Potentiometer: Taifi, Abubuwa, Da Yadda Ake Gudanar Potensiya: Taifi, Abubuwa, Da Yadda Ake Gudanar
Me wani shine Potentiometer?
Potentiometer (ko da ake kira pot ko potmeter) ita ce 3 terminal variable resistor, inda resistance ta zama da yake alaka don kontrolar da gudanar da current. Potentiometer tana iya zama adjustable voltage divider.
Ya Nufin Potentiometer Yadda Ake Iya Binda?
Potentiometer ita ce passive electronic component. Ana iya binda potentiometer tare da ziyartar sliding contact across a uniform resistance. Daga fili, ana iya faɗa input voltage across the whole length of the resistor, kuma output voltage tana zama voltage drop between the fixed and sliding contact.
Potentiometer tana da duka terminals of the input source fixed to the end of the resistor. Don adjustar output voltage, ana iya zama sliding contact along the resistor on the output side.
Wannan shi ne na farko da rheostat, inda haka ana iya fix one end and the sliding terminal is connected to the circuit, as shown below.
Wannan shine wani instrument na biyu na da amfani da ita don compare 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.
Haka, idan 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.
Potentiometer Types
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.
Rotary Potentiometers
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.
Linear Potentiometers
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
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%
