Variable resistors are neat. Unfortunately, I've not been able to find a digital pot that goes to 10 M Ohms. If you check out the ardustat theory you’ll find that the higher the resistance we can use, the finer the resolution on the current control. Luckily, as of version v0002 it is very easy to install a fixed resistor solution.
- Unplug the Ardustat from the usb port.
- On the Ardustat daughter-board, pull the digital pot off. It's the middle chip.
- Place you resistor in the 16 and 17 pin positions. Chip pins follow counter clockwise from the upper left.
- Plug the Ardustat back in.
- Open the ardustat software, and set up as usual, but notice the new option under "Calibrate"
- Select "Fixed" and enter the value of your resistor in k Ohms.
- You're all set!
Note that because the resistance is fixed, you're limited to the dV for current control. Thus the largest possible current you can charge with is
5 V - ADC / Rfixed,
And the discharge current is limited to
ADC - 0 / Rfixed
To return the digital pot, just repeat steps 1 through 4, swapping the pot for the fixed resistor.