What is a Potentiometer?
How Does a Potentiometer Work?
A potentiometer is a passive electronic component. Potentiometers work by varying the position of a sliding contact across a uniform resistance. In a potentiometer, the entire input voltage is applied across the whole length of the resistor, and the output voltage is the voltage drop between the fixed and sliding contact as shown below.
A potentiometer has the two terminals of the input source fixed to the end of the resistor. To adjust the output voltage the sliding contact gets moved along the resistor on the output side.
This is different to a rheostat, where here one end is fixed and the sliding terminal is connected to the circuit, as shown below.
This is a very basic instrument used for comparing the emf of two cells and for calibrating ammeter, voltmeter, and watt-meter. The basic working principle of a potentiometer is quite simple. Suppose we have connected two batteries in parallel through a galvanometer. The negative battery terminals are connected together and positive battery terminals are also connected together through a galvanometer as shown in the figure below.
Here, if the electric potential of both battery cells is exactly the same, there is no circulating current in the circuit and hence the galvanometer shows null deflection. The working principle of potentiometer depends upon this phenomenon.
Now let’s think about another circuit, where a battery is connected across a resistor via a switch and a rheostat as shown in the figure below.
The resistor has the uniform electrical resistance per unit length throughout its length.
Hence, the voltage drop per unit length of the resistor is equal throughout its length. Suppose, by adjusting the rheostat we get v volt voltage drop appearing per unit length of the resistor.
Now, the positive terminal of a standard cell is connected to point A on the resistor and the negative terminal of the same is connected with a galvanometer. The other end of the galvanometer is in contact with the resistor via a sliding contact as shown in the figure above. By adjusting this sliding end, a point like B is found where there is no current through the galvanometer, hence no deflection in the galvanometer.
That means, emf of the standard cell is just balanced by the voltage appearing in the resistor across points A and B. Now if the distance between points A and B is L, then we can write emf of standard cell E = Lv volt.
This is how a potentiometer measures the voltage between two points (here between A and B) without taking any current component from the circuit. This is the specialty of a potentiometer, it can measure voltage most accurately.
There are two main types of potentiometers:
- Rotary potentiometer
- Linear potentiometer
Although the basic constructional features of these potentiometers vary, the working principle of both of these types of potentiometers is the same.
Note that these are types of DC potentiometers – the types of AC potentiometers are slightly different.
The rotary type potentiometers are used mainly for obtaining adjustable supply voltage to a part of electronic circuits and electrical circuits. The volume controller of a radio transistor is a popular example of a rotary potentiometer where the rotary knob of the potentiometer controls the supply to the amplifier.
This type of potentiometer has two terminal contacts between which a uniform resistance is placed in a semi-circular pattern. The device also has a middle terminal which is connected to the resistance through a sliding contact attached with a rotary knob. By rotating the knob one can move the sliding contact on the semi-circular resistance. The voltage is taken between a resistance end contact and the sliding contact. The potentiometer is also named as the POT in short. POT is also used in substation battery chargers to adjust the charging voltage of a battery. There are many more uses of rotary type potentiometer where smooth voltage control is required.
The linear potentiometer is basically the same but the only difference is that here instead of rotary movement the sliding contact gets moved on the resistor linearly. Here two ends of a straight resistor are connected across the source voltage. A sliding contact can be slide on the resistor through a track attached along with the resistor. The terminal connected to the sliding is connected to one end of the output circuit and one of the terminals of the resistor is connected to the other end of the output circuit.
This type of potentiometer is mainly used to measure the voltage across a branch of a circuit, for measuring the internal resistance of a battery cell, for comparing a battery cell with a standard cell and in our daily life, it is commonly used in the equalizer of music and sound mixing systems.
Digital potentiometers are three-terminal devices, two fixed end terminals and one wiper terminal which is used to vary the output voltage.
Digital potentiometers have various applications, including calibrating a system, adjusting offset voltage, tuning filters, controlling screen brightness, and controlling sound volume.
