Thanks for your help!
Just to updated, we haven't done the cooling portion yet but we wanted to let you know about the DC regulator you recommended.
So when we heat up to say 40degC, the peltier only receives anywhere from 7V to 10V, so it's not like it is receiving the full 24V. Because of this we haven't used the peltier yet. Just to let you know
So for a test, we set our temp for 35degC and the peltier will heat up and maintain that temp assuming we cool the other side enough.
So heating is good. But would you be able to point us to the function that is controlling the heating process?
I ask this because we would like to modify it to be able to COOL instead of just heat.
For example, if we set the temp to cool to 10degC (with current temp at 25degC), the temperature sensor actually shows that the peltier tries to cool by a degree or half a degree but then it will shoot up rather crazily.
So I am thinking:
if (desiredTemp < currentTemp)
// Send a control voltage to DPDT relay to send current one direction
// Do cooling function (might need to swap some increment/decrements in existing code)
else
// Stop the control voltage to DPDT relay to have current go the other direction
// Regular Heat.cpp functionality
So basically if we want to heat, then the normal heat function occurs. But if we want to cool, then we select our COOl function.
So I guess the question is where do we find the heat function to edit this?
Just an update, you are legitimately and objectively awesome. We got it to work!
Just to try to help people in the future who have this same problem, in your config file, make sure to set the axis mins and max, otherwise it won't be able to move in the direction where the mins and max aren't defined.
We also set our X axis offset at 50 as a test, but the first print sometimes the X axis won't do the offset, but after redoing the same print file, the X axis offset will work so I think it has to do with the same problem. Will report back with an update
To explain our process for a tool change:
We have dual extruders (in our case actually syringes) but both extruders are connected by a rack system that works as a see-saw.
So, when we do a tool change, we need a macro that will (1.) offset distance between the two syringes and (2.) activate the U-axis motor.
The U-axis motor will be connected to the see-saw/rack system that will rotate to lift one syringe and lower the syringe that will be printing.
See here: https://imgur.com/a/ZSWwyqb
The U-axis will rotate one direction to lift one syringe, and then when the other tool change occurs, it needs to rotate the opposite direction to lift the other syringe.
So...this is just to explain our reasoning.
@dc42 We did not home it first. Our printer technically isn't assembled yet so we are trying to do as much as we can before that.
EDIT:
edited out 
Thank you! The regulator will come in tonight so I will update you tomorrow.
I was not aware of this tbh. Thank you for info!
I honestly thought the the regulator would go inbetween the heater outputs and the peltier, like in series between the two.
I want to make sure I understand, so I drew a picture to clarify.
Does the negative output of the regulator go anywhere? (see X)
Does the Vin of the heater output go anywhere? (see X)
Thanks for the info!
I bought this DC/DC regulator and will update you when it gets in
If we measure the resistance of the peltier (let's just say it is 10kOhms), and then we take a 10kOhm resistor and put it in series with the peltier, voltage divider means that the voltage across the peltier should be half (12V).
Is this viable? Assuming we can find a resistor that can handle up to 6A?
Ahhh this sounds like it may be the reason, thank you for pointing this out! We will experiment...
Our Duet Ethernet is hooked up to a 24V 350W power supply, so as I understand it, that means the extruder heaters/motors/bed heater are all running off this 24V, which is why we chose 24V motors and heaters.
But we want to take your suggestion and run the heaters at 12V instead. Is this something we can change internally (in the firmware or config file) or do we have to swap our 24V power supply with a 12V power supply in order to accomplish this?
The closest thing I can find is this:
M307 V12
source: http://reprap.org/wiki/G-code#M307:_Set_or_report_heating_process_parameters
"Vnnn VIN supply voltage at which the A parameter was calibrated (RepRapFirmware 1.20 and later). This allows the PID controller to compensate for changes in supply voltage. A value of zero disables compensation for changes in supply voltage. "
We actually haven't tried connecting directly to our power supply. We have this one here and were worried that the peltier would draw too much current and burn out. We will try when we meet again though!
