Variable Resistor – Working, Construction, Types & Applications

Variable Resistor – Working, Construction, Types & Applications One of the critical pieces in an electric circuit, the resistor is the most omnipresent electronic component. Available in wide varieties, these resistors can be used in various applications.  Resistors are broadly classified as fixed and variable resistors, according to the type of resistance value they offer. Here, in this article we shall discuss about variable resistors, its definition, types and uses. Let’s start! What is a Variable Resistor? A variable resistor is the type of resistor which changes the flow of current in a controlled manner by offering a wide range of resistances. As the resistance increases in the variable resistor the current through the circuit decreases and vice versa. They can also be used to control the voltage across devices in a circuit too. Therefore, in applications where current control or voltage control is needed, these type of resistors come handy. Figure 1 shows some real life variable resistors. Symbolic Representation The variable resistor is represented by a zig-zag line and an arrow across (or above) it, like that shown in the figure below.   Variable Resistor: Working Principle and Construction When we use the term variable resistor, it means we are talking about linear resistors by default. Linear resistors, as we know are those resistors whose resistance remains constant, even when the voltage and current through it changes. The voltage and current follow ohm’s law and are proportional to each other. A typical variable resistor has 3 terminals. Out of the three, two are fixed terminals at the ends of a resistive track. The terminals are made of conducting metal. The other terminal is a moving terminal, mostly known as the wiper. It is the position of this terminal on the resistive track that decides the resistance of the variable resistor.   These resistors offer a different resistance value, which means their resistance values can be adjusted to different values so as to provide the necessary control of current and/or voltage. To do so, a resistive strip is placed in between two fixed terminals of the device, a third terminal which is a movable one, is made to glide over this strip. Recall your basics of resistance; the resistance of a material is directly proportional to the length of the material.  Yes, that’s exactly what is used here.   The arrow placed on the resistive strip (arc shaped track), indicates the current position of the wiper terminal. Let’s suppose the wiper is placed at position “a”{figure 5(a)}, we can say that it divides the resistive track into two tracks of different lengths, from terminal 1 to point a and the other track being from point a to terminal 3. Our point of focus is the second length, as that is what decides the output of the resistor. As we move the wiper towards terminal 3, we see that the effective length decreases. So what will happen to the resistance offered by the pot? It will decrease. The resistive strip can also be laid down in a straight manner and the wiper in this case is called a slider. Its position cannot be seen or confirmed , therefore a stopping mechanism is needed to be integrated to prevent over rotation. Therefore, the main part of a typical variable resistor is the resistive material. The resistive material can be either of the following types: Carbon Composition:  One of the most common type, this material is made from carbon granules. Its low cost, reasonably low noise and lesser wear than other materials made it popular among the manufacturers. However, their inaccuracies of...
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