Resistors provide a fixed value of resistance that blocks or resists the flow of electrical current around a circuit, as well as producing a voltage drop in accordance with Ohm’s law.
They can be manufactured to have either a fixed resistive value in Ohms or a variable resistive value adjusted by some external means.
Resistors provide a fixed value of resistance that blocks or resists the flow of electrical current around a circuit, as well as producing a voltage drop in accordance with Ohm’s law. They can be manufactured to have a fixed resistive value in Ohms or a variable resistive value adjusted by some external means.
The potentiometer, commonly referred to as a “pot”, is a three-terminal mechanically operated rotary analogue device which can be found and used in a large variety of electrical and electronic circuits. They are passive devices, meaning they do not require a power supply or additional circuitry in order to perform their basic linear or rotary position function. Variable potentiometers are available in a variety of different mechanical variations allowing for easy adjustment to control a voltage, current, or the biasing and gain control of a circuit to obtain a zero condition.
The name “potentiometer” is an acronym of the words Potential Difference and Metering, which came from the early days of electronics development when it was thought that adjusting large wirewound resistive coils metered or measured out a set amount of potential difference making it a type of voltage-metering device.
Today, potentiometers are much smaller and much more accurate than those early large and bulky variable resistances, and as with most electronic components, there are many different types and names ranging from variable resistor, preset, trimmer, rheostat and of course variable potentiometer.
But whatever their name, these devices all function in exactly the same way in that their output resistance value can be changed or varied by the movement of a mechanical contact or wiper given by some external action.
Variable resistors in whatever format, are generally associated with some form of control, whether that is adjusting the volume of a radio, the speed of a vehicle, the frequency of an oscillator or accurately setting the calibration of a circuit, single-turn and multiple-turn potentiometers, trim-pots and rheostats find many uses in everyday electrical items.
The term potentiometer and variable resistor are often used together to describe the same component, but it is important to understand that the connections and operation of the two are different. However, both share the same physical properties in that the two ends of an internal resistive track are brought out to contacts, in addition to a third contact connected to a moveable contact called the “slider” or “wiper”.
When used as a potentiometer, connections are made to both ends as well as the wiper, as shown. The position of the wiper then provides an appropriate output signal (pin 2) which will vary between the voltage level applied to one end of the resistive track (pin 1) and that at the other (pin 3).
The potentiometer is a three-wire resistive device that acts as a voltage divider producing a continuously variable voltage output signal which is proportional to the physical position of the wiper along the track.
When used as a variable resistor, connections are made to only one end of the resistive track (either pin 1 or pin 3) and the wiper (pin 2) as shown. The position of the wiper is used to vary or change the amount of effective resistance connected between itself, the movable contact, and the stationary fixed end.
Sometimes it is appropriate to make an electrical connection between the unused end of the resistive track and the wiper to prevent open-circuit conditions.
Then a variable resistor is a two-wire resistive device that provides an infinite number of resistance values controlling the current offered to the connected circuit in proportion to the physical position of the wiper along the track. Note that a variable resistor used to control very high circuit currents found in lamp or motor loads are called Rheostats.
Variable potentiometers are an analogue device consisting of two main mechanical parts. 1. A fixed or stationary resistive element, track or wire coil that defines its resistive value, such as 1kΩ, 10kΩ, etc, and 2. a mechanical part that allows a wiper or contact to move along the whole length of the resistive track changing its resistive value as it moves. There are many different ways to move the wiper across the resistive track either mechanically of electrically.
But as well as the resistive track and wiper, potentiometers also comprise of a housing, a shaft, slider block, and a bush or bearing. The movement of the sliding wiper or contact can itself be a rotatory (angular) action or a linear (straight) action. There are four basic groups of variable potentiometer.
Rotary potentiometer (the most common type) vary their resistive value as a result of an angular movement. Rotating a knob or dial attached to the shaft causes the internal wiper to sweep around a curved resistive element. The most common use of a rotary potentiometer is the volume-control pot.
