Increasing current ranges to detect mA

[Will Dickson]

@Will-Dickson

I swapped out R7 with a 15 Ohm resistor to create a +/-40 mA range. This appears to work well between about -20mA and 20mA. However, below or above that is starts to fail.

Figure showing the -20 mA to 20 mA range where things look OK.

Figure showing the failure below -20mA and above 20mA.

[Will Dickson]

@Will-Dickson

I've swapped out the 15 Ohm resistor in R7 with two of the 50 Ohms in parallel ( 25 Ohms) to create a +/- 24 mA range. This allows us make use of almost the full range of the ADC. As before it looks good over +/-20 mA range.

It almost works over the full +/-24 mA range. It fails at the negative end right around -22mA.

[williamzerx]

@Will-Dickson Made the resistor change on R8 to 51-Ohm (50Ohm +/- 5%) and then went to the ps_constants.h firmware file and recompiled and uploaded with TEENSY to the Rodeostat. Then went in to the CV.PY sample file and changed the current limit to (12mA) and received a KeyError : u'milliAmpV0.1' ... made a change back to (12000uA) and received the same error. Need to find where the changes need to be made for the mA usage. Any thoughts?

[Will Dickson]

@williamzerx

You need to download the latest version of the Python potentiostat library from the repository here https://bitbucket.org/iorodeo/potentiostat The changes required to the Python library aren't yet in the version on PyPI (installed via pip). You will need to install from source from the repository. The python library is in the software/python/potentiostat sub-directory. You can install from source using "python setup.py install".

[tellis]

@Will-Dickson Hi, trying to do the same thing. Replaced the R8 with a 22 Ohm resistor. Having difficulty finding the changes necessary in the firmware files though. Recon you point me to lines which need changing?

[Will Dickson]

@tellis

In the "firmware/libraries/potentiostat" sub-directory there is file named ps_constants.h there is a #define near the top which is commented out

//#define HARDWARE_VARIANT_MILL_AMP

remove the comments - so it like like this

#define HARDWARE_VARIANT_MILL_AMP

Notes, this assumes that you are changing two resistors - R8 with 50 Ohm and R7 with 25 Ohm. If want to just change one you may need modify lines 92-99 in ps_constants.cpp where the parameters for the current range are set.

[Will Dickson]

@Will-Dickson

Also, when re-programming the firmware make sure the "tools -> optimize" is set to FAST. Right now if it is set to FASTER, which is the default, there will be an issue with the ArduinoJson library.

This is something I'm going to fix soon - just need to upgrade to the latest version of the ArduionJson library and the problem goes away. Then the optimize setting won't matter. There is a little bit of an API change between the new and old version of the library - so I want to test a bit first.

[tellis]

Worked like a charm. You're a gem, thanks so much.

[XanderS]

This post is deleted!

[Artingchen]

@Will-Dickson
Hi Dickson, I am also interested in the Increasing current ranges. Please tell me how to swap out R8 ? I found the R8 resistor has been fixed in the board. Would I add another external trans impedance amplifier feed back path to increase the current range ?