Convert to type K thermocouple  

Re: Convert to type K thermocouple

Bob

There is very little on this, other than what has already been written in these forums (a search should help). There is one change you will need to make in the firmware (the thermistor type setting in Configuration.prusa.h), then a compile and upload. There are a few places you can get a +5V/0V supply from the RAMBo, including on the P3 connector.

The existing firmware will allow up to 300 degrees with the thermistor.

Peter

Re: Convert to type K thermocouple

Thanks Peter,

I think I have it mostly figured out. The two things I'm not sure about are 1) which pin to hook the analog input to, and 2) how to specify that in the firmware.

Any thoughts?

Bob

Re: Convert to type K thermocouple

Bob

Use the same pin as for the current thermistor.

Amend the line:

#define TEMP_SENSOR_0 5

to 20.

Peter

Re: Convert to type K thermocouple

Also, I found this old thread on the subject.

Re: Convert to type K thermocouple

Peter,

Further to the discussion of where to connect the thermocouple amp; due to the lack of any spare analog inputs on mini-RAMBo, it does need to use the same pin as the thermocouple (or another unused thermistor input).

However, I have also been advised that the 4k7 pull-up resistor must be removed. This seems like a pretty important issue to get the thermocouple working correctly.

I'm not too happy about this since it is a non-reversible modification, but I don't see an alternative.

Bob

Re: Convert to type K thermocouple

Bob

// 147 is Pt100 with 4k7 pullup


I don't think it does ... It is already built into Marlin. Type 20 is without the pullup, 147 is with.

Peter

Re: Convert to type K thermocouple

Great news about the software selectable pull-up! I'll give it a try.

Bob

Re: Convert to type K thermocouple

Bob

Just checked and it appears to be present in the correct area of firmware:

#if (THERMISTORHEATER_0 == 147) || (THERMISTORHEATER_1 == 147) || (THERMISTORHEATER_2 == 147) || (THERMISTORBED == 147) // Pt100 with 4k7 pullup
const short temptable_147[][2] PROGMEM = {
// only few values are needed as the curve is very flat
PtLine(0,100,4700)
PtLine(50,100,4700)
PtLine(100,100,4700)
PtLine(150,100,4700)
PtLine(200,100,4700)
PtLine(250,100,4700)
PtLine(300,100,4700)
};
#endif


So it looks like it should work OK.

Peter

Re: Convert to type K thermocouple

Peter,

Further research shows that the pull-up resistor on the thermistor analog pin is a physical one (not selectable in software).

This means that if the pull-up remains in place, there will be the 5v bias as well as the 0-5v signal from the AD595 on this pin. Will this work? I'm thinking not, but it's not my area.

I'm reluctant to remove the pull-up as it is SMD, tiny and hard to reach. It would surely be a non-reversible operation and looks like I could damage the board in the process.

Anybody with experience with this?

Bob

Re: Convert to type K thermocouple

Bob

In all honesty I don't know. I think you may want to contact E3D and ask them about interfacing to a RAMBo mini running Marlin. Or you could also contact Ultimachine.

Sorry, I can't assist further.

Peter

Re: Convert to type K thermocouple

Thought I would post this here vs. starting a new thread. If that's a mistake please let me know...

I decided in the end to use a PT100 instead of the Type K Thermocouple.

I wired the output from the amplifier into the same pins where the thermistor was connected, and adjusted the firmware to

#define TEMP_SENSOR_0 147

based on this comment:

// 147 is Pt100 with 4k7 pullup

When I start up the printer, the LED display shows 300/0 for the hotend temperature. The bed is 24/0.

Obviously something is not right, but I don't know what. Any suggestions will be appreciated!