Increasing current ranges to detect mA



  • Howdy all, I'm just getting into this project and learning my way through python and the arduino interface, very grateful and impressed with the work you are putting into this. My question is: Is there a way to increase the current ranges to register mA? Right now the max range is +/- 1000 uA (=1mA). Is this limit arbitrary or is the teensy not capable of handling higher currents (say, +/- 100 mA)? Many thanks.



  • @rfndzc

    The measurement current can be increased by changing a resistor in the transimpedance amplifier feedback path. However, the current measurement isn't really the main limitation. The control amplifier has an upper limit of 45mA. So with the current design this would represent the absolute maximum current you could source.



  • @Will-Dickson

    Many thanks Will. I believe I can work within the 45 mA limit. I'll look at the schematic for the teensy shield again.



  • @rfndzc

    The transimpedance amplifier feedback resistors are R5, R6, R7 and R8. Each one corresponds to one of the selectable current ranges. Some info on setting the feedback resistor can be found here https://en.wikipedia.org/wiki/Transimpedance_amplifier



  • @Will-Dickson

    I swapped out R8 with 50 Ohm resistor to create a +/-12mA current range. I've attached a figure showing a fullscale range test with a 220 Ohm dummy cell. It looks good over the whole range.

    +/- 12mA range



  • @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.
    +/-40 mA range OK

    Figure showing the failure below -20mA and above 20mA.
    +/-40 mA range fail



  • @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.
    24000uA_current_range_partial

    It almost works over the full +/-24 mA range. It fails at the negative end right around -22mA.
    24000uA_current_range_full


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