Example 1:
A resistor has a marked value of 470Ω.

Determine the tolerance of the resistor if it has a measured value of 455Ω

The difference between the marked and measured values of resistance (the error) is (470Ω - 455Ω) = 15Ω. The tolerance is given by:

Tolerance = error / marked value x 100%

The tolerance is thus 15/470 x 100 = 3.2%

Example 2:
A 12V power supply is to be tested with a 12Ω load resistor.

If the resistor has a tolerance of 10% determine:

(a) the nominal current taken from the supply
(b) the maximum and minimum values of supply current at either end of the tolerance range for the resistor.

(a)If a resistor of exactly 12Ω is used the current will be:

I = V/R = 12V/12Ω = 1A

(b) The lowest value of resistance would be (12Ω - 1.2Ω) = 10.8Ω. In which case the current would be:

I = V/R = 12V/10.8Ω = 1.11A

At the other extreme, the highest value of resistance would be:
(12Ω + 1.2Ω) = 13.2Ω

In this case the current would be:
I = V/R = 12V/13.2Ω = 0.91A