Resistor Tolerance

As you have learned earlier, the last color band of the resistor represents the resistor tolerance. Resistor tolerance is defined as the spread in the value of resistor with reference to its designated resistance value. It is very difficult to make a resistor with exact resistance value that is why resistors contain tolerance to approximate the value of resistor.

Carbon composition resistors and large wire wound resistors may have 5%, 10% or 20% tolerances. Special resistors may have tolerances of as low as 0.01 to 1%. If a resistor has a resistance of 1000 ohms and 10% tolerance, the actual resistance value of this resistor could be anywhere from 900Ω to 1100Ω. This means that the resistance value of a resistor depends on its tolerance.



To get the tolerance value (TV), multiply the designated resistance value (DV) with the tolerance given (T). The tolerance is 10% which is also equal to 0.1 (the decimal equivalent)

TV = DV x T
TV = 1000Ω x 0.1
TV = 100Ω

The minimum value (min), represented by minus sign, is obtained by deducting the tolerance value (TV) to the designated resistance value (DV).

Min = DV – TV
Min = 1000Ω - 100Ω
Min = 900Ω

The maximum value (max), represented by plus sign, is solved by adding the tolerance value (TV) to the designated resistance value (DV).

Max = DV + TV
Max = 1000Ω + 100Ω
Max = 1100Ω


Example 1. A carbon composition resistor has a designated resistance value and tolerance value of 15000Ω +/-5%. What is the range of the resistor value?

Given:
DV = 15000Ω
T = 5% or 0.05

Solution:
TV = DV x T
TV = 15000Ω x 0.05
TV = 750Ω

Min = DV – TV
Min = 15000Ω - 750Ω
Min = 14250Ω

Max = DV + TV
Max = 15000Ω +750Ω
Max = 15750Ω

• The actual resistance value of this resistor could be anywhere from 14250Ω to 15750Ω.

Example 2. A resistor has a value of 50k +/-20%. What is the range of the resistor value?

Given:
DV = 50k
T = 20% or 0.2
Solution:
TV = DV x T
TV = 50k x 0.2
TV = 10k

Min = DV – TV
Min = 50k – 10k
Min = 40k

Max = DV + TV
Max = 50k +10k
Max = 60k

• The actual resistance value could be anywhere from 40k to 60k.