However mechanical potentiometers suffer from some serious disadvantages which make it unsuitable for applications where precision is required. Size, wiper contamination, mechanical wear, resistance drift, sensitivity to vibration, humidity, etc. are some of the main disadvantages of a mechanical potentiometer. Hence to overcome these drawbacks, digital potentiometers are more common in applications since it provides higher accuracy.
Digital Potentiometer Circuit
The circuit of a digital potentiometer consists of two parts, first the resistive element along with electronic switches and second the control circuit of the wiper. The figure below shows both the part respectively.
The first part is an array of resistors, and each node is connected to a common point W, except the endpoints A and B, via a two-way electronic switch. The terminal W is the wiper terminal. Each of the switches is designed using CMOS technology and only one of the switches out of all is in ON state at any given time of the potentiometer operation.
The switch which is ON determines the potentiometer resistance and the number of switches determines the resolution of the device. Now which switch is to be made ON is controlled by the control circuit. The control circuit consists of an RDAC register which can be written digitally using interface such as SPI, I2C, up/down or can be manually controlled by push buttons or a digital encoder. The diagram above shows that of a push-button controlled digital potentiometer. One button is for “UP” or increasing the resistance and the other for “DOWN” i.e. decreasing the resistance.
Generally, the wiper position is at the middle switch when the digital potentiometer off. After power is switched on, depending upon our requirement we can increase or decrease the resistance by a suitable push-button operation. Besides, advanced digital potentiometers also have an inbuilt onboard memory which can store the last position of the wiper. Now this memory can be of the volatile type or permanent type both, depending upon the application.
For example, in the case of volume control of a device, we expect the device to remember the volume setting we used last even after we switch it on again. Hence a permanent type of memory such as EEPROM is suitable here. On the other hand for systems that recalibrates the output continuously and it is not necessary to restore previous value, a volatile memory is used.
Advantages of Digital Potentiometers
The advantages of digital potentiometers are:
- Higher reliability
- Increased accuracy
- Small size, multiple potentiometers can be packed on a single chip
- Negligible resistance drift
- Unaffected by environmental conditions like vibrations, humidity, shocks and wiper contamination
- No moving part
- Tolerance up to ±1%
- Very low power dissipation, up to tens of milliwatts
Disadvantages of Digital Potentiometers
The disadvantages of digital potentiometers are:
- Not suitable for high temperature environment and high power application.
- Due to the parasitic capacitance of the electronic switches, there is a bandwidth consideration that comes into the picture in digital potentiometers. It is the maximum signal frequency that can cross the resistance terminals with less than 3 dB attenuation in the wiper. The transfer equation is similar to that of a low pass filter.
- The nonlinearity in the wiper resistance adds a harmonic distortion to the output signal. The total harmonic distortion, or THD, quantifies the degree to which the signal is degraded after crossing through the resistance.
Applications of Potentiometer
There are many different uses of a potentiometer. The three main applications of a potentiometer are:
- Comparing the emf of a battery cell with a standard cell
- Measuring the internal resistance of a battery cell
- Measuring the voltage across a branch of a circuit
Comparing EMF of Battery Cells
One of the main uses of a potentiometer is to compare the emf of one battery cell with a standard battery cell. Let’s take a cell whose emf is to be compared with a standard cell. The positive terminal of the cell and the same of the standard cell are joined together with the fixed end of the potentiometer resistor. The negative terminal of both cells is joined with the galvanometer in turn through a two-way switch. The other end of the galvanometer is connected to a sliding contact on the resistor. Now by adjusting sliding contact on the resistor, it is found that the null deflection of galvanometer comes for the first cell at a length of L on the scale. After positioning the two-way switch to the second cell and then by adjusting the sliding contact, it is found that the null deflection of galvanometer comes for that cell at a length of L1 on the scale. The first cell is a standard cell and its emf is E. The second cell is an unknown cell whose emf is E1. Now as per above explanation, we can write
As the emf of the standard cell is known, hence emf of the unknown cell can easily be determined.
