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How to select resistor and capacitor components for PCB design?

by: May 14,2021 2045 Views 0 Comments Posted in PCB Design Tutorial

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Summary:       This article walks through some important considerations to keep in mind for selection of major passive electronic components like resistors and capacitors while designing PCB.

OVERVIEW


Choosing appropriate passive electronic components is one of the challenges that novice PCB designers come across with. Inappropriate selection of the components could lead to undesired operation or malfunctioning of PCB and sometimes could even lead to complete failure. Depending on the type of component, there are certain key parameters that PCB designers should look into while selecting the component in the design phase. This article walks through some important considerations to keep in mind for selection of major passive electronic components. 


Selection of Resistors

Resistors come in various sizes, resistance values and tolerances. Apart from the obvious resistance value, it is equally important to consider its tolerance value. There is no Ideal resistor, so tolerance must also be taken into consideration during its selection. Resistors in the market come in several tolerances like 10%, 5%, 1%, 0.1% and so on. The higher the percentage, the higher the resistance can vary. For example, a 100K ohms resistor with 10% tolerance rating, the actual resistance can vary from  90K to 110K ohms. This is a huge variation. For critical circuits like feedback and protections, It is better to choose a resistor with 1% or 0.1% tolerance. In general, lower tolerance resistors are expensive compared to the higher tolerance range.


Another most important rating, which is often overlooked is the power rating. Each resistor is capable of dissipating a certain amount of power. Resistor will burn out if too much power is needed to dissipate than its rated power value. Therefore, it is important to know the actual power dissipation of the resistor that would be subjected to.

The maximum power dissipation of the resistor  can be computed as

Where;

Pd – Max power dissipation of the resistor

Imax = maximum current flowing through the resistor

Vmax = maximum voltage across the resistor

R = resistance value


For example, If the expected maximum power dissipation value is 320mW, then the resistor with the next available power value of 500mW should be selected.


In case of SMD resistor components, the size of resistors are usually correlated with their power dissipation capability. Resistors for a typical logic level circuit (3.3v or 5v), a 1/4 watt resistor will be  more than enough. While designing DC-DC converters or high current/voltage circuits, power rating should be highly  important to consider.


The operating temperature range of the resistor also plays a major role in certain conditions. Especially, If the PCB or the product in which it is embedded is subjected to higher ambient temperatures (over 60 deg centigrade) for a long time. For example, if the expected maximum ambient temperature is 80’° C, then a resistor with an operating temperature of more than 80°C should be chosen. In general, it is advised to consider a buffer temperature of 50% of maximum ambient temperature as resistor body temperature rises due to power dissipation while operation. Meaning a resistor that has a maximum operating temperature of more than 120°C (80+50% of 80) should be selected for an application temperature of 80°C.

A sample datasheet of a Resistor with key parameters is given in the figure.


Selection of Capacitors

Capacitors are used in a wide variety of circuits. Selecting the capacitor component for the PCB design based on just the capacitance value alone is usually not enough in most of the applications. Similar to resistor components, the capacitors also have tolerance factors. The actual capacitance of the capacitor component varies based on manufacturing process, operating temperature, DC biasing and aging. Therefore, the capacitance tolerances should be kept in mind while selecting the capacitor component for the application. The price variation between low tolerance and high tolerance capacitors varies considerably. If price is not a major concern, it is recommended to select a capacitor with tolerance under 10%. However, for most of the low power circuits, the tolerances of 10% or 20% are sufficient.

How much capacitance do you really get?




Capacitors could get damaged by voltage stress or higher (than rated) voltages. A capacitor, in most of the instances, is installed in parallel to a circuit or or a sub circuit or output. The expected voltage drop across the capacitor should be known/calculated.  It is recommended to have a voltage buffer of 50% over expected voltage drop. For example, if the expected maximum voltage drop across the capacitor is 10V, then a capacitor with rated voltage of 15V or higher should be selected. 


Capacitor life or lifetime expectancy is the length of time the capacitor will stay healthy and provides capacitance as designed. This is critical especially for the electrolytic capacitors. The lifetime of a capacitor under normal operating conditions is usually specified by the manufacturer on the datasheet. 


The operating temperature ranges for capacitors should also be considered similar to resistors as mentioned before in the previous section. Based on the type of application, ECR value, ripple current, operating frequency should also be considered for advanced designs, which is out of scope of this article. 

A sample datasheet of a capacitor with key parameters is given in the figure.



I would like to thank PCBWay for providing me the opportunity to write this article.

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