CONTENT

CONTENT PAGE

INTRODUCTION What is a potentiometer?

What is a rheostat?

Difference between rheostat and potentiometer.

EXAMPLE Example for Rheostat: Dimmer switch.

Example for potentiometer:Television

INTRODUCTION

What is Potentiometer?

A potentiometer (also known as a 'pot') is a resistor with a movable tap. Potentiometers can

be used to allow a change in the resistance in a circuit or as a variable voltage divider (in the

case of a volume control). If you have a rotary volume control that doesn't click when rotated,

it is (more than likely) a potentiometer being used as a variable voltage divider. If your

volume control clicks and steps the volume up or down with each click, it's probably a rotary

encoder (a switch), not a potentiometer. A potentiometer generally has 3 terminals. 2 of the

terminals are connected (internally) to the opposite ends of a resistive element. The 3rd

terminal is called the wiper. It is generally located in-between the other 2 terminals. The

wiper is a contact (actually, generally several very small contacts) that makes contact along

the resistive element. The diagram below shows the schematic symbol for a potentiometer.

What is Rheostat?

A rheostat is a variable resistor which is used to control current. They are able to vary the

resistance in a circuit without interruption. The construction is very similar to the

construction of a potentiometers. It uses only two connections, even when 3 terminals (as in

a potentiometer) are present. The first connection is made to one end of the resistive element

and the other connection to the wiper (sliding contact).  In contrast to potentiometers,

rheostats have to carry a significant current. Therefore they are mostly constructed as wire

wound resistors. Resistive wire is wound around an insulating ceramic core and the wiper

slides over the windings.

Rheostats were often used as power control devices, for example to control light intensity

(dimmer), speed of motors, heaters and ovens. Nowadays they are not used for this function

anymore. This is because of their relatively low efficiency. In power control applications they

are replaced by switching electronics. As a variable resistance they are often used for tuning

and calibration in circuits. In these cases they are adjusted only during fabrication or circuit

tuning (preset resistor). In such cases trimpots are often used, wired as a rheostat. But

dedicated 2 terminal preset resistors also exist.

Difference between Potentiometer and Rheostat

A potentiometer is a three-terminal variable resistor, while a rheostat is a two-terminal

variable resistor. Electricians usually use rheostats to adjust current and potentiometers to

adjust voltage. Potentiometers work as rheostats, but rheostats cannot function as

potentiometers. Potentiometers control a circuit's signal level rather than its power level.

Many electronic audio devices feature potentiometers, such as the volume control system on

a television. They also serve as position transducers in joysticks and voltage divider circuits.

Electricians employ rheostats, on the other hand, for handling much higher voltages and

currents. They feature two terminals and a movable wiper that switches from zero to

maximum resistance.

EXAMPLES

Example of Rheostat: Dimmer Switch

Most homes have lamps or fixtures that can be made brighter or dimmer by rotating or sliding

a control on their on-off switch. Years ago this was done using a device called a rheostat--a

large variable resistor. This method wasted electricity and generated a lot of heat. To control

the amount of energy going to the light the rheostat had to throw a lot away, turning it into

heat. For example, at half brightness a 100 watt bulb would waste about 20 watts to heat in

the rheostat.

Modern dimmer switches work an entirely different way. They use a transistor like device

called a TRIAC to switch the electricity on and off very rapidly - 120 times each second.

Because they sort of 'chop up' the electrical power this way they are sometimes called

'chopper switches'.

One cycle of household 60 hertz or 60 cycle AC electrical power is

shown in the figure. (Hertz means cycles-per-second and AC means

alternating current.) It's called alternating current because, for one-

half of the cycle (1/120 second) it's positive and, the other half its

negative or, half the time it's flowing one way and half the other.

And, as you see from the curved shape, it doesn't change suddenly.

It rises and falls or, undulates.

The red line indicates the point in each half-cycle that the dimmer switch turns on. It turns off

each time it reverses direction/sign/polarity, that is, each time it crosses the blue line. The

rotating or sliding control on the switch moves the red line left and right. As the line moves to

the left the light is on more of the time and thus, brighter. As the red line moves to the right,

the light is on less and less of the time and thus gets dimmer and dimmer. Said another way,

the area labelled "on" represents the total power/voltage going to the light and as the red line

moves to the right this area gets smaller.

Actually the knob or slider is also a variable resistor but, in this case it's just used as a signal

to move the turn-on point (red line)--it's not redirecting the flow of current as the old time

rheostat was. With the dimmer circuitry very little energy is wasted. A typical modern

dimmer control is more than 99 percent efficient--less than 1 watt is wasted controlling a 100

Watt bulb.

A modern dimmer switch (left)

Dimmer switch circuit (right)

Simple circuit of rheostat (left)

Example of Potentiometer: Television

Controls in a TV receiver are provided to improve the vision and sound to the desired level.

The potentiometers are used for volume control, bass control and treble control.

On-off switch - There is an on-off control on the front panel to turn the receiver on or off. In

older models, this control was linked with the volume control, but in modern receiver, there is

a separate push button for it.

Contrast control – It adjust the contrast in the video amplifier, controlling its gain. Higher

gain gives whiter pictures and hence increases the contrast.

Brightness control – It adjusts the brightness on the screen by adjusting DC voltage on the

control grid with respect to the cathode. Greater the voltage, greater would be the brightness.

Channel selection – It selects the desired channel. Modern receivers use synthesized tuning

in which the desired channel can be selected by adjusting programmable dividers through

push button switches.

Circuit in a TV remote control (left)

Potentiometers used in a TV (right)