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:
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).
P = I2R or V2/R where P is in Watts
Always make sure the resistor's power rating is not exceeded!
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.
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.