Many types of power products that come into contact with life use various types of resistors, including chip resistors and plug-in resistors, some of which are high-precision resistors and very inexpensive. In circuit design engineering, the resistors are typically derated in the power line design for customers' safety. The derating of the resistor can improve the reliability of the product and achieve the full life of the resistor.


The derating of resistors has its own rules in many product designs. Too much derating of resistors will result in increased production costs and waste of resources. If the resistors are not deducted enough, the life of the products will not be well protected. The following circuit resistor derating principles are usually followed in line design:
 
1. Use a metal film resistor in a circuit with an instantaneous high voltage pulse.
2. Use a wirewound resistor in the case of a large surge current.
3. Continuous power <50% × rated power.
4. Do not use carbon film resistors with a resistance greater than 1MΩ, because   long-term stability is too poor.
5. High resistance and long-term stability are required. Metal glaze resistors should be used.
6. After the thermal shock test, the resistance value must be within ± 5% of the rated range.
7. Fusible resistors, such as fuse resistors, do not rely on the PCB too close to avoid PCB overheating.
8. Try not to use rectangular chip resistors in the ESD protection circuit, because the sharp corners of the rectangle are easy to discharge.
9. In the voltage and current sampling, if using a chip resistor, try to use a resistor with a size of more than 1206.


The above are the most commonly used resistor derating principles for engineers, but the safety requirements for each product are different and the principle of derating of these resistors will vary. Engineers can use different resistors according to their own line rules to design a safe and reasonable power line.