Voltage Divider Calculator

Introduction

This post includes two calculator tools

  • The first finds the voltage drop across each resistor in a voltage divider network consisting of two resistors.
  • The second calculates the resistor values for a specified voltage value between the two resistors. Use this when you want to limit the voltage into an input device.

Calculator 1

Enter

  • Input Voltage Vin
  • R1 and R2 in milliOhm/Ohm/kiloOhm/megaOhm

The tool will calculate voltage drop V1 across R1 and V2 across R2.

Formula

Ohm’s law is used to calculate the voltage drops. The current through each series resistor is the same as the total current through the circuit.

Therefore the total current Itotal is given by

Itotal = Vin/(R1+R2) = V1/R1

V1=Vin*R1/(R1+R2)

Itotal = Vin/(R1+R2) = V2/R2

V2=Vin*R2/(R1+R2)

The largest voltage drop will be across the larger resistor value. Conversely, the smallest voltage drop will be across the smaller.

If you want to calculate the voltage drop across a single resistor set R2=0 and from the equations above, V1 = Vin and V2 = 0 (which makes sense).

Example Calculation

With R1 = 1Ī©, R2 = 3Ī©, Vin = 12V, the voltages

  • V1 = 3V
  • V2 = 9V

The sum of the two voltages = the input voltage. As a check you can see that 3+9 = 12V

Calculator 2

This tool calculates the resistor values for a specified output voltage Vout.

Enter

  • Input voltage Vin
  • Desired voltage Vout
  • Either R1 or R2 in milliOhm/Ohm/kiloOhm/megaOhm.

The tool will calculate the other resistor value.

Formula

Ohm’s law is used to calculate the voltage drop across each resistor. The current through each series resistor is the same as the total current through the circuit.

Therefore the total current Itotal is given by

Itotal = Vin/(R1+R2)

Itotal = Vin/(R1+R2) = Vout/R2

Vout=Vin*R2/(R1+R2)

Example Calculation

Let’s say we want to limit the voltage into a device to a maximum of +3V. For Vin = +5 V and R1 selected to be 2.2 kĪ©

How to Pick Resistor Values

In general, there are many combinations of resistor values that will give the same result. For instance,

  • R1 = 2.2 kĪ© and R2 = 3.3 kĪ©
  • R1 =2.2 MĪ© and R2 = 3.3 MĪ©

Both combinations provide +3 Volt at the output.

In some cases there might be a power or an input impedance constraint (to comply with settling time of an ADC for example). Use the following calculators to see if your chosen impedance values meet the requirements:

Background

What is a voltage divider?

A voltage divider is an electrical circuit used to divide a voltage into smaller, proportional voltages. It consists of two or more resistors connected in series or in series-parallel combinations.

The main purpose of a voltage divider is to obtain a specific output voltage that is a fraction of the input voltage.

The basic principle behind a voltage divider is Ohm’s Law, which states that the voltage (V) across a resistor is directly proportional to the current (I) flowing through it and inversely proportional to its resistance (R). Mathematically, Ohm’s Law is expressed as:

V = I * R

In a voltage divider, when you apply an input voltage (Vin) across resistors connected in series, the total resistance in the circuit determines the current flowing through it.

The voltage across each resistor is then proportional to its resistance compared to the total resistance. This allows you to calculate the voltage at a specific point in the divider.

The voltage across a specific resistor in a voltage divider can be calculated using the following formula:

Vout = Vin * (R2 / (R1 + R2))

Where:

  • Vout is the output voltage across the resistor R2.
  • Vin is the input voltage applied across the entire divider.
  • R1 is the resistance of the first resistor in the series.
  • R2 is the resistance of the second resistor in the series.

Applications

Voltage dividers are commonly used in electronics for various purposes, such as:

  1. Setting reference voltages: They can provide a stable and precise output voltage for use as a reference in analog circuits or to bias components like transistors.
  2. Sensor interfacing: Voltage dividers are used to scale down the output voltage of sensors (e.g., temperature sensors, light sensors) to a range suitable for analog-to-digital converters (ADCs).
  3. Level shifting: Voltage dividers can be used to shift the voltage levels of signals to match the input requirements of other components or microcontrollers. Try the Arduino voltage divider
  4. Biasing components: In amplifier circuits, voltage dividers can be used to bias transistors or other active components to the desired operating point.

It’s important to note that while voltage dividers are simple and widely used, they have limitations, including sensitivity to changes in load resistance and current draw, which can affect their accuracy. Care must be taken when designing voltage dividers to ensure that they operate within the desired parameters and are not subject to significant variations in voltage due to external factors.

Practical Notes

Practical resistor values are never exact.

Resistor tolerance is a specification that indicates the allowable deviation or variation in resistance value from the nominal or specified resistance of a resistor. It is typically expressed as a percentage and helps users understand the range within which the actual resistance of the resistor may fall compared to its rated or marked value.

Use this tool to calculate Resistor Tolerance

For example, if you have a 100-ohm resistor with a tolerance of Ā±5%, it means that the actual resistance of the resistor can be anywhere within the range of 95 ohms to 105 ohms (100 ohms Ā± 5%).

Once you have calculated resistor values in the second calculator for a specified output voltage, plug the min and max values (95 ohm and 105 ohm in the example above) into the first calculator to study the variation of Vout practically.

Does the max output voltage value exceed that allowed by the circuit that it’s feeding?

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