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  • RE: Modifying Rodeostat Current and Voltage range


    The Rodeostat has voltage ranges of +/-1, +/-2 , +/-5 and +/-10V. So it can achieve the voltage you want without modifications.

    The standard current ranges are +/-1, +/-10, +/-100 and +/-1000 uA. Which are a bit lower than you are looking for. We have made modified versions with increased current range. The limit on this was about 24mA (24000uA). There is a topic discussing the modifications here Increasing current ranges to detect mA

    posted in Rodeostat
  • RE: CheapStat Voltage modification


    Ideally you would be able to run from -1.65V to 1.65V. However, in practice with the output cheapstat becomes nonlinear below about -0.9V.

    posted in Cheapstat Potentiostat
  • RE: Measuring Open Circuit Potential

    Hi Brad,

    Unfortunately, just removing the setVolt commands won't be sufficient - as the control amplifier will still be running trying to control the voltage between the reference and working electrode. You really need to disconnect the counter electrode. Right now your best bet is to disconnect it manually. You might put a little switch between the counter electrode and the test cells. You could even use a digital switch activated by one of the DIO lines on the 5x2 header P14.

    In a future revision I would like to put another digital switch on the board so the counter electrode can be disconnected in firmware (without added an external switch).


    posted in Rodeostat
  • RE: LP NF switch


    I think you could achieve pretty much the same result (or better) by filtering the data in software after it is collected - when you are analyzing the data.

    posted in Cheapstat Potentiostat
  • RE: LP NF switch


    There is an optional Sallen-Key lowpass filter on the cheapstat PCB. The NF setting selects "no filter" and LP setting selects the "low pass" filter. The components R10, R11, C20, and C21 can be used to set the filter parameters. By default these components are left empty - so you can put in whatever you like for this. Also, note that the lowpass filtered values come into the microcontroller on a different ADC then the unfiltered values - ADC_LP on (ADC0) vs ADC_NF on (ADC2). So in addition to adding the hardware components for filter you will need to change this setting in the firmware to read the lowpass filtered values rather than the unfiltered values.

    posted in Cheapstat Potentiostat
  • RE: CheapStat Voltage modification


    It might be possible. It would require some minor changes to the firmware. Also, it won't change the overall voltage i.e. (Vmax - Vmin) . You can just shift the range in the towards either the positive or negative directions. Finally, I haven't tried it - so I can't guarantee that it will work. However, it is just a firmware change and I don't think trying it will cause any damage to the instrument.

    The cheapstat is a 0-3.3V device and the mid-point 1.65V is used as the ground for the electrochemical cell. This how the device achieves an effective bipolar voltage range. The value used as the ground for the electrochemical cell is set by the value DAC1 on the microcontroller and is labeled REF on the schematic. DAC1 is a 12-bit digital to analog converter so there are 4096 possible values and it has a 0-3.3V range. It is currently set to 2048 (the mid-point value) which gives an output voltage of 1.65V. This puts the ground for the electrochemical cell midway between 0 and 3.3V. Increasing the value of DAC1 will shift the output voltage range in the negative direction - increasing the range of negative values and decreasing the range of positive values. . Similarly decreasing the value of DAC1 will shift the output voltage range in the positive direction.

    posted in Cheapstat Potentiostat
  • RE: Additional tests for colorimeter (pH, alkalinity, calcium hardness, etc.)


    I can reproduce the issue you are seeing. I will look into it this week and see if I can fix the problem.

    posted in Colorimeter
  • RE: Cheapstat ASV


    You could use the linear sweep voltammetry (LSV) test in the cheapstat firmware to do anodic stripping voltammetry. The settling time parameter can be used to set the time for your preconcentration step.

    There is also a squarewave voltammetry (SWV) test in the firmware. This would almost do what you want for squarewave anodic stripping voltammetry. Unfortunately there doesn't seem to any period at the beginning of the trial which could be used for your preconcentration step. However, I think it would be pretty easy to modify the firmware to add this. The SWV test in implemented in the SWV_test function in cheapstat.c. For an example of how this delay at the start of the trial could be implemented you could look at the LSV_test function which implements the linear sweep voltammetry.

    posted in Cheapstat Potentiostat
  • RE: Increasing current ranges to detect mA


    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.

    posted in Rodeostat
  • RE: Overcurrent


    Yes, I think it is possible to reduce this overcurrent. This is most likely caused by the abrupt change in potential when the cyclic voltammetry test is run with the 'shift' parameter (dimensionless phase shift) set to zero. The cyclic voltammetry test outputs a triangle waveform for the potential and when the 'shift' parameter is set to zero this waveform will start at the minimum voltage as shown in the figure below
    This figure shows two cycles of the potential output for the cyclic voltammetry test when shift = 0. At the start of the test the potential abruptly changes from 0V to the minimum value (in this case -0.4V) at the start of the test. This abrupt change in potential can cause a large current.

    In order to minimize this current it is possible to adjust the shift parameter so that the waveform starts at 0V and then linearly progresses to the minimum value. An example is shown in the figure below

    In this case the shift was adjusted so that the waveform initial goes from 0V to the minimum voltage (-0.4V). The exact value of the shift parameter required to achieve this depends on your minimum and maximum voltages and can be calculated as follows

    shift = 0.5*(volt_max/(volt_max-volt_min)) + 0.5

    Regarding your second question. There is currently no way to disconnect the counter electrode via the firmware e.g., via some sort of digital switch. However, this is something I would like to add to a future revision of the hardware design. At the moment your best option might be to physically disconnect the counter electrode whenever you remove the reference electrode.

    posted in Rodeostat