Hey!
So when we have a tool change, we want to run a macro which is essentially just an offset between the two extruders (in our case syringes).
As part of this macro, we need to move our U axis (already set up/defined and we can move it with normal G1 U50 commands and such).
As a test, we put the following gcode in all the tfree0, tpre0, and tpost0 files:
G90
M83
G1 U50
To clarify, we put the above gcode into tfree0 and tfree1 and then ran a tool change through the gcode console and it didn't activate the macro. We then put it into tpost0 and tpost1 and tried again, with nothing happening. Etc.
To explain further, when we upload and try to print the below gcode print file, the tool change does not run the macro that we defined (in this case it only moves the U axis).
G1 X50
G1 X-50
T0
G1 Y50
G1 Y-50
T1
G1 Z50
G1 Z-50
As I understand from @dc42 's explanation here, when the program sees the T0 or T1 commands, a tool change occurs and should run the gcode in the tpre, tpost, and tfree files, but we are unable to accomplish this.
Any suggestions? Thank you!
Okay we just wanted to make sure the peltier didn't need to be operated in a specific-only way, but if your peltier works with 12V and ground then ours should too.
It is just strange because we can't get the peltier to cool at all, both sides get hot, even with a heatsink and fan, switching polarities doesn't work.
1.) Does this mean that VIN is 24V (we have a 24v PSU) and E0- is 0V?
Just wanting to clarify this point because I believe the peltier needs + and - voltage instead of + and ground/0 voltage, I will double check with the peltier supplier though.
2.) If that is the case, I am assuming the voltage cannot be adjusted to be +12 and -12V because the mosfet pulls the E0- pin to ground and therefore cannot be changed from the original 24V to 0V?
@dc42 We have been working on this, with the same result so far.
Which is why we are looking to see exactly what the pins are/do for the extruder heaters. We want to make sure the + and - pins are compatible with the peltier.
Do you have any additional technical specs of these pins?
We are working on a bio-printer, so we only need to heat up the cell solutions to a maximum of 80deg but we also need to be able to cool it (ideally to about 4deg).
@dc42 Actually, is there a more detailed spec sheet of how the pins on the extruder heaters work?
This is our peltier heater (and technically it should cool): https://tetech.com/product/te-195-1-0-0-8/
1.) When we hook up the peltier directly, we are able to raise the temperature to a set amount (although it does overshoot a bit so is currently unreliable).
2.) No matter the polarity of the peltier, it only gets hot and doesn't get cold.
3.) Also, both sides of the peltier get hot instead of one side getting hot and the other side getting cold.
So...that's why I ask if there is a more detailed spec on the pins of the extruder heaters, we would like to check to make sure the voltage/current/etc being supplied is compatible with the peltier shown in the link above.
Any ideas?
Thank you!
RESOLVED
We were able to resolve this particular issue and want to share what we did.
So we had this following code in our config file:
M584 X0 Y1 Z2 E3:4 U5
But we had it at the end of the file. Once we placed it at the top of the file (literally before anything else) it worked and we were able to move the motor as normal.
For more details, see this link: https://forum.duet3d.com/topic/3604/homing-in-u-axis/6
@dc42 Hope you are doing well
So in our config.g file we added this line to set the E2 drive to become an axis:
M584 X0 Y1 Z2 E3:4 U5
This sets the (formerly) E2 drive on the expansion Duex5 to be the U axis, and this shows up fine in the DWC under "Additional Axes."
However, when we send a gcode command to test the motor (G1 U50, for example), the motor will vibrate but will not spin.
Do you have any suggestions?
Thank you in advance.
EDIT: I won't be around the printer till tonight, but I might have not defined the endstops of the U axis, I know I did all the other stuff like motor current and all that. Will check back.
Just as FYI, this is the peltier we bought and will be using.