Carbon rotary potentiometers are designed to be mounted onto the front panel of a case, enclosure or printed circuit board (PCB) using a ring nut and locking washer. They can also have one single resistive track or multiple tracks, known as a ganged potentiometer that all rotate together using one single shaft. For example, a dual-gang pot to adjust the left and right volume control of a radio or stereo amplifier at the same time. Some rotary pots include on-off switches.
Rotary potentiometers can produce a linear or logarithmic output with tolerances of typically 10 to 20 percent. As they are mechanically controlled, they can be used to the measure the rotation of a shaft, but a single-turn rotary potentiometer normally offers less than 300 degrees of angular movement from minimum to maximum resistance. However, multi-turn potentiometers, called trimmers, are available that allow for a higher degree of rotational accuracy.
Multi-turn potentiometers allow for a shaft rotation of more than 360 degrees of mechanical travel from one end of the resistive track to the other. Multi-turn pots are more expensive, but very stable with high precision used mainly for trimming and precision adjustments. The two most common multi-turn potentiometers are the 3-turn (1080o) and 10-turn (3600o), but 5-turn, 20-turn and higher 25-turn pots are available in a variety of ohmic values.
Slider potentiometers, or slide-pots, are designed to change the value of their contact resistance by means of a linear motion and as such there is a linear relationship between the position of the slider contact and the output resistance.
Slide potentiometers are mainly used in a large range of professional audio equipment such as studio mixers, faders, graphic equalizers and audio tone control consoles allowing the users to see from the position of the plastic square knob or finger-grip the actual setting of the slide.
One of the main disadvantages of a slider potentiometer is that they have a long open slot to allow the wiper lug to move freely up and down along the full length of the resistive track. This open slot makes the resistive track inside susceptible to contamination from dust and dirt, or by sweat and grease from the users hands. Slotted felt covers and screens can be used to minimise the effects of resistive track contamination.
As the potentiometer is one of the simplest ways of converting a mechanical positional into a proportional voltage, they can also be used as resistive position sensors, also known as a linear displacement sensor. Sliding carbon track potentiometers measure a precise linear (straight) motion with the sensor part of a linear sensor being the resistive element attached to a sliding contact. This contact is in turn attached via a rod or shaft to the mechanical mechanism to be measured. Then the position of the slide changes with respect to the quantity being sensed (the measurand) which in turn changes the resistive value of the sensor.
Preset or trimmer potentiometers are small “set-and-forget” type potentiometers that allow for very fine or occasional adjustments to be easily made to a circuit, (e.g. for calibration). Single-turn rotary preset potentiometers are miniature versions of the standard variable resistor designed to be mounting directly on a printed circuit board and are adjusted by means of a small bladed screwdriver or similar plastic tool.
Generally, these linear carbon track preset pots are of an open skeleton design or of a closed square shape that once the circuit is adjusted and factory set, are then left at this setting, being only adjusted again if some changes occur to the circuit settings.
Being of an open construction, skeleton preset’s are prone to mechanical and electrical degradation affecting the performance and accuracy so are therefore not suitable for continuous use, and as such, preset pots are only mechanically rated for a few hundred operations. However, their low cost, small size and simplicity makes them popular in non-critical circuit applications.
Presets can be adjust from its minimum to maximum value within a single turn, but for some circuits or equipment this small range of adjustment may be too coarse to allow for very sensitive adjustments. Multi-turn variable resistors however, operate by moving the wiper arm using a small screwdriver some number of turns, ranging from 3 turns to 20 turns enabling very fine adjustments.
Trimmer potentiometers or “trim pots” are multi-turn rectangular devices with linear tracks that are designed to be installed and soldered directly onto a circuit board either through-hole or as surface-mount. This gives the trimmer both electrical connections and mechanical mounting and encasing the track within a plastic housing avoids the problems of dust and dirt during use associated with skeleton presets.
Rheostats are the big boys of the potentiometer world. They are two connection variable resistors configured to provide any resistive value within their ohmic range to control the flow of current through them.
