Odd temperature readings

Hi!

I use marlin on my prusa. You can see the temperature curve by clicking on the picture below.

The temperatures move simultaniously, but you can clearly see that I only heated the extruder. The dip to 0° was when I unplugged the extruder thermistor. But, when I unplug the thermistor of the heated bed, nothing happens. Where do I have to look for the source of the problem?

Looks like you have the bed thermistor pin set to the same pin as the extruder thermistor input.

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[www.hydraraptor.blogspot.com]

I checked that.. They are not the same. Is Marlin able to distinguish between analog and digital pins? Because it just says pins 0 and 1, but there are also digital pins 0 and 1.

Yes the analogue pins start at 0 but actually correspond to digital pins 24 to 31 or similar.

I had a similar problem when I first enabled an SD card on a Sanguinololu because the SDSS pin was defined incorrectly as one of the thermistor inputs. That did make the temp reading track the other one with with an offset.

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[www.hydraraptor.blogspot.com]

Thanks for the idea.. I checked all the pin assignments, nothing is assigned to these pins (I have a Gen7 1.5 (so the 1.4 pin assignments are the ones for me)). I now also double checked the electronic wirings..

Edited 1 time(s). Last edit at 03/03/2015 08:38AM by GoingForGold.

Found it! I defined the 2 unused sensors 1 and 2 with thermistor type 0. I defined them to 5 and now it workes as it should..Perfect, thank you!

Damn it! Suddenly, the problem is back without changing anything...any more ideas?

The analog inputs where the thermistors are connected are subject to noise pickup that can lead to weird readings. The noise can come from running the thermistor wires too close to motor or extruder or bed heater wires, or AC power wires. Try shutting everything else off and pull other connectors off the board so that only the thermistor inputs are connected. Then add other connections back in, one by one and check the behavior of the temperature measurement. The thermistor wires should be twisted all the way from the bed/hot-end to the controller board. Likewise motor wires- each coil's wires should be twisted all the way from the motor to the controller. Likewise heater wires.

Thanks for the tip. I'm now twisting the wires

Some more information: I activated soft PWM in the firmware and since then, the temperature stopped tracking the other one. However, it startet jumping around (about +-10°C). And whenever I plug something in or out, the temperature changes a bit and then continues on that level. Like everything has an impact.

attached you find a new picture from a print. The printbed of course cools down. I replaced the thermistor with a brand new one. All the wires are twisted.

What could be my next step? Thanks!

My best guess is that you have an intermittent connection somewhere on your PCB between the bed thermistor connector and the pin on the Atmel chip. When not connected the pin could float and tend to track the pin next to it.

Edited 1 time(s). Last edit at 03/20/2015 06:31AM by nophead.

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[www.hydraraptor.blogspot.com]

Bad solder connections will do this sort of thing. The ground pins in particular can be a problem because they connect to a ground plane on the PCB and it sucks heat away from the soldering iron/solder, making it difficult to get a good connection. Check the soldering at the of the thermistor input pins on the controller board. You need a very hot soldering iron with a relatively large tip to make sure you can get enough heat into the connection to melt the solder.

Thanks guys, I am definetly going to look into that and resolder all joints. I have kind of an "adaptor" for my atmega 1248P which allows me to take it out. Maybe the problem lies inside this.