Derating factors of Resistors & Capacitors

Introduction

        Derating is a process to identify the capacity of the component over its life time. For Ex: If a product is using a Resistor , Derating process will identify what will be the tolerance variation of the resistor due to stress over the life of the Product.

Stress on electrical components comes from various types, for example Electrical stress, Thermal stress etc. We need to choose a component which is robust to such stress and meet the life time requirement of the product.

Resistor:

In General below is the tolerance factor associated with different stresses for the component Resistor.

1) Temperature (2%)

2)  Humidity (1%)

3) Solder Tolerance (1%)

 

This is in addition to the component tolerance which is around 5 -10 %. When selecting a Resistor we need to consider combined tolerance of the component, is acceptable for the intended design or not.

Same applicable for other components as well.

Capacitor: 

Supplier create components which will sustain certain amount of stress, for example you could see in the datasheet, the maximum voltage that can be applied to Capacitor or max working temperature of the component or max ripple current etc. 

                Example : Datasheet also contains steady state voltage along with Max voltage, which suggests component will be capable of handling Max voltage but component will de rate if we continue using Capacitor at max voltage. In order to extend the life of the Capacitor or any other component it should be selected in such a way that it is always operated around steady state voltage. 

                Other examples are Peak current vs Steady state current . Capacitance value vs Applied voltage as shown in the below graph. Capacitor should be carefully chosen in such a way that for an application of a particular voltage capacitance of the capacitor is as desired by the product over its life time.      

            Due to these variations of Capacitor over Voltage, temperature etc, Thumb rule is to use a larger capacitance value so that when the capacitance loss is taken into account, you still have enough capacitance for the required functionality of the Product.

Conclusion: It is always recommended to use a component larger than the actual value at the time of design so that even after derating of the component with age, temperature, humidity etc component can withstand required functionality. Endurance test as part  of Qualification post Design will help to understand whether component chosen will withstand the stress over life of the Product.

Below are the guidelines you can refer for Derating concepts

• Mil-Hdbk-338 Military Handbook Electronic Reliability Design Handbook.
• SMC Standard SMC-S-010, US Air Force Space and Missile Systems

Each guideline provides specific guidance for the expected applications. For example Mil Handbook provide guidelines for Military products.