Duet 3 Mini 5+ TMC2209 sense resistors
The recently released schematic for Mini 5+ identifies the stepper driver sense resistor value of R056. The firmware, on the other hand, appears to be using .056 + .02, resulting in .076. Which value is correct and why does the firmware add .02 on top of the physical resistor value?
This came up while I was finalising and cross-checking Klipper configuration for the Mini 5+. Obviously the sense resistor value is important in order for the driver to supply the correct current to the stepper.
Any insight would be appreciated.
0.02 ohms is the internal resistance of the chip, as specified in the TMC209 datasheet.
To be honest, having just reviewed the TMC2209 data sheet yet again, it is still not obvious to me at all that the R_BRxy value they quote in the Sense resistor voltage levels table has to be considered when selecting (or using) the sense resistor value quoted in Chapter 8 Selecting Sense Resistors.
In fact, considering that 20 mOhm R_BRxy is a whopping 27% of the lowest R_SENSE quoted in the table of 75 mOhm, you would think that they would indicate the critical importance of considering R_BRxy in bold font right next to the table in Section 8. In other words, I would interpret lack of such a note as an indication that the 75 mOhm quoted in the table already accounts for R_BRxy when listing the maximum RMS current of 2.4 A. Otherwise I would expect the caption of the first column to be captioned as R_SENSE + R_BRxy [Ohm].
I am curious if you perhaps had any clarifications directly from Trinamic or if you actually measured the stepper current waveform?
Thank you again,
@rext3d I used the formula "RMS CURRENT CALCULATION WITH UART CONTROL OPTIONS OR HOLD CURRENT SETTING" on page 50 of the datasheet. I measured the peak current to confirm that the sense resistor value was correct. We use the lower 180mV value of vsense. Using the 325mV value would result in excessive power dissipation in the sense resistors.
@dc42 Ah, BINGO! It finally makes sense. I am sorry that I missed that! I was also planning to confirm the actual delivered peak current with a scope. Thanks very much for your time!