Resistor Calculator
Calculate total resistance for series and parallel combinations. Enter 2-4 resistor values and optional voltage to calculate current, voltage drops, and power dissipation.
What is the formula for resistors in series?
For resistors in series, the total resistance is the sum of all individual resistances: R_total = R₁ + R₂ + R₃ + ... + Rₙ. Current is the same through all resistors, but voltage divides proportionally across them.
What is the formula for resistors in parallel?
For resistors in parallel, the reciprocal of total resistance equals the sum of reciprocals: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + ... For two resistors: R_total = (R₁ × R₂)/(R₁ + R₂). Voltage is the same across all, but current divides.
How do I calculate voltage drop across a resistor?
Use Ohm's Law: V = I × R. The voltage drop across a resistor equals the current through it times its resistance. In series circuits, individual voltage drops sum to the source voltage.
What happens to total resistance when adding resistors in series?
Total resistance always increases when adding resistors in series. Each additional resistor adds to the total resistance, limiting current flow more. Example: 100Ω + 200Ω = 300Ω total.
What happens to total resistance when adding resistors in parallel?
Total resistance always decreases when adding resistors in parallel, and is always less than the smallest individual resistor. More parallel paths allow more current flow. Example: 100Ω || 100Ω = 50Ω.
How do I find current through each resistor in series?
In series circuits, current is the same through all resistors. Calculate using I = V_total / R_total. This single current flows through every component in the series chain.
How do I find current through each resistor in parallel?
In parallel, use I = V / R for each resistor (voltage is the same across all). Total current is the sum of individual currents: I_total = I₁ + I₂ + I₃ + ... Current divides inversely proportional to resistance.
What is power dissipation in a resistor?
Power dissipated as heat is calculated using P = I²R, P = V²/R, or P = VI. Ensure resistor power rating exceeds calculated power. Example: 10V across 100Ω dissipates P = 10²/100 = 1W.
Can I mix series and parallel resistor combinations?
Yes! For complex circuits, solve step by step: first combine parallel groups, then add series resistors. Or combine series groups first, then parallel combinations. Work systematically from innermost to outermost.
Why would I use resistors in series vs parallel?
Series: voltage division, current limiting, increasing total resistance. Parallel: current sharing, redundancy, decreasing total resistance, load distribution. Choice depends on circuit requirements and desired behavior.
What are standard resistor values?
Standard values follow E-series (E12, E24, E96, E192) with geometric spacing. Common E12 values: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 (multiply by powers of 10). Choose nearest standard value to calculated resistance.
How does temperature affect resistance?
Most resistors have positive temperature coefficient - resistance increases with temperature. Temperature coefficient (α) is in ppm/°C. For precision work, consider temperature effects, but standard calculations assume constant temperature.