Problem configuring my hotend thermistor

hi people, I'm new here and I hope you can help me with this issue.
I bought the MK8 complete assembled extruder, RAMPS 1.4, & Arduino Mega 2560 and got started building my own 3D printer and here comes what I faced:
I successfully uploaded the "Sprinter" firmware by kliment from GitHub and already have ran all of my stepper motors (X-axis, Y-axis, 2 Z-axes, & the extruder motor). Now I don't get my hot end heater working !! and I receive very high readings from the thermistor which I guess it is 100 Ohms one (doesn't exist in the thermistortable header file in the sprinter firmware) ...
for example:
the configuration header file gives these options:

//// Thermistor settings:
// 1 is 100k thermistor
// 2 is 200k thermistor
// 3 is mendel-parts thermistor
// 4 is 10k thermistor
// 5 is ParCan supplied 104GT-2 100K
// 6 is EPCOS 100k
// 7 is 100k Honeywell thermistor 135-104LAG-J01
#define THERMISTORHEATER 2
#define THERMISTORBED 1

for the 10k thermistor (choice number 4) .. the pronterface or reptier reads always 435 Celsius which is higher than the highest value in that thermistor table !!

#if (THERMISTORHEATER == 4) || (THERMISTORBED == 4) //10k thermistor

#define NUMTEMPS_4 20
const short temptable_4[NUMTEMPS_4][2] = {
{1, 430},
{54, 137},
{107, 107},
{160, 91},
{213, 80},
{266, 71},
{319, 64},
{372, 57},
{425, 51},
{478, 46},
{531, 41},
{584, 35},
{637, 30},
{690, 25},
{743, 20},
{796, 14},
{849, 7},
{902, 0},
{955, -11},
{1008, -35}
};
#endif

also when choosing choice number 2 the 200k thermistor, reptier reads 395

what could be the problem ??

I think if I can find a thermistor table for the 100 Ohms thermistor already attached with your extruder, I can add it to the header files of the sprinter firmware and use easily !!

thanks for reading my very long boring message but I deadly need you to reply as soon as possible.
yours,
Osama Kamal
Electronics & Communication engineer
Cairo, Egypt

I don't know what a Mk8 is?

Anyway, you have not got a 100 ohm thermistor. It is more likely a 100k ohm thermistor - so set it to choice number 1 in config.h. If you disconnect the thermistor and check the resistance, you will know for sure. ( if it did read only 100 ohms, then you either have a duff thermistor or your meter is useless.

---

_______________________________________
Waitaki 3D Printer

The thermistors used are rated both for a resistance at room temperature *and* for a specific curve of resistance versus temperature. In order to work with the tables in Marlin, you need the *exact* temperature curve in the table. There are *lots* of different curves for thermistors. You *could* write your own thermistor table from a full data sheet on your thermistor. It's easier to spend the $3 to get one of the right ones.

in my room temperature which may be 22 celsius, my Ohmeter reads 13.5 ohms !!
MK8 is Makerbot8 extruder:
[fficial&tbm=isch&tbo=u&source=univ&sa=X&ei=-p9vUvG6FsOR0AXWkoHwCA&ved=0CCkQsAQ&biw=1525&bih=641&dpr=0.9" target="_blank" rel="nofollow">www.google.com.eg]
anyway, when I choose any configuration from the 7, it reads a value higher than the highest value in any thermistor table !! and the heater doesn't then work.
thanks for your fast replies

If your thermistor reads 13.5 ohms at room temperature, either it's very much the wrong part or your wires are shorted. Try:

1) Measure a known good resistor with your ohm meter to check it. Best to try something in the 50K to 200K range.
2) If your wires are cable tied down, undo the cable ties. Remeasure the thermistor.
3) If it still reads low, pull it out of the hot end and check it. It may be sorted to the hot end it's self. If removal is impossible, check the resistance from each of the thermistor leads to the hot end tip. It should read open circuit.

Only when you have a thermistor that reads 100K at room should you proceed with anything else. You will damage things if you try to run with a bad / shorted thermistor.

(you are unplugging the therm from the ramps board when you measure it -right?)

Edited 1 time(s). Last edit at 10/29/2013 12:30PM by uncle_bob.

um sorry annoying you again, but here what I ended for:
I unplugged the thermistor from the hotend and ramps and tested it ... the ohm value changes when it is heated manually ... but still in the ranges from 13 to 8 celsius when heated.
the crazy thing is that even when the thermistor is unplugged from the ramps reptier and pronterface still reads value higher than the highest value in the thermistor table !! although I'm heating the hotend manually by external voltage !!!!!!!

