Using thermocouples instead of thermistors

As part of getting the Generation 3 electronics working with RepRap Version II Mendel, I created a simple circuit that allows a K-type thermocouple to be used to measure extruder temperature as opposed to the standard thermistor.

The circuit is described here: [objects.reprap.org]

Just wondering how this compares to the MakerBot thermocouple kit, in terms what needs to me modded on the V2.2 of the extruder board?

Thanks

Advantages and disadvantages:

The Makerbot thermocouple board produces a voltage (it's just an amplifier and faked-up cold junction) that you can feed directly into A3 (where the thermistor feeds in) and convert using the Arduino's A-to-D facility. But the chip it uses can be hard to get hold of.

My new board uses a much more widespread chip, and is smaller. But it takes up three pins on the microcontroller as opposed to just one.

I do like thermocouples. Instead of a welded thermocouple, I just jam the K and T wires between the nozzle and barrel; this gives great temperature control with no PID loop, and is fairly failsafe - if the thermocouple starts coming loose, I get spurious temperature readings which trigger a heater shutdown.

I was under the impression that the Thermocouple Sensor v1.0 had to be connected to an other IO ant that we needed to modify the program in the Extruder Controller v2.2.


What hardware are you using wade ? How is you thermocouple connected ?

I'm using the Thermocouple Board 1.0; it just hooks into an analog input on my Arduino. Works great. I'm testing out a new heater on a pinchwheel extrder with the 5d firmware on a Sanguino now; temperature control is the only thing working so far. I should have the rest up and running shortly.

The software for the thermocouple is much simpler; most fIrmwares have it already, you just have to change one of the defines.

Wade

I've got the Thermocouple kit 1.0, which will get attached to the V2.2 extruder board, I have yet to work out exactly what needs to be modified on that board.
I've got that mod to do plus being able to support a DC motor instead of a stepper (default code).

I looked at the code a day or so ago and the function to "getTemperature" was blank, so I dunno what code to put in there for the thermocouple kit and/or Adrian's revised thermocouple adapter circuit

Here's the relevant code for thermocouple temperature getting:

int extruder::getTemperature()
{
return ( 5.0 * sampleTemperature() * 100.0) / 1024.0;
}

In the 5D code, if you comment out the thermistor line:

#define USE_THERMISTOR

in the ThermistorTable.h file, it'll default to the thermocouple code. No calibration requried; just hook your output from the TC board to the temperature pin defined in pins.h. This part of the code I have tested and it seems to work fine; I can't quite figure out the E code section yet though.

You might have to get a more recent version of the code from subversion; I don't think this stuff is released yet.

Wade

Edited 1 time(s). Last edit at 10/06/2009 02:48AM by Wade.

Are you using the thermistor input pin on the Arduino board ?
Were do you define witch analogue input the thermocouple is connected to ?

Luc

I am in the process of putting together a Mendel and I prefer to use a thermocouple for the sensor so ...

I checked the Extruder controller 2.2 and there are two extra analogue pins A6 and A7 brought out to 3 pin headers with 5v and ground (next to the trimpot).

I tweaked the configuration.h to use pin 6 instead of pin 3 and wired in the thermocouple board 1.0 and it works fine.

Two caveats:

- The pinouts are not the same (TC 1.0 has <+5v-Signal-Ground> but the extruder controller has <+5v-ground-signal> so I needed to swap wires in the connectors)

- The silkscreen on the controller is reversed (+5v is at the A6/7 legend, ground is centre and signal is at the opposite end)

No need for any board mods this way.