Jun 26 2010
Basic Resistor Selection Characteristics
Passive components are defined as electrical components that are not dependent on power, such as resistors, capacitors, inductors, and transformers. There are other passive components to consider, but this article focuses solely on resistors. When designing analog or digital circuits, often times it is difficult knowing the best resistor to use. There are thousands of resistors on the market, and all are better than others for a specific application. Different kinds of resistors have different characteristics.
Resistors are elements that, as the name implies, provide resistance to the flow of electrons through a medium. They are used for a vast variety of functions. They are used to limit current, to set voltage levels, to manipulate circuit timing, and for many other applications. Various properties of resistors include their tolerance, voltage rating, power rating, temperature rating, temperature coefficient of resistance, and their frequency response.
Tolerance of a resistor is defined as how much the resistor value varies from its defined value. This parameter is defined as a percentage, usually 1%, 5%, and 10%. Precision resistors are available whose tolerance falls below 1%. The voltage rating of a resistor is the amount of voltage that can be safely dropped across a resistor. The power rating is the amount of power that can be safely consumed by a resistor. Power listed is usually assumed to be the average power that a resistor can consume, rather than instantaneous. As a rule of thumb, it is better to choose a resistor that has a power rating at least double of that needed.
The temperature rating defines the temperature limits wherein a resistor will function properly. The temperature coefficient of resistance is the amount of variance that is exhibited by a resistor as the temperature varies from 25oC and is measured in parts per million (ppm). The frequency response of a resistor is the change in impedance shown by the resistor as a function of frequency. A change in impedance as function of frequency is sometimes desired and sometimes not.
There are many other factors to consider when choosing a resistor, however, for many basic applications, these properties offer a good start. Most vendors who sell resistors make it relatively easy to select the parameters desired. As mentioned previously, there are thousands of resistors on the market, so it helps to be able to filter through the ones needed, depending on the application. All these factors are important in the performance of the circuit.
Nancy has been an electrical engineer for more than 5 years. A majority of her work has been in the selection of electrical components for effective electrical circuits.