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?

Cheers,
Simme

Comments

  • 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?

    Cheers,
    Simeon
  • 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.
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