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nuno:
Nice build.


--- Quote from: jaycee on May 13, 2020, 07:10:30 pm ---(...) metering is not very accurate at the low range because the ADC in the ATMega is not very accurate near ground.
--- End quote ---

Can you elaborate?
jaycee:

--- Quote from: nuno on May 15, 2020, 09:10:51 am ---Nice build.


--- Quote from: jaycee on May 13, 2020, 07:10:30 pm ---(...) metering is not very accurate at the low range because the ADC in the ATMega is not very accurate near ground.
--- End quote ---

Can you elaborate?

--- End quote ---

The ADC in the ATMega, like a lot of ADCs, are not very accurate at the extreme ends of their range. Add in compensation for offset and it becomes an issue. The ammeter on my psu cannot read any lower than 20mA for example, yet it can current limit lower than that so the problem is not in the sense amp. I could have avoided the issue by adding an offset to the sense amps so that they do not go near ground, which would have allowed enough range to both cancel the offset and avoid the nonlinearity.

I attempted to hack it a bit by adding resistors to adjust the range to the front panel display, its not perfect but workable.

I might try a further hack to add the offset I mention. A TL431 should be enough to do the job. At any rate, the power supply works well and it's better than the previous lashups of LM317's etc :)
I'd "Open source" it, but I really dont want some unscrupulous Chinese ebay sellers turning it into a kit.
nuno:
Are you using a current measuring resistor? If so, what value and what amplification (if != 1) before feeding the AVR's ADC?
jaycee:

--- Quote from: nuno on May 15, 2020, 10:57:11 pm ---Are you using a current measuring resistor? If so, what value and what amplification (if != 1) before feeding the AVR's ADC?

--- End quote ---

Yes, 0.1 ohms. There is a sense amp and the output feeds both the ADC and the current control error amp. The sense amp is not the problem.

Full schematic here: http://www.dark-matter.me.uk/files/electronics/psu-18v1a/main_sch.pdf
edit: Actually thats just the main board. The display board is at http://www.dark-matter.me.uk/files/electronics/psu-18v1a/front_sch.pdf

edit 2: My plan is to disconnect AGND from the star ground, and add some offset voltage there. The sense and error amps get shifted away from 0v, but they dont care about that.. the only thing that will see the difference is the ADC which is referenced to DGND
nuno:
I have used an AVR to read small values of voltage and I didn't see any problem with it (reading a PT100, only amplification was ATtiny26's internal differential 20x gain amplifier and its internal ~2.7V Vref *). You are using different Vrefs for the pure analog part and for the display/control part, maybe your problem comes from that, what happens if they differ by, say, 40 or 50mV (or maybe not :D ).


* The schematic is here, it's commented in Portuguese but I think it won't be a big problem, there's nothing fancy:
https://troniquices.files.wordpress.com/2008/07/coscov.png
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