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High range current calibration compensation #100

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prasimix opened this issue Apr 5, 2017 · 0 comments
Open

High range current calibration compensation #100

prasimix opened this issue Apr 5, 2017 · 0 comments

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@prasimix
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@prasimix prasimix commented Apr 5, 2017

Due to heating up, the resistance of current sense resistor is increasing and cause voltage drop. Therefore in CC mode output current will gradually drop. Such drift cannot be measured with ADC since current control loop is in charge of output regulation and with heated resistor, a lower current need to flow for the same voltage drop that is used by control loop circuit. That drift can be up to -0.4%. But it can also be positive when new set current in CC mode is much lower then previous one and output current will starts to increase as resistor is cooling down.
Depending of what kind of end result is desirable we have a few of options here:

  1. Introduce "heating up" down counter in calibration process for high range (5 A) MID and MAX points. For MAX point that could be up to a minute, while for MID we can cut that delay in half. When counting is finished allow entering of externally measured current value.
  2. Asks for two current during calibration, one as soon as load is connected and next one after "heating" delay is over. Using that two values we can guess how much current has to be compensated when output enters CC mode. For example if first measured value is 4.830 A and next one (after minute delay) is 4.815 A that means that in first 60 seconds current has to be gradually increased for 15 mA, i.e. from 4.985 to 5.000 A. If such kind of software compensation is added, we need also to take care about opposite direction: when new set current in CC mode is lower. In that case output current will be lower then set and will gradually increase as resistor is cooling down.
  3. Don't play with "heating up" delay during calibration process but add possibility to enter offset afterwards that can be used for software compensation.
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