Heated Bed thermistor not working for the Heated Bed?

My heated bed's thermistor won't read any temperature.

I've crossed the pins for T1 on the board, the temperature spikes up to 350 just fine so the board is not broken.

I measured resistance of the wire that goes to the heatbed's thermistor, got a result of 95 kohms. So I doubt the thermistor is busted...

Everything is connected, no wire is broken.

But the temp just shows up as 0 degrees in Repetier.

What could be the problem then? Any gotchas I should know about? this is pretty much the last problem I need to resolve before I can start printing reliably I think.

 

Have you tried swapping thermistors to isolate the problem?

 

I only have one thermistor now for the heated bed since the hotend is using a thermocouple.

But I have the thermistor from my E3D which I'm going to use as a replacement for the current heated bed and see what happens.

In the long run I'll probably just get another thermocouple for the heated bed.

 

I meant like plug in the one for the extruder into the bed heater spot and visa versa.

 

The extruder needs 3 pins instead of the two that come in T1, so i can't swap that way.

 

No that's incorrect, they both only need 2 pins. Now if they used a 3 pin connector, that's a different story, but it's definitely possible to swap them.

 

I don't know much about how the electronics work, but the diagram I saw for the AD595 needed pins for 5V, Ground, and the Signal.

 

Ok well thermistors and thermocouples work very differently so you're correct on that not working. I should have remembered you swapped to thermocouples.

How a thermistor works is it's a special kind of ceramic/polymer where the resistance changes depending on the ambient temperature. A 100k thermistor is around 100kohm of resistance at 25C or ambient temp. As you approach 0C the resistance starts to go to infinite resistance, or an open circuit, hence why you read 0C. As it reaches it's max temperature, or a~300C the resistance becomes very non-linear and acts as almost a closed circuit, hence why shorting works.


For the thermocouple it works by having dissimilar metals that when run connected in the correct order will generate a small voltage which is then compared to a reference temperature (probably a thermistor) on the board.

you should probably just contact team robo using the support option and have them send you another thermistor.

For now can you remind me where you're measuring the thermistor resistance?

 

I measure from the end that connects to T1.

I actually have a couple different thermistors here, one from team Robo which was meant to replace the extruder thermistor. I'll be trying the thermistor I got with my E3D as the Heated Bed thermistor, though I think I'll have to change something in the firmware from what I remember so it knows what kind of thermistor is being used.

 

the e3d and robo both use the semitec so you'll be fine with any of them.

 

Well i replaced my heated bed's thermistor with a new one and no temperatures get read...

Don't know what to make of this. Is there something in the firmware I installed perhaps that might be wrong?