Series Resistance

Calculating total resistance when resistors are connected end-to-end.

Quick Reference

Series Resistance Calculation

Total R = ∑(R1...Rn)

More Info

Resistors can be placed in series (end to end), in a circuit like the following:

In this case, the total resistance is the sum of each resistor. So for instance, given the following resistors:

  • R1 - 100Ω
  • R2 -
  • R3 - 1kΩ

Then the total resistance would be:

Total Resistance = 1,000Ω + 100Ω + 5Ω = 1,105Ω

Calculation

Therefore, the equation to calculate total series resistance is as follows:

Total R = ∑(R1...Rn)

Common Current, Different Voltage

Resistors in a series share a common current, that is the amount of amps flowing through each one is the same, since there’s only one path.

So for instance, given the previously calculated resistance of 1,105Ω, if there is a 5V supply of electrical force, Ohm’s law can be used to calculate the current at any given point as:

I = 5V / 1,105Ω = 0.004A = 4mA

So no matter where in the circuit that current is measured, it would be 4mA:

Power Calculation

The P = I^2 * R form of the power law can be used to calculate the power at any resistor (n) in a series:

Pn = I^2 * Rn

Further Reading

For a more in-depth discussion about resistors and resistance, check out Part 4 of the Electronics Tutorial.