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Fubar Solar Power Setup

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ruku:
Thiiis one's special. Not my design, but I need to do analysis on it.



So we have a sensor that needs +/- 5 V power. Threw on a switching regulator to take care of that . Problem is, the output is bipolar, need to make it unipolar. Op Amp you say? Fshaw, silly! Lets do something else.

Split the rails, drop the potential for the AVR microcontroller.

There has GOT to be a plethora of things that are wrong with this. For one, there's no charging protection on the battery, but disregard that. It's the least of my concerns. When the sun is not there, the switching regulator no longer runs, so the microcontroller is back at normal potential.

Can anyone tell me WHY this design is bad? Innately I know this, but I can't explain WHY.

Edit: Fixed the schematic, was wrong on something.

Jimmy:
What are you trying to acheve here

IanB:
I'm confused. I can't make much sense of that drawing.

Does the switching regulator supply the mystery device at the lower right and also the resistor divider?

Where does that regulator get power from if the sun isn't shining?

Where do the +/- 1.65 V taps off the resistor divider go to? Is any current being drawn from them?

What exactly is the mystery device?

ruku:
Haha. Terrible me, I didn't explain it.

TL;DR - We have a sensor (the mystery box) that provides a bipolar signal (+/- 1.65 volts). The problem is the ADC on the AVR can't read anything below 0V (or so).

Instead of properly biasing our signal, we lowered the "ground" potential for the microcontroller. That way, signals at -1.65 Volts appeared to be at 0 V, since the microcontroller's ground was at -1.65 volts.

IanB:
When you analyze circuits, you can't only think about "voltages" (I use the quotes there deliberately). You must also think about where the currents are flowing (or need to flow) in a circuit.

If you have a truly isolated power supply, and you use that for the 0 V power connection to the microcontroller, and you use the independent battery to make the 5 V power connection to the microcontroller, how is an electrical circuit going to be completed for the current consumed by the micro?

Why on earth don't you just connect the AVR power rails to -5 V (0 V) and 0 V (+5 V) on the regulated power supply?

If your sensor output voltage is then in the wrong range for the ADC on the micro you should use a voltage level converter to bring it into the right range.

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