Voltage Divider

Two resistors that turn a big voltage into a smaller, proportional one. The most-used two-component circuit in electronics — and the one whose two failure modes bite everyone exactly once.

The whole idea: the same current flows through both resistors, so the voltage splits in proportion to the resistances:

Vout = Vin × R2 / (R1 + R2)

Equal resistors → exactly half. 12 V battery into a 3.3 V ADC? R1 = 27k, R2 = 10k gives you 3.24 V at full charge.

What it's for: measuring things (battery monitors, sensor scaling, feedback pins on regulators) and biasing things. What it's not for: power. It can only deliver microamps before the ratio collapses.

The two gotchas, both about the same number — the output impedance R1∥R2:

1. Loading. Anything you connect to vout is electrically part of the divider. A 10 k load on this 5 k-output divider shifts the ratio from 0.50 to 0.40 — your reading is just wrong. Keep loads ≥10× the output impedance, or buffer with an op-amp follower.
2. The current never stops. Vin²/(R1+R2) flows 24/7 — the silent battery drain in "why does my project die in a week" designs. Scale the resistors up (100 k–1 M) for battery monitors; just know that high-impedance dividers are slower and noisier into ADC pins.

Exposes: vin, vout (= vin × ratio), gnd.

⚠ An ADC input is not a perfect voltmeter — most want to be driven from under ~10 kΩ. A 1 MΩ battery-saver divider straight into an ADC reads low and jumps around; add a small capacitor on vout or a buffer.