I’m looking for the value of the current adjust potentiometer R9 on the A4983/A4988 breakout boards.
It seems to me that R5 is too low, making the current adjustment very coarse and only use a subset of the potentiometer’s travel.
I’d like to replace it with a more suitable value such that the junction of R5 and R9 sits at 0.8v, the sense voltage which gives maximum current with 0R05 sense resistors.
In order to work out the best value, I need to know the value of R9 which is missing from the published schematics.
while I have your attention, can I suggest adding a ceramic capacitor between VREF and ground? my motors are quite noisy due to this trace picking up RF to the point where I can listen to local AM radio stations if I touch the pot with anything conductive!
Since you have grounds right next to the relevant trace you may not even need to alter the board design!
I’ll try this too when I have some free time for fiddling
Switching R5 with a 68k works beautifully, giving a max. current of ~1.6A when the pololu is supplied with 5v. I’ve experimentally verified this, and it’s awesome. The trimpot is significantly less sensitive to minor adjustments and I get to use its whole range of motion instead of only the first 1/4.
47k should give a max of 1.4A when supplied with 3.3v, or 39k for 1.68A
Which just goes to illustrate that this arrangement makes the breakout rather sensitive to variations in supply voltage in terms of motor current…
Adding a silicon signal diode with its ~0.65v Vf across the pot would work nicely, limiting current to ~1.7A as well as hugely reducing sensitivity to variations in supply voltage. I think that’s the best way to go.
