Two resistors in series across a supply split the voltage in proportion to their resistances. The point between them sits at a predictable fraction of the input, which makes the divider the cheapest way to scale a signal down.
R1 is the resistor between the input and the tap; R2 is between the tap and ground. Only the ratio sets the output voltage — 10 kΩ / 10 kΩ and 100 Ω / 100 Ω both give half the input. What the absolute values change is current draw and how much the divider sags under load.
Choosing the resistor values
The whole chain draws I = V_in / (R1 + R2) continuously, whether anything is reading the output or not. On a 5 V rail, two 100 Ω resistors burn 25 mA and 125 mW forever. Two 100 kΩ resistors draw 25 µA. For battery-powered work, higher values are almost always right.
The counter-pressure is noise and loading. High-value dividers have high output impedance, which makes them slow to settle and easy to disturb. A practical band for most microcontroller work is 1 kΩ to 100 kΩ, with 10 kΩ as the default guess.
The loading problem
The formula above assumes nothing is drawing current from the tap. Connect a load and it sits in parallel with R2, pulling the effective bottom resistance down and the output voltage with it.
A 10 kΩ / 10 kΩ divider on 5 V should give 2.50 V. Hang a 10 kΩ load on it and the bottom leg becomes 5 kΩ, so the output drops to 1.67 V — a third off. The rule of thumb is to make the load at least ten times the divider impedance, which caps the error at roughly a few percent.
Where dividers work and where they do not
They are the right answer for:
- Scaling a 12 V battery voltage down into a 3.3 V ADC range.
- Setting a fixed bias point at a high-impedance input like an op-amp or a MOSFET gate.
- Level-shifting a 5 V logic output down to 3.3 V on a slow signal.
They are the wrong answer for:
- Powering anything. A divider is not a regulator. Its output moves with load and with the input rail. Use an LDO or a buck converter.
- Fast digital signals. The output impedance and the stray capacitance of the target form a low-pass filter that rounds off edges. Above a few hundred kilohertz, use a proper level shifter.
- Anything with varying current draw. The output voltage will move with it.
Common ratios
| R1 | R2 | Ratio | 5 V in | 12 V in |
|---|---|---|---|---|
| 10k | 10k | 0.500 | 2.50 V | 6.00 V |
| 10k | 20k | 0.667 | 3.33 V | 8.00 V |
| 20k | 10k | 0.333 | 1.67 V | 4.00 V |
| 10k | 4.7k | 0.320 | 1.60 V | 3.84 V |
| 100k | 10k | 0.091 | 0.45 V | 1.09 V |