Resistors can't be exactly&reliably measured while being soldered on the board (in-though-experiment:if we remove them we'll get a quite small resistance of several KOhms from the components utilizing 5V).
So measuring them is useful only to verify that they are, say, not 100x greater or 100x smaller than expected. Those numbers shouldn't be used in calculations, compared with datasheets or compared with desoldered resistors.
But their exact
ratio still can be found without desoldering: you write
Micchaleq писал(а):
Between output and ADJ is 0.7V, output is 3,1 V (to ground) and ADJ is 2,4 (to ground).
Assuming that BM1117 is not burned-too-much and its resistance of ADJ pin to other pins is kept huge enough:
(R1+R2)/R1 = 3,1V/0.7V ~= 4.4
Vref = 1.25. So for working BM1117 with same resistors we'll get Vout = Vref * 4.4 ~= 5.5. This is not exact 5v, but the original number of "0.7" was not very exact, so the result looks "precise enough".
I think that with the exact ration known, and inexact measurment results we can calculate the exact values by applying standard basic physic laws about resistance in the complex resister network and some math. However, in practice this is rarely needed.
(I'm a mathematician, so perform some calculations is simpler then desoldering for me))) )