While in theory, any variable potentiometer can be configured to operate as a rheostat, generally rheostats are large high wattage, wire-wound variable resistors, used in high current applications as the main advantage of the rheostat is their higher power rating.
When a variable resistor is used as a two-terminal rheostat, only the portion of the total resistive element that is in between the end terminal and the movable contact will be dissipating power. Also, unlike the potentiometer configured as a voltage divider, all the current flowing through the rheostats resistive element also passes through the wiper circuit. Then the contact pressure of the wiper on this conductive element must be capable of carrying the same current.
Potentiometers are available in various technologies such as: carbon film, conductive plastic, cermet, wirewound, etc. The rating or “resistive” value of a potentiometer or variable resistor relates to the resistive value of the entire stationary resistance track from one fixed terminal to the other. So a potentiometer with a rating of 1kΩ will have a resistive track equal to the value of a 1kΩ fixed resistor.
In its simplest form, the electrical operation of a potentiometer can be considered the same as for two resistors in series with the sliding contact varying the values of these two resistors allowing it to be used as a voltage divider.
In our tutorial about Resistors in Series, we saw that the same current flows through the series circuit, since there is only one path for the current to follow, and that we can apply Ohm’s Law to find the voltage drops across each resistor in the series chain. Then a series resistive circuit acts as a voltage divider network as shown.
In this example above, the two resistors are connected together in series across the supply. As they are in series, the equivalent or total resistance, RT is therefore equal to the sum of the two individual resistors, that is: R1 + R2.
Also being a series network, the same current flows through each resistor as it has nowhere else to go. However, the voltage drop given across each resistor will be different due to the different ohmic values of the resistors. These voltage drops can be calculated using Ohm’s Law with their sum equal to the supply voltage across the series chain. So here in this example, VIN = VR1 + VR2.
A resistor of 250 ohms is connected in series with a second resistor of 750 ohms so that the 250 ohm resistor is connected to a supply of 12 volts and the 750 ohm resistor is connected to ground (0v). Calculate the total series resistance, the current flowing through the series circuit and the voltage drop across the 750 ohm resistor.
In this simple voltage divider example, the voltage developed across R2 was found to be 9 volts. But by changing the value of any one of the two resistors, the voltage can in theory be any value between 0V and 12V. This idea of a two resistor series circuit in which we can change the value of either resistor to obtain a different voltage output is the basic concept behind the operation of the potentiometer.
The difference this time with the potentiometer is that to obtain different voltages at the output, the total resistance, RT value of the potentiometer resistive track does not change, only the ratio of the two resistances formed either side of the wiper as it moves.
Thus the potentiometers movable wiper provides an output which varies between the voltage at one end of the track and that at the other, usually between maximum and zero respectively as shown.
When the potentiometer resistance is decreased (the wiper moves downwards) the output voltage from pin 2 decreases producing a smaller voltage drop across R2. Likewise, when the potentiometer resistance is increased (the wiper moves upwards) the output voltage from pin 2 increases producing a larger voltage drop. Then the voltage at the output pin depends upon the position of the wiper with this voltage drop value subtracted from the supply voltage.
A 270o single-turn 1.5kΩ carbon track rotary potentiometer is required to provide a 6 volt supply from a 9 volt battery. Calculate, 1. the angular position of the wiper on the track in degrees and, 2. the values of the resistances either side of the wiper.
1. Angular position of pots wiper:
Then the wipers angular position is 180o or 2/3rds rotation.
2. Potentiometer Resistance Values:
Then the resistive values either side of the wiper are R1 = 500Ω’s and R2 = 1000Ω’s. We can also confirm that these values are correct by using the voltage divider formula from above:
Then we can see that when used as a variable voltage divider, the output voltage will be some percentage value of the input voltage with the amount of output voltage being proportional to the physical position of the movable wiper with respect to one end terminal. So for example, if the resistance from one end terminal to the wiper is 30% of the total, then the output voltage at the wiper pin across that section will be 30% of the voltage across the potentiometer, and this condition will always be true for linear potentiometers.
