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. So, in this article, I’ll explain what is a resistor? types of resistors, characteristics of Resistors and it’s different parameters related to resistors.
What is Resistor?
Now, as we know this resistor is the passive circuit element that opposes the flow of current and electricity. Symbolically, it can be represented by these symbols ( See on below images).
The property of the resistor to oppose the flow of current is known as the resistance and it is defined by the unit of Ohm. Now, 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.
Now, this 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-characteristic defines the resistance.
The relationship between the voltage, current, and this resistance is defined by the Ohms Low. These resistors are available in various sizes and shapes.
After knowing what is a resistor, now, let’s discuss different characteristics of the resistor as aspects to
- Power Rating
- Temperature Co-efficient
- Frequency Response
- Characteristics of Resistor
1. Power Rating
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=V×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 5W (5V×1A=5W). 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. The commercially available resistors have the power rating of as low as 1/16W or as high as 300W. 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. While selecting a particular resistor, apart from the power rating one should also need to take care of the ambient temperature of the resistor.
Tolerance defines the deviation of the resistance from the nominal value. So, let’s if you have one resistor of 100 Ohm with a tolerance of 1%, 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 a lower tolerance than 0.1%.
3. Temperature Coefficient
It defines the amount by which the resistance value changes with the temperature. This temperature constant may 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=100.02 Ohm. 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. This parameter is particularly critical when the resistor is operated at a high temperature. For any resistor, the value of this temperature coefficient of resistance should be as low as possible.
4. Frequency Response
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. Noise and Stability
The stability defines how stable the resistance value will remain over the period of time. Basically, it defines the stability of the resistor. Every resistor generates its own noise. So, while selecting the resistor for low noise application, one should also need to take care of this parameter.
Now, after knowing what is a Resistor and characteristics of Resistor, let’s know types of resistor:
Types of Resistor
The resistor can be classified into two categories. The one is the fixed resistor and the second is the variable resistors. The fixed resistors mean that once the resistor is fabricated, one cannot change the value of that resistor. While in the case of a variable resistor, the value of that resistor can be changed by changing the knob.
Now. let’s see the different types of fixed resistors which are used commercially.
1. Carbon Composition Resistor
This resistor is made up of carbon particles and the binder like a clay. These types of resistors are used in applications where you are dealing with high energy pulses. But nowadays, these types of resistors are not used because of its poor temperature coefficient and poor stability. Apart from that these type of resistor also produces more noise and they are also less accurate.
So, the Carbon Composition Resistor has been replaced by the film type of resistors.
2. Carbon Film Resistor
If you see the inner structure of this carbon film resistor then you may realize 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, this type of resistor is low-cost resistors and they produce less noise compared to the carbon composition resistor. And the tolerance value of this resistor is also less than the carbon composition resistors. So, this resistor is used in high voltage and high-temperature applications and also available in a wide range of values.
3. Metal Film Resistor
In terms of construction, these resistors are very similar to the carbon film resistor. But here, instead of a carbon film, the thin layer of the metal film has been deposited on a ceramic substrate. This resistor is also low-cost resistors. And in the 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’re quite good. Generally, they are preferred for high-frequency applications.
4. Metal Oxide Film Resistor
In terms of the construction, they’re quite similar to the metal film and the carbon film resistors. But in this, instead of metal or carbon, the metal oxide film is deposited on the ceramic substrate. And generally, the tin oxide is used as a metal oxide layer. Now, this type of resistor is also a low-cost resistor and in terms of stability, noise and 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 Metal Oxide Film Resistors are particularly used in high temperature and high surge applications.
5. Wire Wound Resistor
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 resistor. Generally, the metal alloys like copper and the silver alloy are used for this resistive wire.
This type of resistor provides 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 wire wound resistors are not suitable for the high-frequency applications.
So, these are the different types of axial lead type of resistors which are used commercially.
Apart from that, you might have observed the tiny resistors in various PCBs and various motherboards. This tiny little resistor is known as the Surface Mount Resistors.
6. Surface Mount resistor
If you see the internal structure of this surface mount resistor, then you will find that on the ceramic body, the thin layer of resistive film has been deposited. Now, generally, the metal film or metal oxide film or metal oxide film is used as the 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 will find the metallic contacts. So, this SMD resistor can be soldered on the PCB. As this Surface Mount Resistor is made up of metallic film or metal oxide film, it is possible to achieve very high accuracy and very low tolerance value.
Apart from that, the various types of variable resistors are also used in many applications. So, in this resistor 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 that are used commercially. We covered in this post is what is a resistor? And what are the characteristics of the Resistor? And the types of resistors. Now, I’m sure that you understand what is a resistor and its types.