A voltage divider built from two resistors is one of the most fundamental circuits in electronics. It splits an input voltage in the ratio of the resistors and delivers a smaller output voltage. With the Voltage Divider Calculator you get the result in seconds. This tutorial covers the formula, an example with and without a load, and the most important practical rules.
The basic voltage divider formula
For the unloaded case the output voltage follows the simple divider formula:
- V_out = V_in · R2 / (R1 + R2)
The output is tapped between R1 and R2, while R2 connects to ground. The larger R2 is relative to R1, the closer the output sits to the input voltage. A quiescent current of I = V_in / (R1 + R2) flows through the divider.
Worked example with and without a load
Take an input voltage of 12 V and two equal resistors R1 = R2 = 1 kΩ. With no load connected:
- V_out = 12 V · 1000 / (1000 + 1000) = 6 V
- Quiescent current I = 12 V / 2000 Ω = 6 mA
As soon as a load R_L = 1 kΩ is connected at the output, it sits in parallel with R2. The effective parallel combination is R2 || R_L = 500 Ω, so:
- V_out = 12 V · 500 / (1000 + 500) = 4 V
The output voltage drops noticeably from 6 V to 4 V. This is exactly why an unloaded divider becomes unusable for many applications once it is loaded.
Practical tip: keep R2 small enough
So that the load barely disturbs the output voltage, the divider should be much lower in impedance than the load. A proven rule of thumb is:
- R2 ≤ R_L / 10 for a stable reference voltage.
Smaller resistors, however, mean higher quiescent current and more power dissipation. You always have to balance stability against power consumption.
Where the voltage divider fits
The voltage divider is ideal for reference voltages, measurement dividers (for example scaling a voltage down for an ADC) and level shifting between logic levels. It is not intended to supply loaded consumers: as soon as significant current is drawn, the voltage collapses. For a regulated supply you use a voltage regulator instead of a simple divider.
