Hi,

I'm new to the forum, so apologies if I somehow missed something.

I'm in the process of building my second printer, and I'm almost done. I have however hit a wall regarding my electronics, as my Thermistors are behaving oddly. I'm using a RAMBO v.1.2FP board purchased off eBay running Repetier firmware with two extruders. The board itself works just fine. I can control the motors, switch on and off the extruder heating etc. but the thermistors er not behaving as they should.

When no thermistors are connected, Repetier host shows around 34.5 degrees C rather than showing that nothing is connected. If I short the pins I get 500 deg.C and a message that the thermistor is "broken" and that dry-run will be active until reboot. If I connect my thermister (100k @ 25 Deg.C type B57550G, it has a B25/100 (beta) value of 4092) I get approximately 37 degrees. This value I can change by heating the thermistor using my hands, which to me indicates that it does in fact react to temperature change. I can however guarantee that there isn't 37 degrees where I'm working from. According to a separate thermometer it's about 27 degrees inside.

I have checked the board according to the schematics, and all resistors around the thermistor circuitry have correct values and nothing seems to be missing around there.

As I'm building with two friends of mine I have of course checked three different boards with the same firmware with the same result. Thus I assumed it could be a firmware problem, but I see the exact same result using Marlin firmware as well.

My previous printer used an Azteeg X1 controller board with Repetier firmware controlled using Repetier host as well, where the thermistors showed the correct temperature.

So all in all my question is; have I missed something regarding this RAMBO board? Something I have connected wrong or something else I should have done? All I've done is to upload the firmware to the board, connect one thermistor and connect through Repetier Host.

Thanks in advance - any help is much appreciated.

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Kind regards
Simon

Hi there,
Did you resolve this issue as I am having the same problem.

Any help would be great.

Thanks

I'm not familiar with the repetier firmware, but you most likely can change the thermistor type. If you're lucky your thermistor type is already in the firmware. It will use a different lookup table. Hopefully you can find one that's closer to reality. To make sure it's correct, I'd check with a dvm and a thermo couple to see if the temperature is within the desired range. If not, you could change one of the lookup tables to match your sensor.

You can also try locating a thermistor for which there is a lookup table in the firmware already, that makes it much easier. Buy a couple of spares.

Another way is to download the sensors datasheet (you know the type number so that should be easy), look at the schematics how things is wired up with the thermistor (is there any other resistor, since it indicated a value so there must be a voltage divider somewhere) and look in the firmware how the ADC values are calculated. You can make a table with the values from the datasheet, the voltages that should result into on the ADC pin and what values it results in once digitized. The datasheet should also mention the accuracy of the thermistor. The values in the datasheet are approximations.

Alternatively, you can switch to thermo couples entirely, you have to buy a thermo couple board too, as the signal needs to be converted to a level a microcontroller ADC can properly read it. Thermo couples have the advantage of being much more accurate (within 2 degrees) and more reliable at higher temperatures.