Voltage Divider Calculator
Calculate Output Voltage (Vout), Source Voltage (Vs), or Resistances (R1, R2).
Whether you are biasing a transistor, reading a sensor with an Arduino, or simply trying to drop a 9V battery down to 5V, the voltage divider is likely the first circuit you’ll need.
What is a Voltage Divider?
A voltage divider is a passive linear circuit that produces an output voltage (Vout) that is a fraction of its input voltage (Vin). It distributes the input voltage among two or more components (usually resistors) connected in series.
In its simplest form two resistors connected in series the input voltage is applied across the pair, and the output voltage is measured from the connection point between them relative to ground.
The Voltage Divider Formula
The calculation relies on Ohm’s Law and Kirchhoff’s Voltage Law. The governing equation for an unloaded voltage divider is:
Where:
- Vin (or VS): The source voltage (Input).
- R1: The resistor connected to the voltage source.
- R2: The resistor connected to the ground.
- Vout: The voltage drop across R2.
Rearranging for Resistance
Often, you know the voltage you have and the voltage you want, and you need to find the resistance values.
Practical Applications
1. Logic Level Shifting
Modern electronics often mix voltage levels. You might have a sensor outputting 5V logic, but your microcontroller (like an ESP32 or Raspberry Pi) is 3.3V tolerant. Connecting 5V directly could destroy the pin. A voltage divider drops that 5V signal down to a safe 3.3V.
2. Reading Resistive Sensors
Microcontrollers read voltage, not resistance. To read a resistive sensor (like a photoresistor, thermistor, or flex sensor), you place it in a voltage divider configuration with a fixed resistor. As the sensor’s resistance changes, the V_out changes, allowing the microcontroller’s Analog-to-Digital Converter (ADC) to read the data.
3. Reference Voltages
You can use dividers to create a specific reference voltage for comparators or op-amps. Note: This is only stable if the device drawing the current has a high input impedance (see “The Loading Effect” below).
Critical Engineering Concepts
The “Loading Effect”
A common beginner mistake is assuming V_out stays constant regardless of what you connect to it. This is false.
The standard formula assumes an unloaded circuitmeaning nothing is drawing current from the output. However, as soon as you connect a “load” (like a motor, an LED, or a circuit input) in parallel with R2, the effective resistance of the bottom half of the circuit decreases.
Rule of Thumb: To minimize this error without using an op-amp buffer, the load resistance (R_load) should be at least 10 times greater than R_2.
Power Dissipation & Safety
Resistors have physical limits. A standard through-hole resistor is typically rated for 1/4 Watt (0.25W). If you use low-value resistors with a high voltage source, current will spike, and the power dissipated as heat will exceed this rating, causing the resistor to burn out.
Always check the P_1 and P_2 values in the “Additional Results” section above.
- If Power < 0.25W: Standard resistors are safe.
- If Power > 0.25W: You need higher wattage resistors (e.g., 1W or 5W).
FAQs
Q1. Why can’t I use a voltage divider as a power supply?
A: Voltage dividers are incredibly inefficient for regulating power. The resistors dissipate energy as heat (waste). Furthermore, because of the loading effect mentioned above, the output voltage will fluctuate wildly as the current draw of your load changes. For power supplies, use a voltage regulator (linear or switching) instead.
Q2. Does it matter which resistor is R1 and which is R2?
A: Yes, absolutely. R1 is the “top” resistor connected to the positive source. R2 is the “bottom” resistor connected to the ground. If you swap them, you invert the ratio, likely resulting in a voltage much higher or lower than intended.
Q3. What resistor values should I choose?
- Too Low (e.g., 10Ω): High current flow. This wastes battery life and generates excessive heat, potentially burning out components.
- Too High (e.g., 1MΩ): The circuit becomes high-impedance, making it sensitive to electrical noise/interference. The “loading effect” also becomes severe, leading to inaccurate voltage readings.
- The Sweet Spot: For most 3.3V – 12V logic circuits, resistor values in the 1kΩ to 100kΩ range are ideal.
Sources: Ohms Law Calculator, DigiKey Electronics, Omni Calculator, All About Circuits, Texas Instruments (TI), Circuit Digest, Keysight Technologies, Damien Douxchamps (Electronics Blog), Voltage Disturbance, RapidTables.