Voltage Divider Calculator
Calculate output voltage, resistor values, current draw, and power dissipation for resistor voltage divider circuits.
Voltage Divider Calculator
Calculate Vout or find R2 for desired output
Use k for kilo, M for mega (e.g., 10k, 1M)
Calculated Results
Output Voltage
3.3669 V
Divider Ratio
28.06%
Current
863.3094 µA
Power (R1)
7.4530 mW
Power (R2)
2.9067 mW
Verification: Vout = 12 × 4kΩ / (10kΩ + 4kΩ) = 3.3669V ✓
Tips for Voltage Divider Design
- • Keep divider impedance (R1+R2) much lower than load impedance for accuracy
- • Use high-value resistors (10kΩ-100kΩ) for low power consumption
- • For precision applications, use 1% tolerance resistors
- • Add a buffer amplifier if driving any significant load
Voltage Divider Formulas
Output Voltage (Vout)
Vout = Vin × R2 / (R1 + R2)
Output voltage is a fraction of input, determined by the resistor ratio.
Find R2 (from desired Vout)
R2 = R1 × Vout / (Vin - Vout)
Calculate R2 when you know desired output voltage.
Current Through Divider
I = Vin / (R1 + R2)
Same current flows through both resistors (series circuit).
Power Dissipation
P(R1) = I² × R1
P(R2) = I² × R2
Ensure resistors can handle the power dissipation.
Understanding Voltage Dividers
What is a Voltage Divider?
A voltage divider (also called a potential divider) is one of the most fundamental circuits in electronics. It consists of two resistors connected in series between a voltage source and ground. The output voltage is taken from the junction between the two resistors.
How Does It Work?
The voltage divider works based on Ohm's Law. Since the two resistors are in series, the same current flows through both. The voltage drops across each resistor proportionally to its resistance. The output voltage is the voltage across the bottom resistor (R2).
Common Applications
- ADC Input Scaling: Reduce a higher voltage to match the ADC's input range (e.g., 12V to 3.3V for a microcontroller)
- Sensor Interfaces: Create reference voltages or scale sensor outputs
- Level Shifting: Interface between different voltage domains
- Voltage Sensing: Monitor higher voltages with low-voltage circuits
- Potentiometer Circuits: A potentiometer is essentially a variable voltage divider
Design Considerations
Loading Effect
When you connect a load to the output of a voltage divider, the load resistance appears in parallel with R2. This reduces the effective resistance and changes the output voltage. To minimize loading effects:
- Use low-value resistors (but consider power dissipation)
- Ensure load impedance is at least 10× higher than R2
- Use a buffer amplifier (op-amp voltage follower) after the divider
Power Dissipation
The voltage divider constantly draws current, even with no load. This current causes power dissipation in both resistors. For battery-powered applications, use high-value resistors (100kΩ range) to minimize current draw, but be aware of loading effects and noise.
Example Calculation
Problem: Create a 3.3V reference from 12V
Design a voltage divider to output 3.3V from a 12V supply, using 10kΩ for R1.
Given: Vin = 12V, Vout = 3.3V, R1 = 10kΩ
R2 = R1 × Vout / (Vin - Vout)
R2 = 10kΩ × 3.3V / (12V - 3.3V)
R2 = 10kΩ × 3.3 / 8.7
R2 ≈ 3.79kΩ (use 3.9kΩ standard value)
With 3.9kΩ: Vout = 12 × 3.9 / (10 + 3.9) = 3.37V ✓
Frequently Asked Questions
What is a voltage divider?
A voltage divider is a simple circuit that uses two resistors in series to produce an output voltage that is a fraction of the input voltage. The output is taken from the junction between the two resistors. It's commonly used to create reference voltages, interface sensors with ADCs, and reduce voltage levels.
What is the voltage divider formula?
The voltage divider formula is: Vout = Vin × (R2 / (R1 + R2)), where Vin is the input voltage, R1 is the top resistor (connected to Vin), R2 is the bottom resistor (connected to ground), and Vout is measured at the junction between R1 and R2.
How do I choose resistor values for a voltage divider?
Choose resistor values based on: 1) Desired output voltage ratio, 2) Acceptable current draw (lower resistances = more current = more power wasted), 3) Load impedance (divider impedance should be much lower than load for accuracy). Common practice is to use 1kΩ to 100kΩ resistors for signal conditioning.
Why does my voltage divider output change with load?
When you connect a load to a voltage divider, the load resistance appears in parallel with R2, effectively reducing R2's value and changing the output voltage. To minimize this effect, use resistor values much smaller than the load resistance (typically 10x smaller), or use a buffer amplifier.
What is the maximum power a resistor can handle in a voltage divider?
Standard through-hole resistors are typically rated for 1/4W (250mW) or 1/2W (500mW). Calculate power as P = I²R for each resistor, and ensure you stay below 50-70% of the rated power for reliability. For high-power applications, use appropriately rated resistors or multiple resistors in parallel/series.
Can I use a voltage divider to power a circuit?
Voltage dividers are generally not suitable for powering circuits because their output voltage drops significantly under load. For power supply applications, use a voltage regulator (linear or switching) instead. Voltage dividers are best suited for signal conditioning, reference voltages with minimal current draw, or sensing applications.
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