Jan 3, 2013

Variable Resistors – Working and Applications

Variable Resistors – Working and Applications

This article clearly defines and explains the working of variable resistors, the variable resistance specification, types, and applications.
Variable resistors find its application in most of the electronic circuits used today. We have been getting a lot of comments with questions on how a variable resistor works and so on. Here are the details.


Types of Variable resistors
Types of Variable resistors

What is a Variable Resistor?

A variable resistor is a device that is used to change the resistance according to our needs in an electronic circuit. It can be used as a three terminal as well as a two terminal device. Mostly they are used as a three terminal device. Variable resistors are mostly used for device calibration.

Working of Variable Resistor

As shown in the diagram below, a variable resistor consists of a track which provides the resistance path. Two terminals of the device are connected to both the ends of the track. The third terminal is connected to a wiper that decides the motion of the track. The motion of the wiper through the track helps in increasing and decreasing the resistance.

Variable Resistors
Variable Resistors

The track is usually made of a mixture of ceramic and metal or can be made of carbon as well. As a resistive material is needed, carbon film type variable resistors are mostly used. They find applications in radio receiver circuits, audio amplifier circuits and TV receivers. For applications of small resistances, the resistance track may just be a coil of wire. The track can be in both the rotary as well as straight versions. In a rotary track some of them may include a switch. The switch will have an operating shaft which can be easily moved in the axial direction with one of its ends moving from the body of variable resistor switch.
The rotary track resistor with has two applications. One is to change the resistance. The switch mechanism is used for the electric contact and non-contact by on/off operation of the switch. There are switch mechanism variable resistors with annular cross-section which are used for the control of equipments. Even more components are added onto this type of a variable resistor so as to make them compatible for complicated electronic circuits. A high-voltage variable resistor such as a focus pack is an example. This device is capable of producing a variable focus voltage as well as a screen voltage. It is also connected to a variable resistance circuit and also a fixed resistance circuit [bleeder resistor] to bring a change in the applied voltage. For this both the fixed and variable resistor are connected in series.
A track made in a straight path is called a slider. As the position of a slider cannot be seen or confirmed according to the adjustment of resistance, a stopping mechanism is usually included to prevent the hazards caused due to over rotation.

Variable Resistance Specification

Various parameters like size, type of track and also resistance is used to define a variable resistance. Usually the spindle diameter of a variable resistor is 6mm.
If the variable resistor has a straight track it is defined in the component by the short form LIN representing a linear track. If it is a rotary track it is represented in short as LOG, as for a logarithmic track.
A common representation is given below.
5K6 LIN  – 5.6 kilo ohm with a linear track.
2M LOG – 2 Mega ohm with a logarithmic track.
In a linear track variable resistor, as the wiper is moved along the track the resistance varies constantly. In such resistors, the specification may not be given on the type. In that case, you will have to assume that it is linear.
In a logarithmic track variable resistor, the resistance does not increase/decrease constantly. As the wiper is moved from one end, the resistance changes at a slower rate and when the wiper is brought to the other end, the resistance changes at a fatser rate. This means that when the wiper is at halfway along the track, the resistance is not half the value of the total resistance. This is specifically applied for volume control as the response of the human ear to sound is also logarithmic. That is, a slow change in the beginning and a rapid change towards the end.

Application of Variable Resistors

There are mainly three types of variable resistors. They are
  1. Potentiometer
  2. Rheostat
  3. Presets

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