Thermistor outputting 0 ...

nophead Wrote:
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> Yes 988 is about right for 4.55V, but 4.55V is not
> right for 100K.
>
> With 4.7V and a 4K7 pull up, the voltage across
> the thermistor would be 4.7V * R / (R + 4.7K).
> That makes R = 4.55 * 4700 / (4.7 - 4.55) = 143K.
> That would probably be a temperature below 0.

!!!! This made me wonder and it seems my thermistor is outputting WAY higher values!
In the current temperature here in my workshop the resistance is ~140KOhm, which is pretty high for a 100KOhm thermistor

It still seems to work as its supposed to, lowering when it gets hotter ....

Maybe I could just replace the 4.7KOhm resistor with a different one to get proper readings?

Oh wait, that still would not explain why Sprinter is not giving a proper reading for any other resistance I put across it ..

Yeah i just tried setting THERMISTORHEATER to 2 (which stands for 200k thermistor)
But still I get T:0

For those wanting to try if its the firmware, I tarball'd the source and posted it here : [etc.servehttp.com]

Have you tried lower resistances? E.g. something like 10K. You said a short gave you a reading. I wonder at what point it starts.

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[www.hydraraptor.blogspot.com]

I tried a 5K Ohm resistance, that also didn't give any reading ...

(but it did give a reading in my mini firmware)

Edited 1 time(s). Last edit at 02/23/2012 11:58AM by buZztiaan.

I have just loaded your Sprinter code into my Mega, this is the terminal output and M105 command response:

start
SD init fail
ok T:22 B:0

Sprinter is not to blame.

misan Wrote:
> Sprinter is not to blame.

Can you tell me what resistance you are reading on your thermistor in idle?

109.5K
4.68V at the thermistor
4.88V at Vcc (+5v)

Meh, maybe the thermistor is just 'a bit' broken and needs replacement?
Still wouldn't explain why it doesnt give a reading with a random resistor ....

Thermistor may be wrong but this does not explain while almost always you get T:0.

I'd say the ADC pin is wrong, but then why would it work nice when you measure it with your own code?

What does it happen if you connect your thermistor to the heated bed instead?

I tried putting the thermistor on the bed pin , also B:0 ..

Also I just soldered a wire between analog pins 13 and 12, changed TEMP_0_PIN to 12 tried that, but still, T:0 ..

So I guess Kliment is right in saying I should swap my arduino

It might be worth looking for shorts to pins near the thermistor input. E.g. there could be a short to another pin that is not initialised in your test software bit is in Sprinter.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
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> It might be worth looking for shorts to pins near the thermistor input. E.g. there could be a short
> to another pin that is not initialised in your test software bit is in Sprinter.

Hmm yeah, tomorrow I will update my test software to one-by-one enable all the pins and check the reading on analog

I did check with my DMM before, measuring for shorts between analogpin 13 and all the other pins, but found none ..

Oh I just realized, I made that circuit external of the RAMPS board, so I already know the Shield is not the problem .. as the exact same behavior happened on my breadboard ..

Well, swapped my arduino today, problem fanished for the sun.

Finally i'm getting proper reading

Thanks everybody!