// Nominal fan RPM (for PWM=255)
#undef FAN_NOMINAL_RPM
#define FAN_NOMINAL_RPM 2000
At what current does the change over from mid to high occur? The lowest range is not implemented yet in the software is it?
The Isabellenhütte one is 30 ppm/K so has to be kept cool. The other suggestion looks better at 15 ppm/K. Improving the temperature induced offset comes at a cost, for me it's good enough for now.
However:
During trials I stumbled over the following: With the current range selected to 0.5 A I could not reach the full 500 mA current in the load resistor when increasing voltage. At about 460 mA for power board 1 and 490 mA for power board 2 the CC mode became active. Both channels were calibrated and external measurements were equal to the indication on the psu.
First I suspected the sense resistor for the 500 mA range being to high, R63: 0.082 ohm 1%, but that is not the case. The problem is in the pcb trace that connects R63 with the R65's. This trace carries the 'full' current in the 500 mA range and is part of the sense 'stack'. Although not long, about 9 mm before it divides and connects to the R65's, the trace is rather thin, 16 mill (from eagle), so 0.41 mm. But its resistance value must be below 0.008 ohm to arrive at the wanted total of 0.100 for current measuring in this range. With a resistance calculator (https://www.allaboutcircuits.com/tools/trace-resistance-calculator) I found 0.0108 ohms for this piece of trace. Combined with the tolerance of R63 this is too much. See picture Layout.png
The solution was a simple piece of wire from R63 to R65, picture R65R65C.JPG.
The layout below the three R65's is really nice I think however a bit critical. There is space for a wider trace, certainly when the fork is omitted.
Another reason to minimize the resistance of this trace is the resistance temperature coefficient of copper, about 4000 ppm/K.
I had another issue with the layout during soldering the R65's: Several times the thin measure point traces did not even connect to R65's when I hand-soldered them. Makes the problem above even worse.
So I would like to suggest to change the layout here. A wider trace from R63 to the inside of R65A, and maybe connect the measuring point traces direct to the islands of R65A. Like picture Suggest.png
For the low current range (50 mA) R179 I used a 0.82 ohm resistor, not tested since this range is not yet active in the firmware. But maybe soon...?
The firmware (finally) loaded correctly, but first power up, I got error messages
on Channel 1:
Self-test is not passed
- CH1 IOEXP failed
- CH1 ADC failed
No, something else is probably your problem here since I can easily go over 500 mA in low (in your case middle) range (current range set to Best/Default, and with Autoranging set to on). Did you try to replace R179 with 0R for test before you put that extra wire?
Sorry to bring it up once more, other builders of r5B13 boards may also run into this issue.
When working with the psu in the 500 mA range it seems not possible to reach the full 500 mA with the r5B13 power board as it is. Somewhere between 460 and 490 mA the CC mode takes over, although the set current limit is 500 mA. Seen this with all four power boards I have tested.
Comparing the r5B13 with the r5B12 power board layout the trace between R63 and R65 is much shorter, see picture r5B12.JPG. Also the shunt resistors are twice the value, halving the relative influence of trace resistance.
I soldered R63 and R65, -A and -B on a spare power board r5B13, picture measureC.JPG. While under a load of 500 mA, in series with an external 4 ohm load resistor, I measured these voltages:
left pad of R63, supply from load resistor : 52 mV
other of pad of R63, trace to R65 : 11 mV
left side of R65, -A and -B : 5 mV
right side of R65, -A and -B ,common : 0 mV
I also measured these voltages on a 'life' power board in the psu while in 500 mA range and under almost 500 mA load. They are about the same.
So there is a 6 mV drop over the trace from R63 to R65, apparently a trace resistance of 0.012 ohm here! Total shunt for 500 mA sums up to 0.104 ohm in stead of 0.092. Compare this to a power board r5B12: R65 is 0.02 and R63 is 0.16 ohm. The trace between adds about 0.006 ohm, being less than half length. That is 3% of the total. The 0.012 ohm trace of the 5B13 board adds 13% to the total shunt resistance for the 500 mA range. So the voltage measured is 13% higher than intended... You can calibrate this away but the set value voltage from the DAC is limited, and so the current can not reach the intended 500 mA in CC mode.
Another thing: 13% of the 500mA shunt is the copper trace with 4000ppm/K Temperature Coefficient, significantly worsening the temperature stability of the 500 mA range.
Hi prasimix,
about the serial console, there has three questions, 1) what port should be used, native or programming? 2) does the arduino due should be modify, that means don't powered by usb cable. 3) which baud ratio should be set?
thanks alot
Thanks for more input. I also have 10 mOhm as R65 (with accompanying different R63, R60, R70). I still think that bigger error is announced with R179 then with thiner trace between middle and hi range resistors. Once again, did you try to replace R179 with 0R?