A voltage divider is a passive circuit consisting of two or more impedances connected in series, where the output voltage is taken across one of those impedances (or a subset of them in a tapped network), producing a fraction of the input voltage. The ratio of output to input is determined by the relative values of the series elements, which are resistors in the most common case.
In practice
Voltage dividers appear constantly in embedded hardware design. A typical use is scaling a signal down to fit within an ADC's input range — for example, measuring a 12 V supply rail with a 3.3 V ADC by using a resistor pair such as 10 kΩ and 3.3 kΩ to produce a ratio of roughly 0.25. The formula is straightforward: Vout = Vin × (R2 / (R1 + R2)), where R2 is the lower resistor (connected to ground) and R1 is the upper resistor. Resistor tolerance matters significantly here; a 1% tolerance on each resistor can compound in the worst case, and systematic worst-case or statistical (RSS) analysis is the right approach for quantifying this kind of error budget.
A critical pitfall is loading: the output impedance of a resistor divider is R1 parallel with R2, and any load connected to the output forms another divider stage that shifts the actual voltage. For ADC inputs with high input impedance this is often negligible, but for driving lower-impedance circuits the error can be substantial. As a rule of thumb, keeping the load impedance at least ten times higher than the Thevenin output impedance of the divider limits loading error to roughly 9% or less, though the exact figure depends on the specific loading model; tighter accuracy requirements demand a larger ratio or a buffer amplifier.
Voltage dividers are also widely used for logic-level translation — pulling a 5 V output signal down to 3.3 V or lower before feeding it into a 3.3 V MCU GPIO or UART RX pin. This is a one-way technique suitable for unidirectional signals; bidirectional level shifting requires a different approach, such as the MOSFET-based circuit described in "A simple working I2C (TWI) level shifter." The divider introduces a low-pass RC filter effect if there is any capacitance at the output node, which can limit usable signal bandwidth on fast signals.
Thermistors and potentiometers are commonly placed in one leg of a divider to convert resistance change into a measurable voltage. An NTC thermistor in series with a fixed resistor is a classic temperature-sensing front end for an ADC. In these variable-resistance dividers, the choice of fixed resistor value affects both the sensitivity of the output and the power drawn from the supply, requiring a tradeoff. The Thevenin equivalent circuit of the divider — discussed conceptually in "Stairway to Thévenin" — is the right mental model for reasoning about how such a network interacts with the rest of the circuit.
Frequently asked
How do I choose resistor values for a voltage divider feeding an ADC?
Balance two competing concerns: lower resistance values reduce loading error and noise pickup but increase
quiescent current and self-heating; higher values save power but make the output more sensitive to the
ADC's input characteristics and any stray capacitance. For a typical
MCU ADC, a divider in the 10 kΩ to 100 kΩ total resistance range is a common starting point. Check the ADC's datasheet for its source impedance requirements and acquisition time, and verify that the source impedance of the divider is low enough for the ADC to charge its sampling
capacitor within the allowed acquisition window.
Can I use a voltage divider to power a circuit, not just as a reference or signal scaler?
Generally no, not for anything but the lightest loads. A resistor divider's output voltage shifts with load current because the load appears in parallel with the lower resistor, changing the divider ratio. The Thevenin output impedance of the divider means any current drawn alters the effective ratio and causes a corresponding output voltage shift. For a stable supply rail, use a
linear regulator or switching converter instead. Dividers are appropriate for biasing high-impedance inputs, not for sourcing meaningful current.
What is the bandwidth limitation of a resistor voltage divider?
Any capacitance at the output node — from PCB traces, the load input, or an explicit filter
capacitor — forms a low-pass RC filter with the Thevenin resistance of the divider. The -3 dB corner frequency is f = 1 / (2π × R_th × C), where R_th = R1 || R2. For a 10 kΩ || 3.3 kΩ divider (R_th ≈ 2.5 kΩ) driving a node with 100 pF of parasitic capacitance, the bandwidth is roughly 640 kHz. This is rarely a problem for slow sensor signals, but it can distort fast digital edges or high-frequency AC signals.
Can a voltage divider shift logic levels bidirectionally, for example on an I2C bus?
No. A resistor divider is a passive, unidirectional attenuator. On a bidirectional
open-drain bus like
I2C, a simple divider cannot correctly translate logic levels in both directions. A common solution for bidirectional level shifting between, say, 5 V and 3.3 V I2C buses uses a small-signal N-channel
MOSFET (such as the BSS138) with
pull-up resistors on each side, a circuit covered in detail in 'A simple working I2C (TWI)
level shifter.'
How does resistor tolerance affect divider accuracy, and how do I analyze it?
Each resistor contributes its own tolerance error, and in the worst case these errors add. For a divider with a target ratio of R2/(R1+R2), you should evaluate the ratio at the extreme corners of both resistors' tolerance ranges (high-high, high-low, low-high, low-low). A 1% tolerance on each resistor can yield a worst-case ratio error of roughly 2% in the most sensitive configurations, though statistical (RSS) analysis typically gives a more realistic figure. Systematic worst-case and RSS methods are standard approaches for this kind of component-tolerance budgeting.
Differentiators vs similar concepts
A voltage divider is sometimes confused with a
voltage regulator. A regulator (linear or switching) actively maintains a fixed output voltage regardless of load current, input variation, or temperature. A voltage divider is purely passive and its output voltage shifts with load, input voltage changes, and resistor temperature coefficients. The two are not interchangeable for powering circuits; dividers are signal-conditioning and biasing tools, not power supply elements.
