Electronic Design

The Humble Potentiometer Speaks Your Microprocessor's Language

The digital revolution led to the reduced usage of potentiometers as trimmers. Yet it also has acted as a catalyst for the increased demand of potentiometers as a means for the user or the machine to communicate with the microprocessor.

There are several good reasons to use the humble potentiometer as the real-world information input to your microprocessor. None is more important than the potentiometer's low cost. Nevertheless, the potentiometer has other important attributes, like reliability, the ability to save pins on the microprocessor, built-in memory, and automated and application-specific customization.

Low Cost: Engineers have discovered that there already exists an entire range of customized and commodity products based on potentiometer technology that can help resolve their control and position sensing challenges in applications using microprocessors. The engineer can take advantage of the low cost and reliability gained through the automated assembly of these products. Additional cost savings are realized from the robust materials now specified for product components, such as substrates, inks, and plated materials.

Reliability: In recent years, the potentiometer's performance and reliability have improved dramatically. Specialized potentiometers have a long history of providing extremely long life and dither cycles. But recent improvements in manufacturing have allowed low-cost, mass-produced potentiometers to experience performance increases that were previously limited to the higher-cost specialized versions. Low-cost potentiometers are now available in extended-life versions ranging from 10,000 to as high as 100,000 operations. Specialized versions are available with an operational life of 1 million to 2 million cycles and the potential for 10 million dithers. In addition, today's potentiometers can withstand demanding manufacturing techniques, including solder reflow processes and board washing, eliminating the need for more expensive sealed versions.

Pin Conservation: Potentiometers require only one signal pin to the micro-input. A great way to reduce cost with that pin is to use the potentiometer output as the input to an analog-to-digital converter (ADC). This enables the potentiometer to be implemented as a low-cost replacement to switches. Using a potentiometer this way not only conserves micro-input pins, but also saves on cabling and other component costs.

Built-In Memory: Potentiometers don't require electronic memory. If the potentiometer is set halfway and you turn the micro off, then back on again, it remains halfway. So if it's sensing position, for example, the position is still known when the system comes back on.

Automated And Application-Specific Customization: Automated assembly doesn't mean that the product can't be customized. Potentiometer manufacturers have designed-in the flexibility required to let products be customized to meet a designer's specifications, regardless of the degree of automation, often for the minimal expense of modifying an existing tool or insert.

For instance, electrical output of the potentiometer can be customized with specific electrical angles, cut tracks, special resistance curves, and laser trimming, without jeopardizing the low cost achieved through automation. Potentiometers supplied with detents, which can be used as inputs to an ADC in a micro, are replacing multiposition switches or absolute encoders. In addition, detents are available for applications when "feeling" the fixed position is important. These options can offer cost-effective solutions to many of the engineer's challenges when more exact specifications are required.

Traditionally, potentiometers have mechanical stops that prevent continuous rotation. A new category of low-cost potentiometers is now available with 360º mechanical rotation. This has become an important feature in function control and position-sensing applications, as it provides the operator the option of proceeding forward versus reversing position to return to a previous setting.

Truly unique applications, where a totally custom part is required, can be met by printing the potentiometer directly to the pc board, giving the designer the advantage of combining discrete resistors, switch contacts, and the potentiometer all on the same pc board. By combining this capability with laser trimming and standard or custom-designed wipers, potentiometers can be constructed to meet exact electrical and mechanical specifications, often considerably reducing cost in the overall system.

Some manufacturers are now actually using a pc board as the substrate for a discrete potentiometer. In those cases, many of the advantages outlined above can be achieved in a customized, discrete component.

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