Thermistor questions

After a year ive finally fitted my mendel-parts hotend onto my prusa , hooked it all up to the gen6 board and the thermistor is reading between 790-840 ?

1. whats up with that ?
2. most hot ends ive seen come with some kind of ptfe tubing to cover the wires of the thermistor , i didnt get any , was i supposed to or is there some kind of coating on them to stop from shorting ?
3. does it matter which wire from the thermistor is connected to which pin (3 or 4) in the heater molex connector ?

Edited 1 time(s). Last edit at 01/19/2012 11:27PM by Rooski.

1. It sounds like it is shorting. It can depend on the exact nozzle you have, and what type of thermistor goes with it.

I have the mendel-parts v6, which comes with a bead thermistor (forget which one, and doesn't matter). I had too much trouble with it being so small - and plus for some reason i thought Arctic Silver was a good idea to fill in any gaps with and help contact (it's not).

Also, is this temp shown as soon as you connect or after you turn the heater on? if its just when you turn it on, then it is a short, if its when its hot, someone may have snuck in while you weren't looking and put Arctic Silver in there

Looking at http://reprap.org/wiki/Thermistor#RepRap_Thermistors, your firmware is probably set to the EPCOS 100K Thermistor table (or similar). The first few lines are :
short temptable[NUMTEMPS][2] = {
{1, 841},
{54, 255},
{107, 209},
... etc

which if i remember correctly (been a while since i changed mine - so maybe i have this backwards) a temperature of 841 is a resistence of 0.1Ohm. Therefore, if you are seeing 790 - 840, then you probably seeing very little resistance across your thermistor (and cables).

You can confirm the resistance if you take the 4 pin molex plug out (while it's cold) and use a multimeter across the connector pin things. You should also see it is very small, and NOT around 100K.

2. I understand, to insulate a bead thermistor, you can use kapton tape or stuff like that, or if you are good, you could probably just place it well with some thermal cement or gasket goo or something. I didn't have the patience though.

I ended up redrilling the block by hand and putting a Honeywell 100K Thermistor (135-104LAF-J01) in instead. After some problems, I moved the hole closer to the middle of the block and its ok. I use really small pfte tubing, which "just" fits over normal resistor leads, for the leads that are inside of the block.

3. The direction of the thermistor (and which pins) does not matter - like any resistor.

thanks for the quick reply

i also have the v6 hotend pack from mendel-parts , and if i measure the resistance of the thermistor when cold it does not show the resistance , it stays at 1 , just as if i held the 2 barbs of my meter apart. so i think you got it backwards, or something is wrong with my thermistor.

Edited 1 time(s). Last edit at 01/20/2012 01:37AM by Rooski.

Yeah, that might be right. I'm drawing a blank on it now. Someone with more recent experience will be to pick it straight away, I'm sure.

Having said that, if you have an open circuit, and you definately haven't heated anything up (i.e. no chance of being cooked), then you can track it back, it might just need a resolder of the leads on the thermistor or reseating of the wires to the molex connector.

Also, making no assumptions, do you have the multimeter set to the right range for the thermistor? ie something around 100K (2K - 200K) or something like that (i forget the lingo).

btw. thinking about it, I think saying 0.1Ohm a few posts back is probably 100Ohm so maybe forget that whole paragraph.

Yeah , I removed the thermistor an scraped all the solder off the 2 wires , then measured resistance , I tried the whole range on my meter from 200k to 1 , it does not measure the resistance , stays at the default 1.

An odd Ohm meter. All the ones I have used display 0L when not connected to anything.

---

[www.hydraraptor.blogspot.com]

So if its not measuring anything , does that mean the thermistor is dead?

Edited 1 time(s). Last edit at 01/20/2012 07:14PM by Rooski.

If it is open circuit then it is dead, it should measure about 100K at room temperature. I am not convinced your meter is working though.What does it display when you short the probes together?

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[www.hydraraptor.blogspot.com]

with the meter set at 200k , when i short the 2 probes it reads 00.0 , and to show that the resistance is too high it has a 1 but it is in the far left , away from the decimal. also my meter goes from 200 to 2M , dont know if that is an issue ?

EDIT-------------------------
ok , seems the thermistor wires are coated in something , scraped all the way to the copper and i am getting a reading of about 118 with meter set at 200k at room temp , would i have to change the temperature tables in firmware?

Edited 3 time(s). Last edit at 01/21/2012 02:36AM by Rooski.

118K is probably correct at room temp. The 100K value is for 25C and most rooms are cooler than that. Whether the table is correct or not also depends on the beta value and I have no idea what sort of thermistor you have because they don't normally have insulated wires.

---

[www.hydraraptor.blogspot.com]

After finally getting the thermistor to work and show the temp it stopped all of a sudden , so i pulled it out and the bead was cracked in half .
So i need a new one and was wondering which one i should get that will work with the gen6 electronics/firmware ? also when i start repsnapper the 7ohm resister starts heating up the block without me pressing the nozzle heat toggle button, is it supposed to do that ?

Edited 2 time(s). Last edit at 01/30/2012 01:39AM by Rooski.