Measuring Internal Resistance of A Battery Cell
In this process, one battery is connected across the resistor of a potentiometer through a galvanometer as shown in the figure below. One resistance of known value (R) is connected across the battery through a switch. First, we keep the switch open and adjust the sliding contact of the potentiometer resistor to make the galvanometer current zero. Once the galvanometer shows zero deflection from its null point we take the position of the sliding contact tip on the resistor scale. Say this is L1.
Now we make the switch on. At that condition, a circulating current starts flowing through the battery cell as well as the resistance (R). As a result, there is a voltage drop in the battery itself due to its internal resistance. So now the voltage across the battery cell would be a little bit less than its open circuit voltage or emf of the cell. Now again we adjust the sliding contact on the transistor to make the galvanometer current zero and once it becomes zero that is zero deflection is indicated in the galvanometer, we take the position of the sliding contact tip on the resistor scale and say it is L2.
The internal resistance of the battery cell can be found out by using this below shown formula.
Where r is the internal resistance of the battery cell.
Measurement of Voltage by Potentiometer
The principle of measuring voltage across a branch of a circuit with help of a potentiometer is also simple. Here first we have to adjust the rheostat to adjust the current through the resistor so that it causes a specific voltage drop per unit length of the resistor. Now we have to connect one end of the branch to the beginning of the resistor and other end is connected to the sliding contact of the resistor through a galvanometer. Now we have to slide the sliding contact on the resistor until the galvanometer shows zero deflection. When the galvanometer comes to its null condition we have to take the reading of the position of the sliding contact tip on the resistor scale and accordingly we can find out the voltage across the branch of the circuit since we have already adjusted the voltage per unit length of the resistor.
Rheostat vs Potentiometer
A potentiometer gives variable voltage. A rheostat gives variable resistance. The potentiometer is a three terminal device whereas a rheostat is a two terminal device. Construction wise both of the devices look similar but their principle of operation is entirely different. In potentiometer two end terminals of the uniform resistance are connected to the source circuit. In rheostat, only one terminal of the uniform resistance is connected to the circuit and the other end of the resistance is kept open. In both potentiometer and rheostat, there is a sliding contact on the resistance.
In potentiometer, the output voltage is taken between fixed and sliding contact. In rheostat, the variable resistance is achieved between fixed and sliding terminal. The resistance of potentiometer gets connected across the circuit. The resistance of rheostat is connected in series with the circuit. The rheostat is generally used to control the current by adjusting resistance with the help of sliding contact. In potentiometer, the voltage is controlled by adjusting the sliding contact on the resistance.
tat, the variable resistance is achieved between fixed and sliding terminal. The resistance of potentiometer gets connected across the circuit. The resistance of rheostat is connected in series with the circuit. The rheostat is generally used to control the current by adjusting resistance with the help of sliding contact. In potentiometer, the voltage is controlled by adjusting the sliding contact on the resistance.
Potentiometer Driver Cell
The potentiometer measures voltage by comparing the measuring voltage with voltage across the resistance of the potentiometer. So for operation of potentiometer there must be a source voltage connected across the potentiometer circuit. This cell to provide this source voltage to drive the potentiometer is called driver cell. The driver cell delivers the current through the resistance of potentiometer. The product of this current and the resistance of the potentiometer provides full scale voltage of the device. By adjusting this voltage one can change the sensitivity of the potentiometer. This is normally done by adjusting current through the resistance. The current flowing through the resistance is controlled by a rheostat connected in series with the driver cell. This is to be remembered that the voltage of the driver cell must be greater than the voltage to be measured.
The sensitivity of a potentiometer implies what the small voltage difference can be measured by the potentiometer. For same driver voltage if we increase the length of the potentiometer resistance, length of the resistance per unit voltage gets increased. Hence the sensitivity of the potentiometer gets increased. So we can say sensitivity of a potentiometer is directly proportional to the length of the resistance. Again if we reduce the driver voltage for a fixed length of potentiometer resistance, then also voltage per unit length of the resistance gets decreased. Hence again the sensitivity of the potentiometer gets increased. So the sensitivity of the potentiometer is inversely proportional to the driver voltage.