A voltage divider scales a voltage down with two resistors in series: the output, tapped between them, is the input times the ratio of the bottom resistor to the pair. Vout = Vin× R2 ÷ (R1 + R2). To bring a 5 V rail into an ESP32’s 3.3 V ADC, R1 = 10 kΩ and R2 = 20 kΩ give 5 × 20 ÷ 30 = 3.33 V. Set your own below.
Inputs
Result
3.33 V
output voltage (at the tap)
Add an ADC's input impedance in parallel with R2 if it's not much larger than R2.
Quiescent current through the divider
0.167 mA
The formula
R2 sits between the tap and ground, R1 between the tap and the input. The output is the fraction of the input that R2 claims: R2 ÷ (R1 + R2). Equal resistors halve the voltage; making R2 larger than R1 keeps more of it. The ratio sets the voltage, but the two actual values still matter, because together they set how much current the divider draws and how stiff it is.
From a real board
The OTD L1.05 board reads analog signals on the ESP32’s built-in ADC, which only measures up to roughly its 3.3 V rail (One Thousand Drones, L1.05 design 2026). Anything higher, a 5 V USB rail or a battery, has to be divided down first or it clips, or worse, exceeds the pin’s absolute-maximum. A divider is the simplest way to fit a bigger voltage into that window.
Mind the loading
Whatever you connect to the tap draws a little current and pulls the output down. What that load sees is R1 in parallel with R2, the divider’s output impedance, so keep the resistors modest enough that the ADC’s input does not sag the reading, and add a small capacitor at the tap to give the ADC a stiff, settled voltage to sample. Bigger resistors waste less quiescent current but load more easily; smaller resistors are stiffer but burn more. The calculator shows the quiescent current so you can size that trade.
What it does not do
A divider scales a voltage; it is not a regulator. The output tracks the input, so if the input sags, the output sags with it. For a steady reference you want an actual reference or regulator, not a divider.
References
- Espressif Systems. ESP32-S3 datasheet, ADC characteristics and input range.
- One Thousand Drones. ESP32 analog sensing (L1.05), internal ADC. Build the board.