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Long time reader, first time posting. I have a few projects that I would like to get done, and I realized if I don't start getting them done, I never will. Heh.

I'm making mistakes calculating the resistors I would need for the following. I've done it 5 times now because each time I do the math, I'm coming to different conclusions. This will be my first learnings on surface mount soldering, so I want to be sure the bits are correct, then I can only blame myself for any soldering mistakes. So:

The board has R1=7500ohm, R2=7870ohm, and R3=2100ohm, as a voltage divider sampling filtered output from the 53355 buck inductor (0.6V), and runs at 1.0514V low voltage. At high voltage, R3 is not used, and it runs at 1.172V.

What I would like to do is replace R1 and R2, such that the board, using R1, R2, and R3, runs at 1.172V low voltage, and then using only R1 and R2, runs at 1.25V high voltage. The use of higher voltage is not in question as far as the board is concerned.

I think I'm screwing up the calculations between the different options for R1 and R2:

Vo = 1.17V on low voltage.

R7 = (1.17V - 0.6V) * (7870+2100) / 0.6V

R7 = (0.57V)*(9970)/0.6V

R7 = (55/60)*9970 = 9139ohm for low voltage.

But it has been QUITE some time and I'm not quite sure I am using the formulas correctly. Either way, I think too many numbers are leading me astray.

Anyone have any thoughts? And thank you, in advance!

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# Irritatingly failed resistance calculations

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