What is a Voltage Divider?
A voltage divider is a simple passive linear circuit that produces an output voltage (Vout) that is a fraction of its input voltage (Vin). It is constructed using two resistors (or other impedances) connected in series.
This circuit is fundamental to electronics. Whether you are biasing a transistor, reading a sensor with an Arduino, or setting a reference level for an Op-Amp, you are likely using the voltage divider rule. It allows engineers to step down a high voltage to a lower, usable level using just two components.
How to Calculate Voltage Drop (The Formula)
The voltage divider formula is derived directly from Ohm's Law (V=IR). In a series circuit, the current is constant, so the voltage is dropped across each resistor proportional to its resistance.
Unloaded Voltage Divider Formula
When no external load is connected to the output, the calculation is straightforward:
Vout = Vin × [ R2 / (R1 + R2) ]
Where:
- Vin: Source Voltage
- R1: Upper Resistor (connected to Vin)
- R2: Lower Resistor (connected to Ground)
Loaded Voltage Divider
When you connect a load (RL) across R2 (e.g., plugging in a sensor), it acts in parallel with R2. This lowers the effective resistance of the bottom half, causing the output voltage to drop. This is a common pitfall called the "loading effect."
Req = (R2 × RL) / (R2 + RL)
Vout(loaded) = Vin × [ Req / (R1 + Req) ]
Practical Applications
Voltage dividers are used everywhere in analog and digital electronics:
- Sensor Reading: Resistive sensors like LDRs (Light Dependent Resistors) or Thermistors change resistance with the environment. By placing them in a voltage divider, a microcontroller can read the changing voltage to measure light or temperature.
- Level Shifting: Interfacing a 5V sensor with a 3.3V logic microcontroller (like an ESP32 or Raspberry Pi) requires stepping down the signal voltage to avoid damaging the pin.
- Potentiometers: A volume knob is essentially a variable voltage divider that adjusts the signal level fed into an amplifier.
What is a Voltage Divider?
A voltage divider is a simple linear circuit that produces an output voltage (Vout) that is a fraction of its input voltage (Vin). It is created using two resistors connected in series.
Vout = Vin × (R2 / (R1 + R2))
Common Application: Logic Level Shifting
Many modern sensors and microcontrollers (like ESP32 or Raspberry Pi) operate at 3.3V, while older systems (like Arduino Uno) operate at 5V. Connecting a 5V output directly to a 3.3V input can destroy the chip.
A voltage divider acts as a "logic level shifter" to safely drop that 5V signal down to 3.3V. Using a 1kΩ (R1) and 2kΩ (R2) resistor is a common way to achieve a safe 3.33V signal. You can simulate how these 3.3V signals interact with complex gates in our new Logic Design Sandbox.
[Image showing 5V to 3.3V logic level shifting with a voltage divider]Loaded vs. Unloaded Dividers
The standard formula assumes there is no load connected to Vout. If you connect a device that draws significant current, that device acts as a third resistor in parallel with R2, which causes the output voltage to "sag" or drop. Our calculator allows you to input a Load Resistance to see this real-world effect.
FAQ
How to calculate voltage drop?
To calculate the voltage drop across a specific resistor in a series circuit, use the formula: Vdrop = Vtotal × (R / Rtotal). Alternatively, if you know the current, subtract V_out from V_in to find the drop across the first resistor (R1).
Does the load affect Vout?
Yes, significantly. Any load connected to the output is electrically in parallel with the bottom resistor (R2). This reduces the total resistance between the output node and ground, pulling the voltage down. To minimize this, the load resistance should be at least 10x larger than R2.
Can I use a voltage divider as a power supply?
No. Voltage dividers are very inefficient for power delivery. The resistors dissipate power as heat (I²R losses), and the output voltage will sag heavily if the load draws varying amounts of current. Use a linear regulator (like an LM317) or a buck converter for power supplies.