MakerGear Forum

Howdy!

I'm getting a MINTEMP. Trying to troubleshoot this.

I've measured the voltage of each thermistor terminal (T0, T1, T2, and T3) at 5V. Resistance is 40kohms.

Measures the same at the hotends for both extruder and bed.

40kohms seems too low. Is this correct? If so, what needs to be replaced?

Thanks!

ds7202 wrote:Resistance is 40kohms.

Is that with the thermistor unplugged from the RAMBo? If it's still plugged in, you're also measuring the resistance of the RAMBo circuitry. Turn the power off before unplugging things, though; it's a good habit to get into.

The bare thermistor should be around 100 kΩ at more-or-less room temperature, decreasing as the thermistor warms up. You can change the resistance with the heat of your hand, just to be sure you're looking at the right thing.

If both thermistors measure around 40 kΩ when they're unplugged, then something peculiar is going on...

ednisley wrote:

ds7202 wrote:Resistance is 40kohms.

Is that with the thermistor unplugged from the RAMBo?

I'm measuring the resistance and voltage across the terminal pins at the board. I'm also measuring the resistance and voltage across the connector pins on the hot ends.

And yes, it's fundamental not to measure resistance on an active circuit. I get the 40kohm reading with a completely disconnected board.

I am a bit confused about the word "thermistor," though. I've never heard that word before searching on Google, and my reading led me to believe that the word describes the four "T#" terminals on the RAMBo board. Am I wrong?

Thanks for your response!

Hi,

if I am reading this right, your trying to read the resistance of the board. I'm not sure that is useful. if you look at the schematic, you have a 4.7k resistor, and then a diode to ground and to +5v, so polarity one way your likely to get a different reading then the other with your ohm meter.

what we were expecting is to get the resistance measurement of the thermistor, that is what gets plugged into the board and senses the temperature at the hotend. its resistance will change with temperature and is how the board can sense the temperature.

so, if you measure the resistance of the thermistor, while it is not plugged into the board, with everything being at room temperature, what resistance do you read? if its not measuring the resistance it should at room temperature, then you have a problem with the thermistor, or the wires leading up to it.

I think that looing at the thermistor would be the first place to look before trying to troubleshoot the board.

ds7202 wrote:a bit confused about the word "thermistor,"

Wikipedia, for all its faults, is a good starting point for tech info:
https://en.wikipedia.org/wiki/Thermistor

The thermistors used in 3D printers (nowadays, anyhow) have a room-temperature resistance around 100 kΩ and a negative temperature coefficient: the resistance decreases as the temperature increases.

The two-wire cable from the thermistor to the RAMBo can develop two faults:

• A short circuit when the wires touch
• An open circuit when one wire breaks

A short circuit looks like an exceedingly high temperature, because the RAMBo sees a low resistance and turns the heater off while waiting for the temperature to come down.

An open circuit looks like a very low temperature, because the RAMBo sees a high resistance and turns the heater off because the firmware knows this is a common failure.

Both failures tend to be intermittent, but once you know what to expect, they're easy to diagnose and treat...

Note: a thermocouple is an entirely different component that performs the same task, but requires entirely different circuitry at the microcontroller end of the cable. Thermocouples are much better suited for high-temperature sensing, but we're wedged into using thermistors because they're cheap and easy to interface.
https://en.wikipedia.org/wiki/Thermocouple

innkeeper wrote: what we were expecting is to get the resistance measurement of the thermistor, that is what gets plugged into the board and senses the temperature at the hotend. its resistance will change with temperature and is how the board can sense the temperature.

Thank you! I was indeed measuring the wrong component.

ednisley wrote: Wikipedia, for all its faults, is a good starting point for tech info:
https://en.wikipedia.org/wiki/Thermistor

and thank you for the info. I did not know initially what a thermistor was. Once reading that Wikipedia article, I understood where I should actually measure.

The problem was indeed a faulty thermistor. I replaced it, and the printer is now good as new. Thank you very much!