Resistance issues to consider in PCB circuit design

When designing precision electronics or performing detailed analysis of worst-case scenarios, we need to consider parameters that may not be important in other applications. One of them is the consideration of the tolerance of the resistor. In fact, no matter whether the resistance value of the resistor you use is large or small, tolerance must exist. But it does not limit the maximum and minimum values of resistance in your circuit.Today qihe smt pick and place machine sharing resistance issues to consider in PCB circuit design .

Tolerance: Defines the range of differences between a resistor’s actual resistance and its nominal resistance at the time of manufacture. A resistor with a nominal value of 1000Ω and a tolerance of ±5% has a value in the range of 950Ω to 1050Ω. We want this value to be fixed and not change over time. But this is not the case. Engineers must consider tolerances in the design and ensure that the designed circuit works within the operating time limit.

Temperature coefficient: This coefficient describes the phenomenon that the resistance of the resistor changes with the change of temperature. Its unit is ppm/K (i.e. 10^-6/K); of course, the same is true if K is replaced by °C as the unit; usually The temperature coefficient values are 5, 10, 20, and 100. For example, there is a 1000Ω resistor with a temperature coefficient of 100ppm/K. When it experiences a temperature change of ±60K (assuming the initial temperature is 300K and the change range is 240-360K), its resistance value will change by ±6Ω. Of course, the smaller the temperature coefficient of the resistor, the higher the price.

Calculation of resistance change and temperature change

The resistor heats itself: For high-precision circuits, sometimes the power consumption of the resistor must also be considered. Resistors have specific thermal resistance, whose unit is °C/W. Engineers should be aware of power losses within a resistor; this will cause the resistor’s temperature to rise and ultimately affect the resistor’s resistance.

When determining the maximum and minimum values for the resistors you use, you must consider tolerances, temperature coefficients, and the effects of resistor self-heating. When conducting analysis, you may notice that some parameters can be ignored or not considered accurately, but you must first have a certain understanding of them before you can determine whether they are important.

For some precision circuits (such as gain stages in amplifiers), it is necessary to match the resistors and ensure that their resistances are within the required range and have the same temperature coefficient.

On some circuits, it is also critical to ensure that certain key resistors are positioned so that the temperature is the same on both ends of the resistor. Otherwise, the Seebeck effect also needs to be considered. When using forced air flow, it is necessary to ensure that the resistor is perpendicular to the air flow so that heat from one end is not transferred to the other end and the temperature of the component is equalized.

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