Electronic – Can someone explain this microcontroller ADC interface (for reading solar panel voltage)


I am trying to understand the functionality of a circuit found in TIDA-00121 (you can download the design file from here)
microcontroller interface

solar panel connections

I assume that this has to do with the fact that the PV is not directly tied to ground (reverse current mosfet may be turned off when the solar panel voltage is too low to prevent any reverse current from flowing into the panel)

As for the transfer function (from source code), the voltage at the microcontroller side equals to:

V=0.086045Pv-0.14718475V (PV is the panel voltage).

this was extracted from the fact that Vref=2.39,10 bits ADC and the source code equation:

Panel Voltage = 36.83 * PV – 63

to verify my assumptions, from the source code:

Battery Voltage = BV * 52.44

which yields to voltage at microcontroller side of the battery voltage divider:

V=0.122BV which is the voltage divider ration (14K/100K network)

The question is:

  1. What is the role of the pnp transistor network?
  2. How to calculate the transfer function of the voltage at the microcontroller side?

Thank you very much.

Best Answer

What is the role of the pnp transistor network?

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It's a differential voltage to current converter followed by a load (R34 and R35). The voltage between P+ and P- sets a voltage across R31. This (minus 0.7 volts) sets a voltage across R33 and that causes a current to flow out of the collector (largely irrespective of what load the collector has).

Given the values of R33, R34 and R35, whatever voltage is set across R33 appears across R35 but, reduced by 3:1.

Importantly, this voltage is ground referenced making it suitable for the ADC to make sense of. So there is level shifting involved.