I guess there's something wrong with the ramps or the arduino, but this will definitly cost me much
is there any method to test my pins on ramps and arduino ?? (Note: um a new arduino user)
thanks in advance

I made a test for the analog input pin that takes readings from the thermistor A13 and controlled the output for the heater on pin 10 by this code
[arduino.cc]
the corresponding LED on the ramps is lighten with constant intensity and doesn't vary in its illumination ... I'm really confused now !!

The thermistor is a temperature to resistance device. The A/D converter on the Mega board is a voltage to digits device. The ramps board converts the resistance of the thermistor to a voltage by putting it in a simple bridge circuit.

With no thermistor attached, the bridge will apply maximum voltage to the A/D converter. That would tell the Mega that the thermistor is very cold. A very cold thermistor would mean that a lot of heat is required. There is an "open thermistor" setting that may or may not be enabled in your firmware. If it is, the Marlin firmware will shut things down when you unplug the thermistor.

-----

If you went through all the steps I outlined, you should have determined if your thermistor is broken or not. If it's broken, get a new one.

I figured out something new !!
Um sorry for my dumbness but .. I realized that the temperature sensor in my hot end is not a thermistor !! it is a k type thermo couple !!!!
of course you know the difference between a thermistor and a thermocouple .... is there a solution for such a problem ?? for the RAMPS 1.4 or the Arduino Mega 2560 to read from a thermocouple ???
thanks in advance

There are custom add on modules for thermistors. You can hand modify the Marlin code to work with them.

The next problem you will have is that K thermocouples are supposed to have a very specific characteristic. I'd bet your hot end is from China. None of the low cost thermocouples I have seen from China has the "right" voltage vs temperature characteristics. You would need to hand calibrate the thermocouple and put that table into Marlin with all of the ones I have seen.

I'd spend the $50 and get a real hot end ....

Quote
uncle_bob
I'd spend the $50 and get a real hot end ....

the best advice from a wise man
thanks uncle bob

...... advice from somebody who seems to have a *lot* of hot ends lying around ...

I've got the AD595 IC and built the PCB following the instructions of this:
[reprap.org]

but I still have very bad readings of temperature !!
it always reads from 450 to 499 celsius !!!!

I modified the sprinter firmware to cancel the analog pin that reads a thermistor and let A3 pin to read a thermocouple.
also I modified the repetier firmware to work with AD595 but it gives the same results !!!

are there any considerations on connections ?? any components needs to be removed or jumpers to put ??

You now need to construct a table in the firmware to match the output of the AD595. That's not your real problem though. Use a DVM to check the AD595 connections. Either the thermocouple isn't connected or there's something else wrong with the wiring (like pins 8 and 9 aren't connected together).

A temperature reading in the 400's indicates that either you have a thermocouple table in there and the input is at the +5 rail, or that you have a thermistor table in there and the input is at ground.

well I got it working correctly thanks god ... the problem was bad connections in my circuit ... I did it with an already drilled PCB and wires soldered to it.
but now I face another strange problem !!
I think the reading of temperature has caught NOISE !!!
if the reading is stable at 160 degree with no extrusion. then when I try to extrude some plastic then the readings fluctuate from 130 to 450 !!!
attached is a picture of what happens on repetier


Edited 1 time(s). Last edit at 12/10/2013 07:18PM by Osama.

I'd say that looks like a loose connection. It may be at the thermocouple junction inside the hot end.

I hate to necro such an old thread, but I had to give you props uncle_bob!

Quote
uncle_bob
If your thermistor reads 13.5 ohms at room temperature, either it's very much the wrong part or your wires are shorted. Try:

1) Measure a known good resistor with your ohm meter to check it. Best to try something in the 50K to 200K range.
2) If your wires are cable tied down, undo the cable ties. Remeasure the thermistor.
3) If it still reads low, pull it out of the hot end and check it. It may be shorted to the hot end itself. If removal is impossible, check the resistance from each of the thermistor leads to the hot end tip. It should read open circuit.

Only when you have a thermistor that reads 100K at room should you proceed with anything else. You will damage things if you try to run with a bad / shorted thermistor.

This was exactly the problem I was having, shorted against the block!

Thanks,
Ian