Designing Voltage Divider Circuits: A Step-by-Step Guide

Voltage divider circuits are fundamental components in electronics, given their usefulness for bringing down voltage to a desired level. Sure enough, voltage dividers see widespread use for such purposes as the creation of reference voltages, the scaling-down of signals for microcontrollers (like reading sensors), and the biasing of transistors. These circuits are relatively straightforward to […]

Voltage divider circuits are fundamental components in electronics, given their usefulness for bringing down voltage to a desired level.

Sure enough, voltage dividers see widespread use for such purposes as the creation of reference voltages, the scaling-down of signals for microcontrollers (like reading sensors), and the biasing of transistors.

These circuits are relatively straightforward to design, given that they typically require just two resistors. But of course, there is much more you will need to know about the design process than this. So, in this guide, we have sought to outline the crucial steps.

Firstly… What Actually Is a Voltage Divider Circuit?

Before we begin explaining the step-by-step design process, let’s be absolutely clear what a voltage divider circuit is.

In summary, a voltage divider is a simple circuit that uses two or more resistors (R₁, R₂, etc…) in series to create an output voltage (V_out) that is a fraction of the input voltage (V_in).

This kind of circuit involves the total voltage (V_in) being divided between the two resistors, based on their resistance values. The output voltage (V_out) is taken from the point between the two resistors.

The formula for the output voltage across R₂ is: V_out = V_in x R₂ / (R₁ + R₂).

It is also important for those designing voltage dividers to understand the concept of the resistance ratio. In the event of R₂ being larger than R₁, the output voltage will be higher. If, though, R₁ and R₂ are equal, the output voltage will be half of the input voltage.

The 5 Steps of Designing a Voltage Divider Circuit

Follow these steps to design a voltage divider circuit that suits your needs:

Step 1: Define Your Requirements

Prior to choosing components, you will need to know these three key values:

• The input voltage (V_in), the source voltage you are starting with (for example, a 9V battery).
• The required output voltage (V_out), the target voltage you require for your load (such as 3V for a sensor).
• The maximum current draw (I_max), which is the current your load or the rest of your circuit requires.

Step 2: Calculate The Required Resistor Ratio

Rearranging the aforementioned formula can enable you to find the ratio needed between R₁ and R₂: V_out / V_in = R₂ / (R₁ + R₂).

Inputting the numbers we suggested above would give us a calculation of 3V / 9V = 1 / 3. This tells us that the resistance of R₂ must be one-third of the total resistance (R₁ + R₂).

Remember that this calculation doesn’t necessarily need to be done manually. You can work out ratios using the voltage divider circuit calculator on the RS website, for instance. This could help you speed up the process and verify the accuracy of your manual calculations.

Step 3: Select Appropriate Resistor Values (Choosing Total Resistance)

Although the ratio is fixed by your voltages, you will have a certain amount of flexibility when setting the actual values of R₁ and R₂. The total resistance (R₁ + R₂) determines how much total current the divider draws from the source.

• Low resistance values (for example, R₁ = 100Ω and R₂ = 100Ω) draw a greater amount of current. Such resistance values are associated with higher power consumption and heat, but also less susceptibility to noise and load changes.
• High resistance values (such as R₁ = 1 megaohms, or MΩ, and R₂ = 1MΩ) draw a minimal amount of current. This lends itself to better battery life, but it may introduce noise. Furthermore, it may be easily affected by the input impedance of the connected load.

A common “rule of thumb” is that the current flowing through the divider should be a minimum of 10 times greater than the current drawn by the load it is supplying.

So, let’s go back to our 9V-to-3V example, and aim for a total current of around 1 milliamperes (mA). The relevant calculation for total resistance would be R_total = V_in/I_total = 9V/0.001A = 9000Ω (or 9kΩ).

Step 4: Calculate Individual Resistor Values

For this step, we need to use the ratio from step two, and the total resistance from step three:

• Calculate R₂ with the formula: R₂ = 9kΩ x 1/3 = 3kΩ.
• Calculate R₁ with the formula: R₁ = R_total – R₂ = 9kΩ – 3kΩ = 6kΩ.

Step 5: Select Standard Resistor Values

Again, presuming you get the above numbers from your calculations, the chances are that you won’t find exact 6kΩ and 3kΩ resistors already available in standard E-series ranges in the UK.

So, you will probably need to choose the closest standard values you can get for your voltage divider circuit. Using standard values will slightly alter your output voltage, so it may be necessary to recalculate to make sure the new V_out is within your tolerance.

Wrapping Up: May You Thrive with Your Voltage Divider Projects!

Voltage dividers are unquestionably among the “unsung heroes” of electronics, thanks to their combination of affordability, simplicity, and usefulness.

By following this step-by-step guide carefully, you can help build your self-confidence with the process of designing such circuits, whether for a school project, a commercial prototype, or any other application.