Resistors can offer creative solutions to design problems
Application-specific resistors (ASRs) are one of the most significant of a wealth of developments resulting from improvements in resistor materials and processing techniques. Specific resistors can offer lower cost, higher performance, and smaller size than off-the-shelf resistors. ASRs will be applied to a wide variety of design problems, including size, cost and noise reduction and temperature coefficient compensation. The 1990s will see a continuation of the trend toward tighter tolerances, as well as continued size reduction, without corresponding power rating reduction in many cases. Surface-mounted resistors will be the dominant technology, but leaded resistors will continue to be used for special requirements.
Resistors are generally perceived as the most mundane of electronic components. The typical design engineer thinks of them as simple devices that have remained unchanged since he was in school. However, there are continuing significant developments in resistors and other resistive components that deliver cost-effective solutions to new and existing circuit demands.
Improvements in resistor materials and processing techniques have led to a host of developments. I think the most exciting development is the application-specific resistor (ASR). Range extensions at both ends of the resistance scale have expanded–gigaohms at one end and fractional ohms at the other–with each incremental extensions opening up new application areas.
Specific resistor types with higher performance, lower cost, and smaller size have replaced earlier styles or more costly resistor types. In the past, for example, a close-tolerance power resistor had to be wirewound. Now, for power ratings up to 10 W, film devices are every bit as stable. During the ’90s, that 10-W figure will undoubtedly go higher.
I expect to see the continuation of tighter tolerances. Tolerances of 5% and 2% will fade away as production processes improve. The 1% tolerance will become the leading figure for film resistors.
Size reductions will continue, often with no reduction in power rating. I expect the 1/8-W size to displace the 1/4W size as the volume leader during the next decade. Surface-mounted resistors will continue to get smaller and command an increasingly large share of the market. The continued downsizing will increase the use of passive networks. The parts are becoming so small that even the best automated equipment has great difficulty handling them individually. As a result, the passive network will become the form factor of choice in mounting these units onto circuit boards. Instead of users handling 15 or 20 things that resemble little specks of pepper, they’ll handle 2 or 3 networks that have those specks on them. The networks will also get smaller, but not small enough to present a problem.
Although surface-mounted resistors will dominate the industry, leaded resistors will continue to be in demand for special requirements, such as high power, fuseability, ultra-precision, and customized impedance.
The ASR will emerge as an extremely cost-effective solution to many knotty design problems. In the past, design engineers didn’t usually think of ordering custom resistor products and resistor makers didn’t push them very much. The reason was that we didn’t have much to offer. Thanks to various developments in ceramics, filming techniques and materials, and coating techniques and materials, we can now offer inexpensive ways out of numerous design difficulties.
One case that exemplifies this point concerns an engineer who built a prototype of a piece of high-frequency communications equipment with standard off-the-shelf resistor at a critical point in the circuit. When it came to volume production, similar resistors caused the circuit to malfunction. Although it apparently was identical, the production resistor had a slightly different capacitance, and hence as slightly different rf impedance. We could adjust the geometry of our standard product to give that designer the impedance his design needed.
Design engineers are constantly pressured to reduce the cost and size of their circuits. ASR components, where appropriate, are an effective means to achieve both objectives.
We’ve built resistors with a dab of nickel-bearing epoxy painted over the regular coating to form a resistor with a bypass capacitor connected to one lead. That’s an extremely cost-effective way to reduce noise in a sensitive high-gain amplifier. Another ASR was built with a particular temperature coefficient (TC) to compensate for a capacitor’s TC. The RC combination then maintained a very stable time constant over temperature. The solution was elegant and much cheaper than the brute-force approach of trying to get both TCs down to zero.
There’ll be more such innovation, whether it’s special resistor in a spark plug or a lamp ballast. Engineers must be aware of the possibility of solving problems with a custom resistor. Most of them won’t consider discussing problems with resistor application engineers. And that’s too bad, because these products won’t be found in any catalog.