On Resistors

On the surface, a resistor's function appears undesirable: It is a purely passive element that converts all of its input power into "wasted" heat. Turns out it's not that simple.

While resistance is an undesirable characteristic of many components, such as inductors, transformers, motors, wiring, and electrical transmission lines, leading to inefficiency and loses, it is the core function of the resistor. In juxtaposition, the resistor is probably the most frequently used electronic component.

The simple mathematical relationship of Eq. 1 describes the relationship between the current through a resistor and the voltage across its terminals.

V = I * R

Eq. 1: Ohm's Law

This simple relationship is the beauty of the resistor. Knowing any two of the variables makes easy work of calculating the third. Simple analog components can sense the voltage across a known resistor to determine the current flowing through it. Many of the LED current controllers in vogue today use this technique to sense and maintain the proper LED current.

Uses

Resistors serve many useful functions. They can protect circuits by limiting the maximum current flow during voltage surges or start-up. In combination with fixed voltage sources, they can limit the current in a chain of LEDs. Together with energy storing elements such as inductors or capacitors, they form frequency filters allowing certain frequencies to pass while attenuating others. The power input section of electronic circuits frequently uses filters to control conducted emissions and meet FCC requirements. Resistors can also provide voltage references and set the gain in an op amp circuits.

There are many different varieties of resistors. Some feature very tight tolerances, and some can dissipate a lot of power (need I say without suffering damage). Others can accept power surges many times above their normal ratings for very brief periods to protect circuits from short, high-voltage spikes.

Shortfalls

Some shortfalls can cause resistors' performance to diverge from Ohm's Law. Resistors can have parasitic (unwanted but inherent in their construction) inductance or capacitance; their resistance tolerances can be large; or they may have limitations on the power they can dissipate.

Resistance can vary with respect to time or temperature (known as the temperature coefficient of resistance). In some cases, negative or positive temperature coefficient resistors are desirable, primarily for use in temperature-sensing and control circuits. They are specially designed for the purpose. The resistance of a negative temperature coefficient (NTC) resistor goes down as temperature increases, while for a positive coefficient (PTC) resistor, the resistance increases with temperature. NTC and PTC resistors have well-defined and predictable curves of resistance versus temperature. However, in resistors not specifically designed for it, temperature coefficient is usually not well-defined.

Varieties

Resistors are also available in many different physical embodiments. There are through-hole resistors, chassis-mount resistors, and surface-mount resistors. They can consist of many different types of materials such as carbon composition, thick film, thin film, and wire-wound. Understanding these features and the limitations they imply is important when selecting a resistor for an application.

In his recent white paper entitled "The Right Choice of Current-Limiting Resistors for Constant Voltage LED Drivers," Phil Ebbert, VP of engineering at Riedon, discusses that company's high-power, current-limiting resistors in LED chains operating from voltage sources. In many of these applications, the power dissipation in the resistor can be significant, and the white paper discusses some of the different high-power packages (up to 35 Watts) and constructions (wire-wound, film) available from Riedon for these applications. The article gives a good overview and discusses the important factors in resistor selection. The guidance applies to other resistor manufacturers that offer similar products as well.