RUMBA Thermocouple Upgrade

Hi all,

I'm new to this forum and to 3D printing, so hello everyone! I recently purchased the Prusa i3v kit with RUMBA controller from Makerfarm and have it mostly assembled. I am making a few mods and I have a few questions.

I am changing out the extruder thermistor for a K-type thermocouple (want to print >300C) using Adafruit's AD8495 analog amplifier. The amplifier board has 4 pins: +5V, Vout, and 2 ground pins. I am assuming that the +5V and Vout pins can be connected to the RUMBA in place of the thermistor, with the Vout being routed to the ADC on the AT Mega chip. But I'm not sure how to connect the ground. My knowledge of electronics is just enough to be dangerous...can I connect the ground pins to any ground on the RUMBA board? Has anyone used this board with RUMBA (or RAMPS/RAMBO)?

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EDIT: I did find a schematic of the AD8495 board from Adafruit...not sure why they can't just post an image (only available in Eagle CAD format). I keep looking at the RUMBA schematics for the thermistors and they just don't make sense...I've compared to some other boards and I think there is a connection missing on the diagram (or I don't know all the conventions). I've made changes (light blue boxes) showing more explicitly how I think it works...can anyone tell me if this is correct? If so, I'm not sure how to wire the the amplifier board to RUMBA as there is no +5V on one side and a return on the other. I'm thinking pins A9 and A10 might be the way to go, available on the EXP3 header. But I was really hoping to just use the thermistor connections. Could I attach the Vout and grounds from the amplifier to the thermistor connectors and power the board from somewhere else?
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EDIT 2: I got some circuit simulation software and with the way the electronics are set up on the thermistor connectors, I don't think it is possible to use those connectors for a signal from the thermocouple amplifier board. I am going to use A9 and A10 for the first and second thermocouples (adding a second extruder) on EXP3, which will also provide power and ground. The next revision of RUMBA needs to have a jumper to select whether to use the RC filter (for a thermistor) or have a direct line to the ADC (for a thermocouple), maybe with a third pin for 5V power.
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I'm also looking for a high-temp thermal compound to use with the thermocouple and cartridge heater. I've found Timtronics 611HTC which is rated up to 360C but runs $30 for 3 cc's. Anyone know of any alternatives? Heat sink compound won't work, isn't rated high enough.

That's all for now, thanks in advance for your help!

Edited 2 time(s). Last edit at 02/24/2015 11:03AM by jwesolow.