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Resistor selection: much more than just Ohm’s law

23 Oct 2018
By Tom Morris, Senior Field Applications Engineer, TT Electronics, Sensors and Specialist Components
Resistor selection

 

“What’s the big deal? It’s just a resistor” are phrases you’ll hear from designers whose application demands are routine and modest (a situation where it’s OK to say that), or those who are inexperienced and naïve (where it is not OK). Yes, a resistor’s world is constrained by two simple equations: V = IR and R = I2R – but there is much more to resistor selection than just its ohmic value and power rating. Certainly, for routine, non-critical applications such as proving a pull-up for an open-collector output at a few volts and milliamps in a benign environment, the choice of resistor type and technology to use is fairly easy.

But there are a large number of situations that call for understanding and insight about which fundamental type of resistor construction and technology to use, in order to get the performance needed beyond just the basic resistance and dissipation values. These include applications where higher voltages will be seen across the resistor terminals, or where there will be high-current surges and pulses. Or perhaps the ambient operating temperature will be fairly high, or superior precision and stability are needed for a measurement front end. Maybe the resistor is part of a circuit-protection function against undesired by possible current-surge events.

For these reasons, any decision about what type of resistor to use, other than those in low-end non-critical applications, begins with an understanding of the unique challenges of the specific installation. Doing so establishes the context for selecting the most suitable type resistor construction.

There are six important application areas for these targeted, specialised resistors: 1) pulse/surge situations; 2) current sensing; 3) signal conditioning and instrumentation; 4) circuit protection; 5) high power and dissipation; and 6) high voltage. To meet the diverse requirements of these areas, specialised resistor manufacturers rely on a variety of advanced physical configurations and construction techniques using highly purified materials, including:

 

Wire wound

Wire wound, where a metal wire is wound around an insulating rod, then welded to metal termination caps.

 

Bulk Metal

Bulk metal, where a thicker metal wire or strip forms a self-supporting structure without substrate.

 

Thick Film

Thick film, where paste containing glass and metal particles is fired to form a resistive layer.

 

Thin film

Thin film, with a very thin layer of conductive material applied to substrate.

 

composition

Composition, where the resistive-element material is distributed throughout a substrate. 


Each of these approaches has additional variations which further enhance their performance in a given application situation.
 

Making the choice: Sometimes, often not

Of course, while there are general “likely best fit” guidelines to match application priority requirement with an advanced resistor technology, there are always tradeoffs among performance factors. Among the tradeoff factors in resistor selection are the ability to fabricate relatively low and high resistance values, tolerance, temperature coefficient of resistance (TCR), power rating, voltage rating, long-term stability, overload tolerance, surge-withstand capability, high temperature performance, and, of course, cost. While the latter is almost always an issue, for many leading edge or extreme designs, it is not the highest priority (unlike the case for low-end consumer applications) while satisfying the challenging performance criteria is the foremost issue.

Still, balancing the competing and conflicting aspects of these tradeoffs is often not easy. In most cases, consulting with application experts at a broad-range vendor offering whose product line includes multiple resistor classes and materials is usually the best way to avoid making a less-than-optimal choice. There’s a very informative and readable overview presentation of the challenges, alternatives, techniques, and tradeoffs associated with these resistors posted at Proper Resistor Selection for Optimum Performance, Reliability, and Price.”