Library / Electrical / ELE-16
Electrical
Potential divider
Fraction of a supply voltage that appears across one resistor in a series pair.
- Ref
- ELE-16
- Rev
- v1
- Field
- Electrical
- Result
- Vout [V]
Vout is measured across R2 with nothing connected to the output. Use the same unit for R1 and R2 (the calculator uses kΩ).
Two resistors in series share the supply voltage in proportion to their resistances. The voltage across one of them is its share of the total resistance times the supply. It is the simplest way to make a lower voltage from a higher one.
Calculator
- Divider ratio R2/(R1 + R2)0.3333 ×
- Divider current0.8 mA
- Power drawn from the supply9.6 mW
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- The output depends only on the ratio of the resistors times the supply. Scaling both resistors by the same factor leaves Vout unchanged but changes the current drawn.
- A load connected across R2 sits in parallel with it and lowers the output. To keep that error small, make the load at least ten times R2.
- The divider current is the supply over R1 + R2. Large resistors save power but make the output more sensitive to loading and noise.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (V) | Local step | Change in result |
|---|---|---|---|---|---|---|
| R2 Lower resistor | 1 to 100 kΩ | 1.091 | 10.91 | 9.818 | +10% | +6.45 % |
| R1 Upper resistor | 1 to 100 kΩ | 0.5714 | 10 | 9.429 | +10% | −6.25 % |
| Vsupply Supply voltage | 3 to 24 V | 1 | 8 | 7 | +10% | +10 % |
Sweep of Lower resistor
| Lower resistor (kΩ) | Output voltage (V) |
|---|---|
| 1 | 1.091 |
| 1.778 | 1.812 |
| 3.162 | 2.883 |
| 5.623 | 4.319 |
| 10 | 6 |
| 17.78 | 7.681 |
| 31.62 | 9.117 |
| 56.23 | 10.19 |
| 100 | 10.91 |
Sweep one variable
Variables and how each one contributes
Output voltage
What the equation gives you. Everything else feeds it.
Supply voltage
Voltage across the two resistors in series. Directly proportional: the output follows the supply at a fixed ratio.
Upper resistor
Resistor between the supply and the output. Raising R1 lowers the output, because it takes a larger share of the supply.
Lower resistor
Resistor the output is measured across. Raising R2 raises the output, with diminishing effect once R2 is much larger than R1.
Worked example with the values above
- Total resistance: R1 + R2 = 10 + 5 = 15 kΩ.
- Fraction across R2: R2/(R1 + R2) = 5 / 15 = 0.3333.
- Vout = Vsupply × 0.3333 = 4 V.
Assumptions and limits
- Output unloaded: no current drawn from the junction.
- Ideal resistors at constant temperature.
- Steady (DC) supply, or AC where the same resistance ratio applies.
Common mistakes
- Putting R1 on top of the fraction. The resistor the output is measured across goes on top.
- Ignoring a load connected to the output, which changes the ratio.
- Mixing ohms and kilohms between R1 and R2.
Statistics and uncertainty
Resistor tolerance sets the accuracy of the output. With k = R2/(R1 + R2), the error in Vout is δV/V = (1 − k)(δR2/R2 − δR1/R1). At a ratio of one third, two ±1% resistors give a worst-case error of about ±1.3%.
Where it is used
- Scaling a sensor or battery voltage down for an ADC input.
- Biasing transistors and amplifiers.
- Reading a variable resistor (potentiometer) as a position or level.
Maths you need
- Fractions and ratios: Vout is a fraction of the supply
- Ohm’s law: the same current flows through both resistors in series