- Tue Apr 28, 2009 8:56 pm
#71622
Thanks to previous feedback, I have redesigned the power supply section for my upcoming solar powered wireless weather station. Essentially I went with symmetric circuits for both the solar panel and battery that allow voltage sampling via 2:1 divider to an ADC and the ability to individually connect/disconnect the solar panel and battery to the regulator.
Couple of issues that came up with my redesign:
1- I used 100K resistors in the voltage divider to limit current drain. I know this impacts the ADC sampling time. By my calculations, I need about 5ms to be certain the sample/hold capacitor is properly charged. Are there any other implications to using large resistors for the divider? Could I go larger if the sampling time is not an issue?
2- There is one gotcha in that my design won't actually power up without help. I used pulldown resistors on the N-MOSFETs which act as level shifters. I cannot use pullups because that would exceed the voltage limits of the microcontroller. Any clever idea on how I get this circuit to startup with minimal extra parts?
Couple of issues that came up with my redesign:
1- I used 100K resistors in the voltage divider to limit current drain. I know this impacts the ADC sampling time. By my calculations, I need about 5ms to be certain the sample/hold capacitor is properly charged. Are there any other implications to using large resistors for the divider? Could I go larger if the sampling time is not an issue?
2- There is one gotcha in that my design won't actually power up without help. I used pulldown resistors on the N-MOSFETs which act as level shifters. I cannot use pullups because that would exceed the voltage limits of the microcontroller. Any clever idea on how I get this circuit to startup with minimal extra parts?