Hello all,
I have a Prusa I3 printer with Melzi 2.0 board.
I had some issues with the heater cartridge I took it out for cleaning once and I broke the thermistor or so. Since then I had this DEF error both on HEAT BED and HEAT END .
I now have replaced the thermistor with a 100Kohm one and when measuring at room temp I get about 100Kohm on both thermistors, for bed and HEAT END.
Problem is that I already reset the board a couple of times and no change. The HEAT BED works fine and when I start the printer the temp shows room temp or 0 degrees. But the HEAT END shows 245 degrees even when it’s off.
That’s the problem the HEAT END shows 245 degree’s the thermistor works perfect it is new I triple checked it and gives a good value. I resest the printer several times by pressing the reset button for more then 10 second.
The red LED light after resetting the printer does NOT remains on, so my guess is that it can read the thermistors good ? But why does it says its 245 degrees when is actually 0 or room temperature ?
How can I fix this problem ?
Please advice greets
sveive
2
I had a problem one time when both temperatures show the “DEF” error. Both tremistor was good. So I changed the Melzi board to a new one and it worked agien
Could have damaged the processor on the melzi by shorting voltage into the temperature port. This has happened plenty of times when people try and mess with the extruder not realizing how delicate the thermistor is. Power off and read resistance across the thermistor inputs on the melzi to make sure it reads good all the way there. Which ever one reads 245 is the bad one.
So what do you suggest where do you got this info ?
I have problems only with my HOT end not the heat bed. Heat bed works fine. So I know that my hot end isn’t ok.
If I measure the input pins on the board for my hot end what ratings should i get ?
245 ? ohm ? 245 kohm ? what was that about 245 … ?
I mentioned 245 degrees celsius showing up when it’s actually room temperature.
Strange thing is that when I raise up the hot end temp to say 260 because it’s the maximum the hot end heats up to +200 degree celcius havent measured it yet but it was really hot it carbonized my PLA filament.
I was hoping for a firmware approach on this issue.
Bu please tell me the exact ratings on the hot end input pins on the board. So I can check it.
greets
Thermistors have either a positive or negative temperature coefficient meaning that some increase in resistance at higher temperature (positive temperature coefficient ) and some decrease resistance at higher temps (negative temperature coefficient). Not sure what our printers use but it means that the Melzi is either seeing an open or shorted thermistor. My guess would be an open wire in the harness which would mean a positive temp coefficient. The board sees the open wire as high resistance and thinks the temp is high
I thought you mentioned that you used 100k thermistor. That is what the hotend and hotbed uses so at room temperature you usually read around 98k ohm. I just assumed you knew that since you said you checked it and it read 100k. The 245c is what the board shows after reading the resistance / voltage that the thermistor creates on the hotend circuit. I’m pretty sure the last person that had this problem had it reading around the same temperature and it was a bad board. Somwitg the power off measure across the two pins on the hotend thermistor cable on the cable with it unplugged and on three board with it plugged in. You should also measur from each pin to ground and 12v pins with the power turned off (on resistance) and see if any of the pins is shorted to +12. One of the pins should be tied to ground and will probably read a short to ground. It could be a short in the cable chain or if the wires broke and touched something. If you want to see if the other sensor works you can swap the two connectors. The hotend will then read a temperature on the bed and the other way around.
Any chance the leads from the HBP and hot end got reversed on the Melzi?
if i swap the heat end with heat bed sensor it will remain the same issues since they have same values.
The thermistors are fine it is one port the heat end port that reads wrong or interpret the information wrong at firmware level OR hardware somewhere is broken. I was hoping for a firmware kind of fix.
245 degrees celsius also means about 10kohm in resistance … So I was thinking maybe a resistor got broken first.
I don’t see how a mcu could broke down. there are allot of parts in between. Yes if I change the entire board it will fix the issue but that doesn’t mean it is the mcu. It could also be something more simple to fix like resistor on board or something else. I was curious what that might be. Any of you measured across the input pins on the heat end ? without the thermistor attached to it offcourse . What value does it have … ? I get 172 ohm on the heat end input pins on board with power off
greets
