Use the Resistor Voltage Drop Calculator to quickly determine the voltage dissipated across each resistor in a simple series circuit or voltage divider configuration, confirming Kirchhoff’s Voltage Law.
Resistor Voltage Drop Calculator
Resistor Voltage Drop Calculator Formula
The calculation is based on the fundamental Voltage Divider Rule, which is derived from Ohm’s Law and Kirchhoff’s Voltage Law.
$$V_{Rx} = V_{S} \cdot \frac{R_{x}}{R_{1} + R_{2}}$$
Where $V_{Rx}$ is the voltage drop across the resistor of interest ($R_{1}$ or $R_{2}$).
Formula Sources:
All About Circuits: Voltage Divider Circuits |
Electronics Tutorials: Series Circuits
Variables
The calculator requires three primary inputs to solve for the individual voltage drops and current:
- Source Voltage ($V_S$): The total voltage supplied by the power source (e.g., a battery). Measured in Volts (V).
- Resistor 1 Value ($R_1$): The resistance of the first resistor in the series circuit. Measured in Ohms ($\Omega$).
- Resistor 2 Value ($R_2$): The resistance of the second resistor in the series circuit. Measured in Ohms ($\Omega$). This resistor is often where the output voltage is taken (for a voltage divider).
Related Calculators
What is a Resistor Voltage Drop?
A voltage drop occurs when the electrical potential energy is dissipated as current flows through a component, such as a resistor. In any closed loop circuit, the sum of all voltage drops must equal the source voltage, a principle known as Kirchhoff’s Voltage Law (KVL). Resistors are components specifically chosen to convert electrical energy into heat, thus creating a predictable and desired voltage drop, which is essential for applications like current limiting and establishing reference voltages.
The size of the voltage drop across a specific resistor in a series circuit is directly proportional to its resistance value relative to the total resistance in the series path. In simpler terms, the resistor with the largest resistance value will “take” the largest share of the source voltage.
How to Calculate Voltage Drop (Example)
- Identify Inputs: Assume a Source Voltage ($V_S$) of 12V, $R_1 = 100 \Omega$, and $R_2 = 50 \Omega$.
- Calculate Total Resistance ($R_{total}$): Since the resistors are in series, $R_{total} = R_1 + R_2 = 100 \Omega + 50 \Omega = 150 \Omega$.
- Calculate Total Current ($I_{total}$): Use Ohm’s Law: $I_{total} = V_S / R_{total} = 12 V / 150 \Omega = 0.08 A$.
- Calculate Voltage Drop across $R_2$ ($V_{R2}$): $V_{R2} = I_{total} \cdot R_2 = 0.08 A \cdot 50 \Omega = 4 V$.
- Calculate Voltage Drop across $R_1$ ($V_{R1}$): $V_{R1} = I_{total} \cdot R_1 = 0.08 A \cdot 100 \Omega = 8 V$.
- Verify: Check KVL: $V_{R1} + V_{R2} = 8 V + 4 V = 12 V = V_S$. The drop across R2 is 4V.
Frequently Asked Questions (FAQ)
What is the difference between voltage drop and output voltage?
Voltage drop is the voltage *dissipated* across a component. Output voltage (often $V_{out}$) is the voltage *available* at a specific point in the circuit, usually measured from that point to the ground reference. In a standard voltage divider, the voltage drop across the second resistor ($R_2$) is typically the same as the output voltage.
Why is the voltage drop higher across a larger resistor?
According to Ohm’s Law ($V = I \cdot R$), when the current ($I$) is constant (as it is in a series circuit), the voltage ($V$) is directly proportional to the resistance ($R$). Therefore, the resistor with the higher value will incur a proportionally larger portion of the total source voltage drop.
Can the total voltage drop be greater than the source voltage?
No. According to Kirchhoff’s Voltage Law (KVL), the sum of all voltage drops in a closed loop must be exactly equal to the source voltage. If your calculation or measurement suggests a drop greater than the source, there is likely an error in the circuit analysis or measurement.
Does a switch or wire have a voltage drop?
Ideally, wires and switches have zero resistance and thus zero voltage drop. In practical terms, they possess a tiny resistance, resulting in a small, non-zero voltage drop, which is usually considered negligible in most circuit designs.