Resistor Characteristics and Types of Resistors

 

The resistor is the most used passive circuit element in the circuit design. And in fact, it is hard to imagine any circuit without the resistors. Let's find out briefly about these resistors, different Characteristics or different parameters related to these resistors.

And see the different types of resistors which are used commercially.

Now, as we know this resistor is the passive circuit element which opposes the flow of

current or electricity. And symbolically, it can be represented by this symbols.

 The property of the resistor to oppose the flow of current is known as the resistance. And it isdefined by the unit of Ohm. For the given resistor, if 1V of voltage is applied across that resistor and if 1A current is flowing through that resistor then we can say that the resistance of that resistor is 1 Ohm. The resistor is the linear element. It means that as we increase or decrease the voltage that is applied to this resistor, the current that is flowing through this resistor will also increase or decrease. And the slope of this V-I characteristic defines the resistance.

The relationship between the voltage, current and this resistance is defined by this Ohm's Law.

Let's see the different parameters which affect the resistance of the resistor. For any resistor, the resistance can be given by this expression.

 

As we can see over here, it is directly proportional to the resistivity of the material. So, if we change the material, then the resistivity of that material will also change and accordingly the resistance of that material will also change. Then this resistance is directly proportional to the length of the resistor and it is inversely proportional to the cross-sectional area of this resistor. So, as we increase the length of the resistor, then the electrons which contribute to the flow of current will see the more resistance. Similarly, if we increase the cross-sectional area of the resistance, then these electrons can move easily across the resistor. So, if we increase the cross-sectional area then the resistance of the resistor will reduce. So, in this way, by changing these three parameters we can change the resistance of the resistor.

Characteristics of the Resistor

Resistors are available in various size and shapes. But before we see the different types of resistors, let's see the different parameters or the different characteristics of the resistor which we should consider before selecting the particular resistor for a particular application. So, here is the list of different characteristics or the different parameters of the resistor.

1.    Power Rating

The first factor is the power rating of the resistor. And it defines the maximum power which can be handled by the resistor. Now, the power which is going to get dissipated across the resistor can be given by the simple expression of P is equal to V X I. So, if 5 Volt of voltage is applied to the resistor and if 1A of current is flowing through that resistor then we can say that the power that is going to get dissipated across the resistor will be equal to 5 W. So, the power rating of that resistor should be more than the 5W. Now, as a general rule of thumb, the resistor which we are going to use should have a power rating at least 2 to 4 times the maximum power which is going to get dissipated across that resistor. Now, the commercially available resistors have the power rating of as low as 1/16 W or as high as 300 W. Now, apart from this power rating sometimes the manufacturers also provide the power derating curve. So, this curve basically defines, with temperature how the maximum power which is going to get dissipated across the resistor will change. So, while selecting particular resistor, apart from the power rating one should also need to take care the ambient temperature of the resistor.

2.    Tolerance

The second parameter of this resistor is the tolerance. So, basically, this tolerance defines the deviation of the resistance from the nominal value. So, let's if you have one resistor of 100 Ohm with tolerance of 1 percent, it means that the value of the resistance will be in between 101 Ohm and 99 Ohm. Now, the commercially available resistors have a tolerance value of as low as 0.1% up to 20 %. And it is also even possible to achieve the lower tolerance than the 0.1 %.

3.    Temperature Coefficient

Then the third parameter associated with the resistor is the temperature coefficient of the resistance. So, basically it defines the amount by which the resistance value changes with the temperature. Now, this temperature coefficient can be either positive or negative. And generally, it is defined by the unit of ppm/ C. So, let's say we have one resistor of 100 Ohm and it is operated at 25 C. And the temperature coefficient of the resistance for that resistor is 50 ppm /C. Now, suppose if this resistor is operated at 29 C, then the value of the resistance R will be equal to 100.02 Ohm. So, basically here this 50 ppm will get multiplied by the change in temperature along with the value of that resistor. So, let's say for some resistor, if the value of this temperature coefficient of resistance is 500 ppm /C, in that case, just by a 4-degree change in the temperature, the value of the resistor will change by 0.2 Ohm. So, this parameter is particularly critical, when the resistor is operated at high temperature. So, for any resistor, the value of this temperature co-efficient of resistance should be as low as possible.

4.    Frequency Response

Now, the next parameter which is associated with this resistor is the frequency response of the resistor. Now, generally, we assume that the resistor which we are using is purely resistive in nature. But depending upon the construction of this resistor, it also has some inductance and the capacitance. And because of that, the maximum frequency at which this resistor can be operated will be limited. So, while selecting a resistor for high-frequency application, one should also need to consider this parameter.

