The Voltage Divider Rule

Two resistors in series share the input voltage in proportion to their resistances. The output taken across R2 is Vout = Vin × R2 / (R1 + R2). Because only the ratio matters, the divider is the standard way to scale a voltage down — the heart of sensor interfaces, level shifters, and reference circuits.

Designing a Divider

First choose the ratio you need: for a 3.3 V output from a 5 V supply, R2/(R1+R2) = 0.66, so R1 should be about half of R2. Then scale both values to control the current: larger resistors waste less power but make the divider sensitive to a load connected at Vout.

The Load Problem

A real load in parallel with R2 reduces the effective lower resistance and drags Vout down. As a rule, keep the load at least ten times R2 so the error stays under 10% — or buffer the output with an op-amp, which is what professional circuits do when accuracy matters.