What Is a Voltage Divider?

A voltage divider is a simple circuit that uses two resistors to produce a lower voltage from a higher voltage.

             R1
Vin ───────/\/\/\──────┬──── Vout
                       │
                       │
                      R2
                       │
                      GND

The output voltage is taken from the junction between R1 and R2.

Voltage Divider Formula

For an unloaded two-resistor voltage divider:

Vout = Vin × R2 / (R1 + R2)

Where:

• Vin = input voltage
• Vout = output voltage
• R1 = upper resistor
• R2 = lower resistor

Voltage Divider Calculator

Example 1 — 12 V to 6 V

Suppose a 12 V supply is connected to two equal resistors:

Vin = 12 V

R1 = 10 kΩ

R2 = 10 kΩ

Because R1 and R2 have the same resistance, the input voltage is divided equally.

Vout = 12 × 10000
       ─────────────
       10000 + 10000

Vout = 6 V

Example 2 — 12 V to Approximately 5 V

Suppose we want approximately 5 V from a 12 V supply.

If R2 is 10 kΩ, we can determine R1 from:

Vout = Vin × R2 / (R1 + R2)

Rearranging:

R1 = R2 × (Vin / Vout - 1)

For 12 V, 5 V and R2 = 10 kΩ:

R1 = 10000 × (12 / 5 - 1)

R1 = 14000 Ω

R1 = 14 kΩ

A practical standard resistor value can then be selected according to the required accuracy and load.

Calculate R1 for a Desired Output Voltage

Calculate R2 for a Desired Output Voltage

If R1 is already known, R2 can be calculated using:

R2 = R1 × Vout / (Vin - Vout)

Divider Current

The current flowing through an unloaded voltage divider can be calculated using the total resistance:

I = Vin / (R1 + R2)

This current flows through both resistors because they are connected in series.

Divider Current Calculator

Resistor Power

Both resistors dissipate power. The power should be checked when selecting resistor values.

For R1:

P1 = I² × R1

For R2:

P2 = I² × R2

The resistor power ratings should provide appropriate margin above the calculated dissipation.

Loaded Voltage Divider

The basic voltage-divider formula assumes that the output is unloaded. If a circuit is connected to Vout, the load introduces an additional resistance.

             R1
Vin ───────/\/\/\──────┬──── Vout
                       │
                      R2
                       │
                       ├──── RL
                       │
                      GND

The load resistance RL effectively appears in parallel with R2.

R2effective = R2 || RL

The voltage divider must then be calculated using R2effective rather than R2 alone.

Loaded Divider Calculator

Why Does the Load Matter?

A voltage divider is not an ideal voltage source. Connecting a load draws current from the divider and changes the output voltage.

For this reason, very high resistor values can be unsuitable when the divider must drive a relatively low-resistance load.

Choosing Voltage Divider Resistor Values

When selecting R1 and R2, consider:

• Required output voltage
• Input voltage
• Required accuracy
• Divider current
• Load resistance
• Resistor power rating
• Resistor tolerance
• Noise sensitivity

High-Value Resistors

Using very large resistor values reduces divider current and therefore reduces power consumption.

However, high resistance also makes the divider more sensitive to loading, leakage currents, noise and measurement effects.

Low-Value Resistors

Using lower resistor values makes the divider less sensitive to some loading effects, but increases current consumption and resistor power dissipation.

The correct values therefore depend on the application.

Voltage Divider for ADC Input

A voltage divider is commonly used to reduce a voltage before feeding it to an analog-to-digital converter.

Higher Voltage
      │
      ▼
    R1
      │
      ├────► ADC Input
      │
    R2
      │
     GND

The divider must be designed so that the ADC input voltage remains within the permitted range.

The ADC input impedance and leakage characteristics should also be considered.

Voltage Divider for Microcontrollers

A voltage divider can be used to measure a higher voltage with a microcontroller ADC, provided the resulting voltage remains within the ADC's allowed input range.

For example, a divider can reduce a battery voltage to a suitable ADC measurement range.

Voltage Divider for Logic Signals

A resistor divider can sometimes be used for reducing a logic signal voltage.

However, it should only be used when the receiving input characteristics and required logic levels are suitable.

For reliable level conversion, a dedicated level-shifting circuit may be preferable.

Voltage Divider and Potentiometer

A potentiometer can be considered as an adjustable voltage divider.

        +V
         │
         │
      ┌───────┐
      │  POT  │
      └───┬───┘
          │
          ├──── Vout
          │
         GND

Moving the wiper changes the ratio of the upper and lower resistance sections and therefore changes the output voltage.

Voltage Divider Errors

The actual output voltage can differ from the ideal calculation because of:

• Resistor tolerance
• Load resistance
• Input leakage
• Temperature coefficient
• Supply voltage variation
• Measurement error

Common Voltage Divider Mistakes

• Using the wrong resistor position.
• Forgetting that the load is in parallel with R2.
• Using excessively high resistor values for a loaded circuit.
• Ignoring resistor tolerance.
• Ignoring resistor power dissipation.
• Applying a voltage higher than the receiving circuit can tolerate.
• Assuming the divider output behaves like an ideal voltage source.

Voltage Divider Troubleshooting

If the measured output voltage differs from the calculated value, check:

1. Input voltage.
2. Actual resistor values.
3. Resistor tolerance.
4. Load connected to the output.
5. Wiring.
6. Ground connection.
7. Measurement instrument.

Voltage Divider Faults

Key Points

• A voltage divider uses two resistors to produce a fraction of an input voltage.
• The output is normally measured across the lower resistor R2.
• The basic formula is Vout = Vin × R2 / (R1 + R2).
• The divider output depends on the connected load.
• Lower resistor values increase divider current.
• Higher resistor values reduce current but increase sensitivity to loading and leakage.
• Resistor tolerance affects the actual output voltage.
• Voltage dividers are commonly used with ADC inputs.
• A potentiometer is an adjustable voltage divider.
• Resistor power dissipation should be checked during circuit design.