Variable Resistors and Its Types
A variable resistor is a resistor with a resistance value that can be adjusted. Essentially, variable resistors are electromechanical transducers that normally work by sliding a contact over a resistive element. When a variable resistor is used as a potential divider by using three terminals, it is called a potentiometer. When only two terminals are used, it is known as a rheostat and functions as a variable resistance. There are also electronically controlled variable resistors, which can be controlled electronically instead of by mechanical action. In these cases, the resistor is called a digital potentiometer. In this blog, we will discuss potentiometers, rheostats, digital potentiometers, and their functions.
Potentiometers are the most common type of variable resistor. They function as potential dividers and are used to generate a voltage signal depending on the position of the potentiometer. This signal can then be used in a broad range of applications, such as: amplifier gain control (audio volume), measurement of distance or angles, tuning of circuits, and more. When variable resistors are used to tune or calibrate a circuit or application, trimmer potentiometers, or trimpots, are used. These are small potentiometers mounted on the circuit board that can be adjusted using a screwdriver. Potentiometers are rarely used to directly control more than a watt of power, as the power dissipated in the potentiometer would be comparable to that of the controlled load.
Rheostats are similar to potentiometers in terms of constructions, but are used as a variable resistance rather than as a potential divider. Additionally, while potentiometers use three terminals, rheostats use only two. One connection is made at one end of the resistive element and the other is made at the wiper of the variable resistor. In the past, rheostats were used as power control devices in series with the load, such as in a light bulb. Currently, rheostats are not used as power control, as they have proven to be less efficient than more efficient switching electronics. Preset variable resistors, wired as rheostats, are used in circuits to perform tuning or calibration.
3. Digital Potentiometer
Lastly, a digital potentiometer is an electronic device that mimics the functions of an analog potentiometer. Through digital input signals, digital potentiometers can adjust the resistance between two terminals, just like in a traditional potentiometer. There are two main types of digital potentiometers: volatile and non-volatile. Volatile digital potentiometers lose their set position if power is removed and are designed to initialize at the minimum position. Non-volatile digital potentiometers retain their set position using a storage system similar to flash memory.
Use of digital potentiometers is far more complex than standard potentiometers and they have many limitations. Despite this, they are widely used for factory adjustment and calibration of equipment, especially in instances where limitations of mechanical potentiometers present problems. In general, digital potentiometers are immune to the effects of moderate long-term mechanical vibration or environmental contamination to the same extent as other semiconductor devices. They can also be secured electronically against unauthorized tampering by protecting the access to its programming inputs by various means.
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