In the simple voltage divider example above, we have calculated the values for R1 and R2 as 500Ω’s and 1000Ω’s respectively, to produce a voltage at the wiper terminal (pin 2) of 6 volts with a wiper angular position of 180o. We have assumed here that the potentiometer is unloaded and producing a linear straight line output, so VOUT = θVIN.
However, if we were to load the wiper terminal by connecting a resistive load, RL, the output voltage would no longer be 6 volts as the load resistor, RL is effectively in parallel with R2, the lower 1000Ω’s part, and thus affects the total resistive value of the load part of the voltage divider network.
Consider what would happen if we connected a 3kΩ load resistance to the wipers output terminals.
So we can see that by connecting a load across the terminals of the potentiometers output, the voltage has decreased in this example, from the required 6 volts to just 5.4 volts as the loading effect of the 3kΩ resistor gives a parallel equivalent resistance, RP of 750Ω’s instead of the original 1kΩ’s.
Obviously, the higher or lower the resistance of the connected load the greater or lesser the loading effect on the wiper. So a load resistance in the mega-ohms range would have very little effect compared to one that was just a few ohms in value. Thus, to return the output voltage back to the original 6 volts would require a small adjustment of the potentiometer wiper position (18o in this case) as now RT is equal to 1250Ω’s (500 + 750).
Thus far we have seen that a variable resistor can be configured to operate as a voltage divider circuit which is given the name of potentiometer. But we can also configure a variable resistor to regulate a current, and this type of configuration is commonly known as a Rheostat.
Rheostats are two-terminal variable resistors which are configured to use one end terminal and the wiper terminal only. The unused end terminal can be either left unconnected or connected directly to the wiper. They are wirewound devices which contain tight coils of heavy duty enamelled wire that changes resistance in step-like increments. By changing the position of the wiper on the resistive element, the amount of resistance can be increased or decreased thereby controlling the amount of current.
Then the rheostat is used to control a current by changing the value of its resistance making it a true variable resistor. The classic example of the use of a rheostat is in the speed control of a model train set or Scalextric were the amount of current that passes through the rheostat is governed by Ohm’s Law. Then rheostats are defined not only by their resistive values but also by their power handling capabilities as P = I2xR.
In the diagram above, the effective resistance of the rheostat is between end terminal pin 3 and the wiper at pin 2. If pin 1 is left unconnected, the resistance of the track between pin 1 and pin 2 is open-circuited and has no effect on the value of the load current. Conversely, if pin 1 and pin 2 are connected together, then that part of the resistive track is short-circuited, and again has no effect on the value of the load current.
As rheostats control a current, then by definition they should be suitably rated to handle that continuous load current. It is possible to configure a three-terminal potentiometer as a two-terminal rheostat, but the carbon based resistive track may not be able to pass the load current. Also the wiper contact of a potentiometer is normally the weakest point so its best to draw as little current through the wiper as possible.
Note however that the rheostat is not suitable for controlling a load current if the load resistance, RL is much higher than the full value of the rheostat resistance. That is RL >> RRHEO. The resistive value of the load resistance must be much lower than that of the rheostat to allow load current to flow.
Generally rheostats are high-wattage electro-mechanical variable resistors used for power applications and whose resistance element is usually made of thick resistance wire suitable to carry the maximum current, I when its resistance, R is minimum.
Wirewound rheostats are mainly used in power control applications such as in lamp, heater or motor control circuits to regulate the field currents for speed control or the starting current of DC motors, etc. There are many types of rheostat but the most common are the rotary toroidal types which use an open construction for cooling, but enclosed types are also available.
Tubular slider rheostats are also available which can be found in physics labs and laboratories in schools and colleges. These linear or slide types use resistive wire wound around an insulating tubular former or cylinder. The sliding contact (pin 2) mounted above, is manually adjusted left or right to increase or decrease the rheostats effective resistance as shown.
As with rotary potentiometers, multi-gang type slider rheostats are also available. In some types, fixed electrical connections are made to the resistive wire to give a fixed value of resistance between any two terminals. Such intermediate connections are generally known as “tappings”, the same name as those used on transformers.