5.    Stability And The Noise

Apart from these parameters, the other parameters related to this resistor are the stability and the noise. So, this stability defines how stable the resistance value will remain over the period of time. So, basically, it defines the stability of the resistor. Now, every resistor generates its own noise. So, while selecting the resistor for low noise applications, one should also need to take of this parameter.

So, these are the different parameters which one should need to take care of while selecting the resistor.

Different types of Resistors

So, now let's see the different types of resistors which are used commercially. So, these resistors can be classified into two categories.

1.  Fixed Resistor

2. Variable Resistors.

The fixed resistors mean that once the resistor is fabricated, one cannot change the value of that resistor. While in case of a variable resistor, the value of that resistor can be changed by changing the knob. So, now let's see the different types of fixed resistors which are used commercially.

 Different types of fixed resistors

i.              Carbon Composition resistor

So, the first type of resistor is the carbon composition resistor. So, this resistor is made up of carbon particles and the binder like a clay. So, these type of resistors are used in applications where you are not used because of its poor temperature coefficient and poor stability. Apart from that these type of resistors also produces more noise and they are also less accurate. So, these type of resistors has been replaced by the film type of resistors

ii. Carbon Film resistor. 

So, the first type of film resistor is the Carbon film resistor. So, if you see the internal structure of this carbon film resistor then you will find that on the ceramic substrate the thin layer of this carbon film has been deposited. And this film has been deposited in the form of a helix. So, just by changing the pitch of the helix, one can change the resistance value of this carbon film resistor. Now, these type of resistors are low-cost resistors and they produce a less noise compared to the carbon composition resistor. And the tolerance value of this resistor is also less than the carbon composition resistors. So, these type of resistors are used in high voltage and high temprature applications. And also these type of resistors are 

available in wide range of values.

iii.           Metal Film Resistor

So, the next type of film resistor is the metal film resistor. So, in terms of the construction, these resistors are very similar to the carbon film resistor. But here, instead of a carbon film, the thin layer of metal film has been deposited on a ceramic substrate. So, these type of resistors are also low-cost resistors. And in terms of the noise and tolerance, they are better than the carbon composition resistors. Apart from that in terms of the stability and the temperature coefficient of resistance they are quite good. And generally, they are preferred for the high-frequency applications.

iv.           Metal Oxide Film Resistor

Then the next type of film resistor is the metal oxide film resistor. So, in terms of the construction, they are quite similar to the metal film and the carbon film resistors. But here, instead of metal or carbon, the metal oxide film is deposited on the ceramic substrate. And the generally the tin oxide is used as a metal oxide layer. Now, these type of resistor is also low-cost resistor and in terms of the stability, noise and the tolerance, the performance of the metal oxide film resistor is poor than the metal film resistors. But if you compare in terms of the carbon composition resistor, then they are much better. Also in terms of the temperature coefficient of resistance, these resistors are poor than the metal film resistors. But these type of resistors are particularly used in high temperature and high surge applications.

v.            Wire Wound Resistor

Then the next type of resistor which is used quite frequently is the wire wound resistor. Now, if you see the internal structure of this wire wound resistor, then the metallic resistive wire is wound around the ceramic material. So, the thickness or the gauge of the metallic wire decides the resistance of this wire wound ressitor. And generally, the metal alloys like copper and the silver alloy are used for this resistive wire. Now, these type of resistor provides the very high accuracy and also they have a very low-temperature coefficient of resistance. And because of that, they are quite suitable for high precision applications as well as for the high power applications. But the htese type of resistors are not suitable for the high-frequency applications. So, these are the different types of axial lead type od resistors which are used comercially.

vi.           Surface Mount Resistors (SMD)

Apart from that, you might have observed the tiny resistors in various PCBs and various motherboards. So, these tiny little resistors are known as the Surface Mount Resistors. Now, if you see the internal structure of this surface mount resistor, then you will find that on ceramic body, the thin layer of resistive film gas been deposited. Now, generally the metal film or metal oxide film is used as resistive element for this surface mount resistor. And on top of this resistive film, the thin layer of the insulating layer has been deposited. Now, on both sides of this surface mount resistor, you will find the metallic contacts, so that this SMD resistor can be soldered on the PCB. Now, as these surface mount resistors are made up of metallic film or the metal oxide film, it is possible to achieve very high accuracy and the very low tolerance value. So, these are the different types of fixed resistors which are used in commercial applications.

Variable Resistor

Apart from that, the various types of variable resistor are also used in many applications. So, in these resistors just by changing the knob, it is possible to change the value of the resistor. So, this is all about the different types of resistors which are used commercially.