Exploring Resistor Tolerance and Variations | Understanding Nominal vs. Actual Values

Resistors are key components in electronic circuits, used for controlling the flow of current. They are manufactured to specific resistance values and tolerances, indicating the permissible variation from their nominal value. This article will delve into whether resistors can exceed their specified tolerance values and explore the factors affecting their resistance.

Understanding Nominal and Actual Values

When a resistor is manufactured, it is designed to have a specific resistance value, known as the nominal value. The tolerance, expressed as a percentage of the nominal value, defines the allowable deviation from this specified value. For example, a resistor rated at 100 ohms with a tolerance of ±5% means the actual resistance could vary between 95 ohms and 105 ohms.

Example Calculation

If you have a resistor with a nominal value of 100 ohms and a tolerance of ±5%, the actual resistance can range as follows:

Minimum Value: 100 ohms - 5% of 100 ohms 95 ohms Maximum Value: 100 ohms 5% of 100 ohms 105 ohms

Therefore, the actual resistance must be within this range. If the resistance falls outside this range, the resistor is considered out of specification or defective.

Can Resistors Exceed Their Tolerance?

The question "Can resistors have greater values than their tolerance?" is often unclear. Tolerance is typically given as a percentage of the nominal value and not as an absolute maximum. For instance, a resistor with a nominal value of 100 ohms and a tolerance of ±120% means the actual resistance could vary between 70% and 220% of the nominal value. However, this scenario is rare for resistors.

Factors Influencing Resistor Variations

Several factors can cause deviations from the nominal resistance value:

Temperature Effects

The thermal coefficient of resistance can cause significant changes in resistance values. The equation R Rrt T - Tref x C, where C is given in PPM, can be used to calculate the variation due to temperature changes.

Aging Effects

Resistors may shift over time due to aging. The rate of aging is typically given as PPM per year, and other environmental factors like humidity can also affect resistance over time.

Contact Issues

Excessive heat, bad soldering, or exposure to extreme temperatures can permanently shift the resistance of resistors. These factors can push resistors beyond their nominal plus or minus the worst-case tolerance.

Conclusion

In summary, resistors are manufactured with specific tolerance values, and their actual resistance must fall within this range. While resistors can exhibit significant changes in resistance due to temperature, aging, and environmental factors, these changes are not within the specified tolerance. Understanding and accounting for these variations is crucial for the design and optimization of electronic circuits.

Key Takeaways:

Resistor tolerance defines the allowable range around the nominal value. External factors like temperature and aging can affect resistance values. Resistors that fall outside their tolerance range are considered defective.

Further Reading:

Understanding Tolerance in Electrical Components Resistor Aging and Environmental Effects Temperature Compensated Resistors