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JochiSt / OpenFlowMeter

Measuring Flow
GNU General Public License v3.0
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Non linearity of current and voltage #5

Closed JochiSt closed 2 years ago

JochiSt commented 2 years ago

calibration_20220307_190727_CH0_100

When looking at the calibration, there is a non linearity observable, which effects both current and voltage.

JochiSt commented 2 years ago

The current measured by the Multimeter and the DAC setting have a quite linear dependence. calibration_20220307_190727_CH0_100_MMvsDAC

JochiSt commented 2 years ago

This behaviour might be caused by the 3V3 Zener diodes, which limit the voltage to the ADC inputs.

JochiSt commented 2 years ago

3V3_zener_voltage_current

together with the 1k current protection resistor, this might cause the observed behaviour.

JochiSt commented 2 years ago

grafik

By using a simulation, it is possible to reconstruct this issue.

JochiSt commented 2 years ago

By removing the Zener Diode and replacing R27,R28,R39,R40 with 56k, the result looks quite linear. calibration_20220308_191237_CH0_10_MMvsDAC

JochiSt commented 2 years ago

grafik

Also with more points, the behaviour stays linear.

But, on the other hand, if no load or a very high ohmic load is connected, this might damage the controller.