What is the substitution principle when selecting resistors?

In the process of electronic component procurement and circuit design, engineers often encounter situations where specified resistors are out of stock or need to reduce costs and increase efficiency. In this case, the selection of alternative resistors becomes a key skill. Reasonable substitution is not simply finding a resistor with similar parameters to replace it, but requires following a rigorous set of principles to ensure the long-term stability and reliability of the circuit.
When replacing resistors, the core is to systematically compare the key parameters of the new and old resistors to ensure that the substitute can meet or exceed the requirements of the original model in terms of electrical performance and physical characteristics.
1、 Core electrical parameters are the basis for substitution
1. Resistance and accuracy: The nominal resistance of the substitute resistor must be consistent with the original resistor. Accuracy (tolerance) is another key point, and the accuracy of the substitute resistor should not be lower than that of the original resistor. For example, the original circuit uses a resistor with 1% accuracy, which cannot be replaced by a resistor with 5% accuracy. However, it is possible to consider using a resistor with higher accuracy (such as 0.5%) to ensure circuit performance.
2. Rated power: This is the most easily overlooked and dangerous parameter in resistor selection and replacement. The rated power of the substitute resistor must be greater than or equal to the maximum power consumption that the original resistor can withstand in the actual circuit, and it is recommended to leave sufficient margin (such as 20% -30%). Insufficient power can cause the resistor to overheat or even burn out.
3. Temperature coefficient (TCR): It represents the stability of resistance as a function of temperature. In temperature sensitive applications such as precision measurement and reference voltage circuits, it is necessary to choose resistors with equal or better TCR as substitutes. A resistor with a larger TCR may introduce unexpected errors when the ambient temperature changes.
2、 The consideration of packaging and process materials cannot be ignored
1. Packaging size and installation method: The physical size of the resistor (such as 0201, 0402, 0603 for surface mount resistors) must match the solder pads on the PCB board. Meanwhile, pay attention to the installation method, whether it is patch or plug-in, to ensure smooth installation.
2. Types of Resistance Technology: Different types of resistors have their unique application scenarios. Ordinary thick film surface mount resistors can replace carbon film resistors, but in high-frequency or high-precision applications, thin film resistors or metal film resistors may need to be used as substitutes. For high-power surge current applications, winding resistance or metal oxide resistance should be considered.
3、 Special requirements for specific application scenarios
In some special circuits, the replacement of resistors requires additional attention to their high-frequency characteristics or pulse tolerance.
High frequency circuits: It is necessary to pay attention to the parasitic inductance and distributed capacitance of resistors, and thin film resistors are usually a better choice than wound resistors.
Pulse load circuit: For example, in power buffering or surge suppression circuits, the resistance's ability to withstand pulse power is more important than the average rated power, and must be confirmed by consulting the data manual.
summary
In summary, the selection of alternative resistors is a rigorous balancing process. A successful alternative solution means that the new resistor can meet the requirements in core parameters such as resistance, accuracy, power, temperature coefficient, and packaging, and is compatible with the original application scenario in terms of technical type and high-frequency/pulse characteristics. The safest approach is to conduct sample testing before batch replacement to verify its performance in real circuits. Developing a systematic habit of parameter comparison is the key to ensuring the success of resistor substitution and guaranteeing product quality.