Resistors selection is very simple, but it plays a critical role in limitting the performance of any design. In this post, I will discuss about various parameters that shall be considered during the selection of resistors. Before selecting a resistor, you’d better learn about the non-ideal characteristics of resistor.
Practical resistors are NOT available at all exact values you needed for your desgn. You need to design circuit with the resistance values available in market. Select resistance values from EIA E6 (for 20% tolerance), E12 (for 10% tolerance), E24 (for 5% tolerance), E48 (for 2% tolerance), E96 (for 1% tolerance), or E192 (for 0.5, 0.25, or 0.1% tolerance) series standard values.
Resistor wattage rating is dependant on package size. In general, a 0805-size resistor will have wattage rating of 1/8W, and a 0402-size resistor will have wattage rating of 1/16W.
If you need a resistor with higher wattage rating, you have to choose a bigger package.
Tolerance of resistors can be typically 10%, 5%, and 1%. For non-critical applications such as pull-ups, etc, 5% or 10% resistors can be used. For critical applications such as analog filters, current sinking resistors, voltage divider resistors in power regulators, 1% or less tolerance resistors ae preferred.
Nowadays, surface mount packages are peferred by most of the engineers, as they occupy lesser space and very good choice for complex designs that use multi-layer PCBs.
SMD resistors have standard naming convention (eg. 0402, 0805, etc.) for the footprints. 0402 resistor would have approximate length of 40 mils and width of 20 mils. Similarly, 0805 resistor would have approximate length of 80 mils and width of 50 mils.
Use resistor pack or R-C packs to save board space, but use them ONLY when there is no control needed over individual component values. Resistor packs are mainly useful for mass pull-up, pull-down applications typically where whole bus needs to be terminated, pulled-up, etc. It can save space upto 50% i.e. a resistor pack of 4 resistors may save space of 2 resistors if all of them were discrete SMD0603 type.
Consider the power derating curve of the resistor against temperature.
Temperature Coefficient of Resistance (TCR)
Consider the TCR value as critical parameter for precision circuits.
A 1 megohm resistor with 200PPM will have a change its resistance upto 20 kilohm is possible with increase in 100 °C temperature.
Current Rating of Zero-Ohm Jumper
Consider the current rating of a zero ohm resistor during selection. Ensure that the current rating for the resistor is adequate for the application.
Zero ohm resistors may be used in power supply circuitry to select between different voltages. Though the resistor value is 0, the current handled cannot be infinite.
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