Voltage dividers: measuring higher voltages safely
How a resistor voltage divider works, how to scale a battery or 12 V signal for an ADC, the loading effect, and why a divider must never be used to power a circuit.
How it works
Two resistors in series share the voltage in proportion to their values: Vout = Vin × R2 / (R1 + R2). It is the standard way to bring a battery voltage or a 12 V signal down to the 0 to 3.3 V range of an ADC.
Choosing values
Low values draw a constant current from the source; on a battery device, 12 V across 37 kΩ wastes 0.32 mA continuously. High values (hundreds of kΩ) save current but become sensitive to the ADC's own input current and to noise; a 100 nF capacitor from Vout to ground helps. Use 1 % resistors: two 5 % parts can make the reading 10 % wrong.
The loading effect
The formula assumes nothing draws current from Vout. Anything connected there acts as a third resistor in parallel with R2 and pulls the voltage down. That is why a divider is fine for an ADC input (which draws almost nothing) and useless for powering anything: the output voltage collapses as soon as current is drawn. Use a regulator for that.
Measuring batteries accurately over temperature, isolating measurements from high voltage or mains, and designing ADC front-ends with the right filtering and protection are part of our feasibility and schematic work.
Parts and tools to practise
| Item | What to look for | Buy |
|---|---|---|
| Resistor assortment, 1 % metal film | E24 or E12 values from 10 Ω to 1 MΩ, 1/4 W. Sorted in labelled bags or a box. | View on Amazon ↗ |
| Solderless breadboard kit with jumper wires | Full-size breadboard, pre-cut jumpers and flexible Dupont wires. | View on Amazon ↗ |
| ESP32 or similar development board | To try pull-ups, ADC readings, buses and PWM for real. | View on Amazon ↗ |
| Digital multimeter | Auto-range with continuity beeper, CAT III rated. You will use it every day. | View on Amazon ↗ |
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