Techzone thermocouple Help

hi all,
first post here. i have just completed the mechanical build of my techzone LC mendle, and started hooking up my electronics. i have all the boards talking to each other and all the stepper motors moving using repsnapper. i also have the hotend heating on demand through the AD board. however the thermocouple is not reading properly. i am getting either 0 or 2000 degrees in repsnapper. the AD board is working as the heater works so i am thinking it is a problem with the thermo couple. any help with this would be fantastic. its so frustrating to be so close to finishing.

... measure the voltage, then check if you use the right input-pin and configuration for calculating the temperature from the received value ...

---

Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

i dont seem to be getting any voltage drop across the thermocouple. so i assume that the thermocouple is dead. is there any way of confirming this?

... measure the resistance - should be nearly short ... maybe you'll sense a slight difference, when cold or hot ...

---

Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

it is defiantly close to a short. i couldn't get a noticeable difference when hot but i am using a cheap old DMM. i checked all my wiring for continuity and they all seem to be ok.

dose the the thermocouple being a short mean that is defective or is that how it is meant to be?

A thermocouple is just composed of two metals twisted/welded together at one end. They are metals chosen for a specific property, but there is nothing otherwise special about thermocouple wire. If you are testing continuity (short verses open circuit), then the wire should behave similar to ordinary wires joined together at one end.

Thermocouples use the Seebeck affect to detect temperature differences. At least one source describes the principle in terms of generating a current rather than generating a voltage. Try using an amp meter instead of a volt meter? Maybe that will be the key difference in detecting a temperature response. If you consider ohms law (V=IR) and you have no resistance in your circuit (wire offers an insignificant amount of resistance) then it makes sense that you would detect no voltage because you have very close to 0 ohms of impedance. No mater what current your thermocouple might be generating, I*0ohms = 0 volts.

... "0ohms" mean short, but with measuring the termocouple with an Ohm- or Volt-meter you measure against the internal resistance of the meter, what's in the MOhm-range, so there should be a small change in voltage, when changing temperature.

When measuring Ohms, the resistance of the meter is near 0Ohms and the meter measures the voltage-drop along this internal shunt-resistor, what's mostly in the range of some milliVolts with common thermocouples.

So if you can't sense any difference, then either you have the wrong material pair, wrong wiring, or your temperature difference is not high enough ...

---

Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Thermocouples require an amplifier to get the correct reading from the metals, you also need conectors of the same material to read them correctly and the amp must be very close to these connectors, the most popular thermo type is K type with a range of -200C to +1350C.
The voltage produced by these devices are very small and most DVMs would just see a flutter on the millivolt range, if you get zero ohms then all that tells you is the 2 metals are connected.

I'm no expert on the mendel kit yet but I thought we used thermisters which are resitance verying tempreture devices.

Stephen.

Edited 1 time(s). Last edit at 01/19/2011 11:51AM by Qbert.

Qbert Wrote:
-------------------------------------------------------
> I'm no expert on the mendel kit yet but I thought
> we used thermisters which are resitance verying
> tempreture devices.
>
> Stephen.

Most people do use thermistors. The idea behind thermocouples is that they are less fragile and more accurate, as well as requiring less code to read. Thermistors are less expensive and easier to implement though.

No!
The idea behind thermocouples is that it is the right way to do temperature monitoring for the extruder hot end.

Thermistors are bad for this and WILL fail! Because they are not designed for continuous use at high temperatures, approx 150 degrees continuous, which is fine for a heated bed.

It is a fundamental called 'thermal run away'.

I convinced Zach to do this for MakerBot, took a while.

i eventually re flashed the extruder board. this seems to have cleared things up. i dont know why as the TechZone boards are meant to work out of the box and for the most part did. just waiting for some PLA and then it's time to print.

WAAHHOOOO

Yes I see why you would go for thermocouples now, thx for the update.

I was just wondering throu my old projects and I found one that used a pt100 ( to monitor the flow of solder paste), there cheap (around £3) and the temp range is well within the range of melting plastic (-70C to +500C).

Has anyone used this device?

Quote

Thermistors are bad for this and WILL fail! Because they are not designed for continuous use at high temperatures, approx 150 degrees continuous, which is fine for a heated bed.

Simply not true for the ones recommended which are rated for 300C and do not fail in my experience of running them at 250C for thousands of hours. The wires are delicate though.

Thermocouples are more expensive and have stiff wires, so not the best for moving head machines. You also need to insulate them if your heater block is grounded. If it isn't grounded then if your heater shorts you have a problem unless you insulate it.Thermistors are already insulated.

---

[www.hydraraptor.blogspot.com]

nophead Wrote:
-------------------------------------------------------
> Thermistors are bad for this and WILL fail!
> Because they are not designed for continuous use
> at high temperatures, approx 150 degrees
> continuous, which is fine for a heated bed.
>
> Simply not true for the ones recommended which are
> rated for 300C and do not fail in my experience of
> running them at 250C for thousands of hours. The
> wires are delicate though.
>
> Thermocouples are more expensive and have stiff
> wires, so not the best for moving head machines.
> You also need to insulate them if your heater
> block is grounded. If it isn't grounded then if
> your heater shorts you have a problem unless you
> insulate it.Thermistors are already insulated.

I don't deny that many people will have flawless problems with thermistors, however overall they are not really suited for this purpose. You, Adrain and I think even Vik have not had thermistors break down.
Some will think, well i'll just pop down to the electrnics shop and get a thermistor, pop it on and it fails a week after, for example. There have been so many posts all over the place where thermistors have failed, half of which are from MakerBot users.

I can't remember if you have encountered thermal runaway, I have when developing my extruder on my RepStrap, where the temperature goes well over 300 degrees.
As a rule of thumb, I suggest taking what the manufacturer says as max, and use this value as continuous.

Yes, ideally the thermocouple should be bolted on and not taped on with Kapton tape, meaning good grounding is necessary.

not sure if this is the place to ask, but i just wanted to ask a quick question because this has been killing me, and its very close situation as the OP.
im using techzone parts, with the newest 5d firmware, i can connect and move the machine in replicatorg and reprap host as well as read temp, but when i use repsnapper, i can move the 3axis(s) as well as the external stepper extruder driver but cannot get a temp reading at all. im using a 100k thermosistor, reads fine using all other softwares but repsnapper refuses to see it. what am i doing wrong?
thanks

RepSnapper and RepRap host use different temp reading methods. You can't use the official 5D firmware with RepSnapper. You need to use triffid_hunter's firmware or similar, such as 5D on Arduino to be able to use RepSnapper.

@NewPerfection, thank you very much, changed firmware and all works, i'm actually able to use techzone defualt 5d firmware the unit ships with to use repsnapper out of the box.