saying, “Gee, 1.3V is kind of a small margin for an electrolytic.” Well, you’d be right, and there’s a great post on that subject at Stack Exchange[0]. Anyways, if you’ve read this far then I hoped this was entertaining and that we all learned something.

Happy printing!

[0] Is it a problem to use a capacitor at or near its rated DC voltage? - Electrical Engineering Stack Exchange