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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.

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