Voltage Divider Calculator: Circuit 3 (Calculate Vo)

Calculate the output voltage (Vo) of a series resistive voltage divider circuit.

Formula Used

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

What is a Voltage Divider Circuit?

A voltage divider, also known as a potential divider, is a simple and fundamental electronic circuit used to produce an output voltage (Vo) that is a fraction of its input voltage (Vin). This is achieved by distributing the input voltage across two or more components, most commonly resistors, connected in series. The most basic form consists of two series resistors, as used in this calculator. This configuration is often generically referred to as “circuit 3” in educational contexts. The main purpose is to scale down a higher voltage to a lower, desired level, which is a frequent requirement in circuit design.

The Voltage Divider Formula (for Circuit 3)

To calculate the output voltage (Vo) across the second resistor (R2) in a simple two-resistor series circuit, the following formula is used. This formula is derived directly from Ohm’s Law.

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

This equation shows that the output voltage is directly proportional to the input voltage and the ratio of the resistors.

Formula Variables

Variable	Meaning	Unit	Typical Range
Vo	Output Voltage	Volts (V)	0 to Vin
Vin	Input/Source Voltage	Volts (V)	mV to kV
R1	Resistor 1 (closer to Vin)	Ohms (Ω)	1 Ω to 10 MΩ
R2	Resistor 2 (closer to Ground)	Ohms (Ω)	1 Ω to 10 MΩ

Practical Examples

Example 1: Powering an LED

Imagine you have a 5V power source but need to provide a voltage of around 2V for a small LED. You can use a voltage divider to achieve this.

• Inputs: Vin = 5V, R1 = 3000 Ω, R2 = 2000 Ω
• Calculation: Vo = 5 * (2000 / (3000 + 2000)) = 5 * (2000 / 5000) = 5 * 0.4
• Result: Vo = 2V. This lower voltage is safer for the LED.

Example 2: Reading a Sensor

Many microcontrollers can only handle input voltages up to 3.3V or 5V. If you have a sensor that outputs a signal up to 12V, you must scale it down. This is a common practical application of a voltage divider.

• Inputs: Vin = 12V (from sensor), R1 = 8700 Ω, R2 = 3300 Ω
• Calculation: Vo = 12 * (3300 / (8700 + 3300)) = 12 * (3300 / 12000) = 12 * 0.275
• Result: Vo = 3.3V. This voltage is now safe for the microcontroller’s input pin.

How to Use This Voltage Divider Calculator

1. Enter Source Voltage (Vin): Input the total voltage supplied to the series circuit.
2. Enter Resistor 1 (R1): Input the value in Ohms for the resistor connected between Vin and the output tap.
3. Enter Resistor 2 (R2): Input the value in Ohms for the resistor across which you are measuring the output voltage (Vo).
4. Interpret Results: The calculator instantly provides the output voltage (Vo), total circuit resistance, circuit current, and the voltage drop across R1. The chart also updates to give a visual comparison. For help with component selection, you might consult a guide on how to choose resistor values.

Key Factors That Affect Output Voltage (Vo)

Source Voltage (Vin)

The output voltage Vo is directly proportional to Vin. If you double Vin, Vo will also double, assuming the resistors remain the same.

Resistor Ratio (R2 / (R1+R2))

This is the most critical factor. The ratio of the second resistor to the total resistance determines the fraction of the input voltage that becomes the output.

Magnitude of R1

Increasing R1 will decrease Vo, as a larger portion of the total voltage will be dropped across it.

Magnitude of R2

Increasing R2 will increase Vo, as it makes up a larger portion of the total resistance.

Load Impedance

A critical real-world factor. If you connect another component (a “load”) in parallel with R2, it changes the equivalent resistance of that part of the circuit, which in turn lowers the actual Vo. This calculator computes the “no-load” voltage.

Resistor Tolerance

Real resistors have a manufacturing tolerance (e.g., ±5%). This means their actual resistance can vary, causing the measured Vo to be slightly different from the calculated value. For precision, it’s important to understand the applications of voltage dividers and select low-tolerance resistors.

Frequently Asked Questions (FAQ)

1. What does “Circuit 3” refer to?

In many physics or electronics textbooks, “Circuit 3” is used as a generic label for a standard two-resistor voltage divider in a problem set. It refers to the fundamental circuit topology this calculator is based on.

2. Why is my measured Vo different from the calculated value?

This is usually due to the “loading effect.” The device you use to measure (like a multimeter) has its own internal resistance, which acts as a load. This, along with resistor tolerance, can cause slight deviations.

3. Can I use any resistor values I want?

Theoretically, yes, as long as the ratio is correct. However, in practice, you must consider power dissipation (P = V²/R). Very low resistance values will draw a lot of current, wasting power and generating heat. Very high values can be susceptible to noise and loading effects.

4. What happens if R1 is zero?

If R1 is 0, the formula becomes Vo = Vin * (R2 / R2), so Vo = Vin. The output is directly connected to the input.

5. What happens if R2 is zero?

If R2 is 0, the formula becomes Vo = Vin * (0 / R1), so Vo = 0. The output is connected directly to ground.

6. How do I choose the right resistors for my circuit?

Start with the voltage ratio you need. A common rule of thumb is to ensure the current flowing through the divider (I = Vin / (R1+R2)) is about 10 times the current required by the load you plan to connect to Vo. This minimizes the loading effect. Then select standard resistor values that approximate your required ratio.

7. What is the main purpose of a voltage divider?

Its main purpose is to create a reference voltage or to scale down a signal voltage to a level that is compatible with another component, such as a microcontroller’s analog-to-digital converter (ADC).

8. Does the order of R1 and R2 matter?

Absolutely. The formula assumes R1 is the resistor connected to the positive side of the supply (Vin) and R2 is the resistor connected to ground, with Vo measured across R2. Swapping them will give you the voltage drop across the other resistor.