The most popular type of variable resistor and potentiometer is the linear type, or linear taper, whose resistive value at pin 2 varies linearly when adjusted producing a characteristics curve that represents a straight line. That is the resistive track has the same change of resistance per angle of rotation along the whole length of the track.
So if the wiper is rotated 20% of its total travel, then its resistance is 20% of maximum, or minimum. This is mainly because their resistive track element is made from carbon composites, ceramic-metal alloys or conductive plastics type materials which have a linear characteristic across their whole length.
But the resistance element of a potentiometer may not always produce a straight line characteristic or have a linear change in resistance across its whole range of travel as the wiper is adjusted, but instead can produce what is called a logarithmic change in resistance.
Logarithmic potentiometers are basically very popular non-linear or non-proportional types of potentiometers whose resistance that varies logarithmically. Logarithmic or “log” potentiometers are commonly used as volume and gain controls in audio applications where the attenuation changes as a logarithmic ratio in decibels. This is because the sensitivity to sound levels of human ear has a logarithmic response and is therefore non-linear.
If we where to use a linear potentiometer to control the volume, it would give the impression to the ear that most of the volume adjustment was restricted to one end of the pots track. The logarithmic potentiometer however, gives the impression of a more even and balanced volume adjustment across the full rotation of the volume control.
So the operation of a logarithmic potentiometers when adjusted is to produce an output signal which closely matches the nonlinear sensitivity of the human ear making the volume level sound as though it is increasing linearly. However, some cheaper logarithmic potentiometers are more exponential in resistance changes rather than logarithmic but are still called logarithmic because their resistive response is linear on a log scale. As well as logarithmic potentiometers, there are also anti-logarithmic potentiometers in which their resistance quickly increases initially but then levels off.
The all potentiometers and rheostats are available in a choice of different resistive tracks or patterns, known as laws, being either linear, logarithmic, or anti-logarithmic. These terms are more commonly abbreviated to lin, log, and anti-log, respectively.
The best way to determine the type, or law of a particular potentiometer is to set the pots shaft to the center of its travel, that is about half way, and then measure the resistance across each half from wiper to end terminal. If each half has more or less equal resistance, then it’s a Linear Potentiometer. If the resistance appears to be split at about 90% one way and 10% the other then chances are it’s a Logarithmic Potentiometer.
In this tutorial about potentiometers, we have seen that a potentiometer or variable resistor basically consists of a resistive track with a connection at either end and a third terminal called the wiper with the position of the wiper dividing the resistive track. The position of the wiper on the track is adjusted mechanically by rotating a shaft or by using a screwdriver.
Variable resistors can be categorised into one of two operational modes – the variable voltage divider or the variable current rheostat. The potentiometer is a three terminal device used for voltage control, while the rheostat is a two terminal device used for current control.
We can summarise this in the following table:
|Three Terminals||Two Terminals|
|Number of Turns||Single and Multi-turn||Single-turn Only|
|Connection Type||Connected Parallel with a Voltage Source||Connected in Series with the Load|
|Quantity Controlled||Controls Voltage||Controls Current|
|Type of Taper Law||Linear and Logarithmic||Linear Only|
Then the potentiometer, trimmer and rheostat are electromechanical devices designed so that their resistance values can be easily changed. They can be designed as single-turn pots, presets, slider pots, or as multi-turn trimmers. Wirewound rheostats are mainly used to control an electrical current. Potentiometers and rheostats are also available as multi-gang devices and can be classified as having either a linear taper or a logarithmic taper.
Either way, potentiometers can provide highly precise sensing and measurement for linear or rotary movement as their output voltage is proportional to the wipers position. The advantages of potentiometers include low cost, simple operation, lots of shapes, sizes and designs and can be used in a vast array of different applications.
However as mechanical devices, their disadvantages include eventual wear-out of the sliding contact wiper and/or track, limited current handling capabilities (unlike Rheostats), electrical power restrictions and rotational angles that are limited to less than 270 degrees for single turn pots.