What’s a String Pot?

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A string potentiometer is a type of linear measurement device that uses a spring-loaded lead or “string” on a coil. These typically generate an electrical signal based on the amount of wire being pulled from the reel. Sometimes called “string pots” for short, the string pot is common in many technical applications.
These devices were first developed for the aerospace industry in the 1960s. This technology allows aircraft engineers to closely monitor the position of control surfaces, such as ailerons and rudders. The use of string pots goes far beyond aircraft, however, and potentiometers are often used in industrial automation and robotic systems.

All types of potentiometers work like variable resistors. This means they are capable of creating many different amounts of electrical resistance, depending on their location. High levels of resistance slow the flow of electric current, while low levels allow electricity to pass easily. The standard dimmer knobs on lights, for example, are variable potentiometers that allow users to manually adjust the current flow. A string potentiometer works using the same principle.

The lead inside a string potentiometer is connected to a spring. When the lead end is pulled, it unwinds from the coil and rotates the internal potentiometer. The spring return mechanism maintains tension on the cord to prevent slack and also to pull the cord back onto the spool when the end is released.

Applications using string potentiometers typically have the coil securely mounted in one place. The end of the cable is usually connected to a moving object, such as a sliding hydraulic rod or the arm of an industrial machine. The electrical resistance of the internal potentiometer increases as the lead is pulled and decreases as it returns.

The signals from a string potentiometer can be used in several ways. Computers and industrial controllers can read the level of resistance and convert it into a distance measurement. A computer readout, for example, can display the height of a hydraulic cylinder in centimeters or inches. The resistance level generated by a string potentiometer can also be used directly in an electrical circuit to switch a device once a preset threshold is reached.

Not all string potentiometers have the same accuracy. Some use very precise internal potentiometers and are capable of measuring changes in distance of less than a centimetre. Typically, the more accurate devices are also more expensive. Many are housed in protective cases, allowing them to operate in wet or dirty environments without damage.

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