Electronics

Voltage Divider Calculator

Calculate voltage divider output or solve for any unknown resistor. Enter input voltage and two known values to find the third.

Quick Answer

V_out = V_in× R2 / (R1 + R2). Two equal 10kΩ resistors on a 12V supply give 6V output. The ratio R2/(R1+R2) determines the fraction of the input voltage that appears at the output.

Solve For

Input Voltage (V)

R1 (Ω)

R2 (Ω)

Circuit:

V_in ──[ R1 ]──┬── V_out
│
[ R2 ]
│
GND

Results

6.0000 V

Output Voltage

0.600 mA

Current Draw

0.5000

Divider Ratio

7.20 mW

Total Power

About This Tool

A voltage divider is one of the most fundamental circuits in electronics. It uses two resistors in series to produce an output voltage that is a fraction of the input voltage. This calculator handles all three modes: solving for the output voltage when both resistors are known, or solving for either resistor when the desired output voltage is specified.

The Voltage Divider Formula

The output voltage is V_out = V_in× R2 / (R1 + R2). This formula assumes no load on the output, meaning whatever you connect draws negligible current compared to the current flowing through the divider itself. In practice, the load impedance should be at least 10 times R2 to keep the output voltage within about 10% of the unloaded value.

Common Uses

Voltage dividers appear everywhere in electronics. They create reference voltages for comparators and ADCs, set bias points in transistor circuits, read sensors that change resistance (thermistors, photoresistors, strain gauges), and level-shift signals between circuits running at different voltages. A potentiometer is essentially a variable voltage divider in a single package.

Loading Effects

The ideal voltage divider formula assumes zero current at the output tap. Real loads draw current, which effectively places a resistance in parallel with R2 and lowers the output voltage. To minimize loading effects, keep the divider resistances low compared to the load resistance. However, lower resistances mean higher current and more power waste. There is always a design tradeoff between accuracy and efficiency.

Choosing Resistor Values

Start by choosing the desired output voltage and divider ratio, then pick resistor values that produce reasonable current draw. For signal-level dividers, 10k to 100k ohm resistors are common. For power supply dividers feeding an ADC, 1k to 10k ohms may be appropriate. Always check the power dissipation in each resistor and ensure it stays within the resistor rating.

Frequently Asked Questions

What is a voltage divider used for?

Voltage dividers create a specific fraction of an input voltage. Common uses include reading sensor values with microcontrollers, setting reference voltages for comparators, biasing transistors, and level-shifting signals between 5V and 3.3V logic systems.

Can I use a voltage divider as a power supply?

Not recommended. Voltage dividers can't regulate voltage under changing loads. When the load draws more current, the output voltage drops. For powering devices, use a voltage regulator (linear or switching) instead. Voltage dividers work best for signal conditioning with high-impedance loads.

What happens when I connect a load to the output?

The load resistance appears in parallel with R2, reducing the effective R2 value and lowering the output voltage. To minimize this effect, the load resistance should be at least 10 times R2. If your load resistance is comparable to R2, you need to account for it in the calculation.

How do I choose between high and low resistance values?

Lower resistances provide a stiffer voltage (less affected by loads) but waste more power. Higher resistances save power but are more sensitive to loading and noise pickup. For battery-powered circuits, use higher values. For precision measurements, use lower values.

Can voltage dividers step up voltage?

No. A resistive voltage divider can only produce an output voltage lower than or equal to the input voltage. To step up voltage, you need an active circuit like a boost converter or charge pump.

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