Variable Resistor – Working, Construction, Types & Applications

john September 22, 2017 No Comments

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 - Symbol

Variable Resistor – Symbol

 

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.

 

Variable Resistor Terminals

Variable Resistor Terminals

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.

Ohms Law Graph

Ohms Law Graph

 

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 operation lead to the makers look for other alternatives.
  • Wire wound An insulating substrate is wound by nichrome wire.  They are mostly used in high power applications, have a long life and are precise. Their only drawback is that they have limited resolution.
  • Conductive plastic: Due to its resolution, they are often used in high end audio applications. Their use is limited as they are really costly, and can be used in low power applications only.
  • Cermet:  A very stable type of material, it has a low temperature coefficient and is highly resistant to temperature.  However, it has a short life and can burn a hole in your pocket.

Now that the working principle has been discussed, let’s have a look into the characteristics of a variable resistor.

Variable Resistor Characteristics

The most important characteristic of a variable resistor is given by the relation between the mechanical position of the moving terminal and the resistance ratio. It is marked on the resistor as its taper. Mostly two types of taper are marked, namely linear and logarithmic taper. Linear taper indicates that the relation between the two is linear, meaning the resistance ratio will be directly proportional to the mechanical position. This when drawn in a graph, will have a straight line with a constant slope.

The other type of taper is the logarithmic taper. This means the relation between the mechanical position and the resistance ratio is logarithmic, when plotted in a graph. The resistors with this type of taper are mostly used in audio control.

Tapers

Tapers

 

There’s another important characteristic of a variable resistor which one needs to know before choosing a resistor for a specific application. It is known as the resolution of the resistor. The resolution is nothing but the smallest value of resistance through which a  variable resistor changes. A variable resistor with a resolution 0.005 means, that the smallest value by which the resistance changes is 0.005 ohm. A high resolution is a favourable characteristic of a variable resistor.

Now that we have discussed the main characteristics of the variable resistor, let’s now have a look into the types of variable resistor.

Variable Resistor – Types

The connection and purpose of a variable resistor in a circuit, determines the type of variable resistor. Some of the popular types of variable resistors are:

Potentiometer:

When all the three terminals are used in a circuit, and the output voltage is taken from the moving terminal, the variable resistor is known as a potentiometer. It looks like a voltage divider circuit, as shown below.

Potentiometer Circuit Diagram

Potentiometer Circuit Diagram

 

Here, the two fixed terminals are connected across a voltage source. This means the voltage drop across the whole resistive track, is nothing but equal to the voltage source. The output circuit is connected across the moving terminal. This way, by controlling/ changing the position of the moving terminal, we can change the resistance and hence the voltage across the load.

This is particularly used in circuits where control of voltage is needed.

The resistive track can be of an arc shape or can be a straight one. It is this feature which decides the geometry of a potentiometer.

The potentiometer in a circuit diagram is represented as shown in the figure below.

 

 

Potentiometer Symbol

Potentiometer Symbol

Rheostat : When a variable resistor is used in a circuit to control the current flow, it is known as a rheostat. Here one of the fixed terminals and the moving terminal is used. The third fixed terminal is left unused. Connecting in this way helps to reduce or increase the current through the circuit by just changing the position of the moving wiper. As the resistance changes, the current changes inversely. That is if there is an increase in resistance, the current through the circuit will decrease.

Since these resistors have to carry a significant amount of current, they have to be mechanically strong enough to handle variations in current flow through them. Therefore, wire-wound resistive material is the most common choice when variable resistor is used as a rheostat.

We can connect any three terminal variable resistors (mostly potentiometer) as a rheostat too.  It is done by wiring the other unused fixed terminal and the moving terminal together, as a single terminal.

The rheostat is represented by the symbol shown in figure:

Rheostat Symbol

Rheostat Symbol

 

  1. Presets: A preset variable resistor is the micro version of a variable resistor and has three legs or terminals. It can be directly mounted on circuit and mostly its value is adjusted only once during the calibration process of the circuit. It has an adjustable screw attached to the resistor, which is adjusted using a screw driver, to get the desired resistance. The resistance here varies in a logarithmic manner.

The preset is represented by a symbol shown in figure below:

Preset Symbol

Preset Symbol

Applications of Variable Resistors

Variable resistors are mostly used where voltage control and current limiting is needed. In applications where voltage control is needed, the potentiometers are preferred as their connection suits for the same. Here the voltage source is connected across the resistive track that is it is connected across the two fixed terminals. The load circuit here is connected across the wiper terminal. As the resistance of the resistive track changes, the voltage across the load changes.

In other applications where current is to be limited, rheostats are used. Here

one end of the resistance track and wiper terminal are connected with the circuit, so that the current through the resistor limits according to the position of the wiper contact on the resistance track. As the wiper contact slides away from the connected end of the resistance track, the resistive value of the resistor increases and current goes down through the circuit.

Presets are mostly mounted in calibration circuits. Variable Resistors can also be found in Audio control, Television, Motion control, Transducers, Computation, Home Electrical Appliances, Oscillators, sensors etc.

 

 

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