Resistor Theory

Resistors provide an obstruction to the flow of electricity around a circuit. A voltage is dropped across a resistor, dependant on the current flowing through the resistor. Here are some useful formulae:

Voltage Dropped across a Resistor:

V = IR, where I is in Amps, and R in Ohms

This can be re-arranged to give Current or Resistance if Voltage is already known (see ohms law).

Power Dissipated in a Resistor:

P = I²R or V²/R where P is in Watts

Always make sure the resistor's power rating is not exceeded!

Resistors in Series:

Putting resistors in series increases the total resistance:
R = R₁ + R₂ = R₃ .....
Note that the total resistance should be greater than that of any of the individual resistors.

Resistors in Parallel:

Putting resistors in parallel reduces the total resistance:
(1/R) = (1/R₁) + (1/R₂) + (1/R₃) .....
If you have only 2 resistors in parallel you can use:
R = (R₁ R₂) / (R₁ + R₂)
Note that the total resistance will be less than that of any of the individual resistors.