Series Resistor Calculator

In a series circuit, current flows through each resistor in sequence. The total resistance is simply the sum: Rtotal = R1 + R2 + ... + Rn. The same current flows through all resistors, and each one drops a share of the supply voltage proportional to its resistance.

The voltage divider is the most important application of series resistors. Vout = Vin × R2/(R1+R2) lets you create any fraction of the input voltage. Series resistors also appear as current limiters (for LEDs and inputs), protective series resistors for op-amp inputs, and impedance-matching elements.

Power must be considered when sizing resistors. Each resistor dissipates P = I² × R watts. Make sure the total power across all resistors in a chain is within the power ratings. Higher-wattage resistors (¼W, ½W, 1W) are required for higher currents.

Key Points

• Rtotal = R1 + R2 + ... + Rn — always larger than any single resistor
• Same current flows through all: I = Vin / Rtotal
• Voltage divides proportionally: Vn = Vin × Rn / Rtotal
• Power: P = I² × R — check each resistor rating

Applications

• Current limiting for LEDs and sensor inputs
• Voltage divider for ADC input scaling
• RC filter component design
• Pull-up and pull-down resistor networks