Temperature dips

Since a few months I have a strange behavior of Extruder temperatures: Every few minutes the temperature reading drops to 0°C.

I first thought that it would b an issue with the PT-100 or the amplifier board I had in my hotend. Now I just found my old thermistor again, connected it to another input than the PT-100 - and it shows the same behavior.
I even tested the thermistor directly plugged into the mainboard, so that no wire breaks or loose connectors could be the reason.
This is with PT-100:

And this is now with my trusty old thermistor:

Any Ideas?



  • The bed is also not looking good. The PT-100 also swings too much even if you ignore the dips. So I'd say anything reading analog values is not as stable as it normally is. So I think the Vref which is used to get voltage to value conversion is somehow not stable. Have you tried with main PU disabled and only usb power if the temps are stable? If so after which action do they start to get the ripples or even drops.
  • Hmm, thanks for the hint, but it seems to be caused by something different.
    In the following graph I heated the extruder, then switched it off and disconnected the main PSU.
    So even when the Arduino (Due) was only powered via USB (from raspberry pi with repetier-Server), the same dips occur.

    Any further ideas? 
    Would it help to attach a capacitor to the connector of the thermistor?

  • I'm quite sure someone had the same problem just do not remember reason/solution. Capacitor would at least stabilize the voltage at the pin so should at least not hurd and may in deed help. There is always some variance which is why we average the values before outputting, but in your case there seem to be several low readings in a row. I still think it is a hardware problem but one affecting all inputs is hard to reason. You could try a different analog input if you have a unused one. Just to see if it makes a difference or also has the same pattern. Bed has at least less drops so it is not identical for all inputs.
  • Have you found a solution gor this. I have the same Problem. Please help me.
  • These can happen with defect sensors or cables. Shorts and disconnects cause temperature swing to the extremes like you see in the image.
  • I changed all Cabels + the sensor and sensorboard.
    Shorts maybe but where?
  • Could that be that the heating bullet make shorts and because of this the resistance isnt static?
  • No that cant be, even if i dont heat i have the dips.
  • Typical defects are that the cable starts to break due to all the movements or the sensor is under stress when you heat and starts to get problems. Both cases are resolved with a new cable + sensor.

    Does this also happen when motors are off and you are not heating? These may cause cross talk changing resulting analog value, also smoothing normally reduces the effect.

    Last thing would be to change the analog input port in case the port has some problems.
  • I changed the port already.
    And i have used shilded cabels + the printer was in a park position
  • I will try it out with just the usb cabel next.
    If that dosent work il try to use a capacitor
  • It is not the usb cable! I'm talking about the cable to the thermistor. I do not think it is a communication problem.
    BTW: What temperatures does it drop to? I mean if just one letter is missing it could be communication, but if it always drops to 0 or whatever minimum is for the thermistor table it is the analog input getting the bad voltage.
  • Like from 21° ich to 5° then bach to 21°.
    Ore from 200° to 5° and then back.
    So should i just try a Capacitor parralle to the Voltage input of the Pt100 sensorboard?
  • Here is a Picture of my Temp. 
    At first it was in the Park position with Motors turned off including with the capacitor.
    After that i heated up and after cooling down i comlietlie removed the Thermistor.
    Then i just powerd the board via USB.
    And now its clearl than not my Hardware has a Problem. Its the Firmware.
    But what can i do?

  • The sensor works like voltage divider. Without connection we get 5V and high temperature. With increasing temperature or short resistance drops towards 0V. So what is clear is that the drops are because the chip measured 0V from time to time. When this happens without sensor as well it is in deed not the sensor. 

    Did it work before or is this since the beginning? If it is only since some time my guess would be a defect on the chip, otherwise you would need to check if you do not use the same pin e.g. as output somewhere to be sure.
  • I have a solution, you need to turn your baudrate Up. To like 250000.
    It works in my Case, Iam so Happy :).
    Thanks for your help
  • So serial communication interferes with measured voltage. Ok and higher baud rate reduces time so it not dipping. 
    Glad it is now working.
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