Pronterface showing temperature spikes

Hi everyone,

I am one of the co-creators of the Duct Tape RepStrap. We are using a J-head hotend that we bought off the internet with a homemade extruder, running on Marlin firmware and Pronterface.

We have completed all of the hardware, but are having trouble getting the hotend working. The thermistor does not register any temperature, even though the hotend heats up, and we get a MINTEMP error. We measured the resistance across the thermistor and it was around 100k, so it's not broken or shorted out. We tried with a different thermistor (EPCOS 100k) and we faced the same problem, so I'm pretty sure it's a software issue. Also bear in mind that we're new at this, and neither of us has ever built a 3D printer before, so it's absolutely possible that we made a stupid mistake. Attached are an image of the Pronterface temperature graph and the settings that we use for Marlin.

Thanks!

Carl

The two obvious possabilities are

1) The thermistor is plugged into the wrong place on the Ramps
2) The Ramps (or Mega) is broken

First check would be with a DVM. One thermistor lead should be at ground. The other one should be at about 0.25 below the 5V supply (not at 5V).

Next check (since you already have mintemp). Short the thermistor leads together. That should trigger maxtemp. If it does not, you are on the wrong input.

Likely a faster check - plug the therm into the other thermistor inputs untill you see a room temp reading ....

Thanks for the reply to our topic Uncle_Bob. I am the other co-creator of The Duct Tape RepStrap. We have made sure of a few things:
1) The thermistor is indeed plugged into the right place.
2) We can't get the thermistor to short and show MAXTEMP
3) Using other firmwares has yielded a similar, if not exactly the same, error.
4) Plugging the thermistor into other thermistor ports on RAMPS has not shown room temp.
5) Testing the current across the thermistor ports shows there is no current through them.
6) We tested the resistance between the pin that connects RAMPS to our Arduino and the non-grounded thermistor pin and got little to no resistance (showing that there is no fault in the RAMPS board)
After 5) we decided we had a deeper understanding of the problem, however we still can't figure it out.
We can't find a problem in the way we've configured Marlin. We think it may be a problem with how the pins are allocated (is that even possible?)...

Any more help would be greatly appreciated!

PLEASE NOTE!
Since our last post we have also switched out the Arduino Mega just to make sure it wasn't an Arduino problem. Still getting MINTEMP...

The circuit is pretty simple:

1) You need +5 on the Ramps board to enable the bridge
2) You need the 4.7K resistor on the Ramps to make up half of the bridge
3) You need the thermistor to make up the other half of the bridge
4) You need the ADC in the Mega connected to the center of the bridge (thermistor / 4.7K junction) to measure the bridge voltage

With no therm, there should be 5V on the the thermistor pins (+5 through the 4.7K to one pin, ground to the other).
With no therm, the ADC will read +5V and it will call it mintemp
With a shorted input, the ADC will read 0V and it will call it maxtemp

The firmware decides what the voltages mean based on the pointers in configuration.h. They are commonly set to 1 for an Epcos for the thermistors you are using. Those pointers reflect to thermistortables.h where the actual data is soared.

You can check the bridge with a DVM and watch the voltage change as the temperature on the thermistor is varied. If there is no voltage then you either:

1) Don't have +5 to the Ramps from the Mega
2) Don't have a 4.7K resistor
3) Have a short across the thermistor