Resistance is one of the most important factors in the world of electronics and electricity, this makes resistors one of the most important components in electronic circuits design, even if its functionality and theoretical analysis could be called “simple”. It is fair to say that resistors might be the most used component in electronics and electricity, because of this, there are different types of them depending on the situation in which they are placed.
As the name implies, the job of a resistor is to “resist” or regulate the flow of current that passes through them depending on the material used in its construction and its quantity. Applying a potential difference between the terminals of a resistors produces a voltage drop which sets the current obeying the Ohm’s Law, this is very useful since it allows us to set the current or voltage how we want it to be in certain situations.
Resistors can be made of different materials and sizes depending on the application in which they are going to be used, this way, the resistors are able to comply with particular situations and characteristics. For instance, a general-purpose resistor can’t be used in high power application since it wouldn’t be able to sustain the amount of current going through it.
Modern resistors can be classified in four bigger groups which have their own sub classification, this are:
- Carbon composition resistors
- Film or cermet resistors
- Wire-wound resistors
- Semiconductor resistors
This classification is made according to the way they are constructed, and each of them have their own advantages and disadvantages due to the electrical characteristics given by the materials and manufacturing applied to each type of resistor, some of the characteristics taken into account are temperature rating, power, noise, tolerance, frequency response and others.
Carbon Composite Resistor
Along with wire-wound resistors, carbon composite resistors were one of the first types of resistors, were widely used until the 1960s, when electronic design fields started to need a better performing resistor. They were principally made with powdered carbon and an insulating material, usually ceramic and resin, this mixture was then pressed into small rods, to which then were added leads to each side.
They are characterized to have very high tolerances due to difficulties to manufacture them to a given value, they were produced mostly with ±20% tolerance, but ±10% and ±5% were also available. As for the power that they could sustain, only ¼, ½, 1 and a less common 5 watt versions were made. For today’s standards the only advantage found in these resistors is the ability to withstand high energy pulses without much trouble.
These types of resistors are made by depositing a film of the material of choice inside an insulating ceramic rod or substrate, then a helical cut is added into the film to control the level of resistance that it will have. These types of resistors are used in general purposes applications as long as their maximum ratings and characteristics are met.
Carbon Film Resistor
This one was the one that took the place of the carbon composite resistor as the most used type of resistor at some point in history, though it was later replaced for other types of film resistors. Its advantages go around being smaller and having the opportunity of getting a tolerance as low as ±2%, but there were ±5%, ±10% and ±20% versions available.
They had a much smaller max noise than the carbon composite resistors so they were a huge improvement from the transistor-based circuits that were taking off at the moment. Today they can still be seen for general prototyping and learning applications.
Metal Oxide Film Resistor
This, as the name imply, are made with metal oxide, these were characterized by having even lower tolerance than the carbon film resistor, going as low as ±1%, and being susceptible to much less noise, it was just a basic improvement from the carbon film resistor, and can still be seen in learning and basic prototyping applications.
Metal Film Resistor
This is the last step in the evolution of the general-purpose application resistor, this is the definitive standard for resistors used today, they provide a good level of tolerance, stability and temperature coefficient, these are almost always going to be the right answer. Most of them are available with ±1% and ±2% tolerance, but they can be found as well in ±0.1%, ±0.25% and ±0.5% tolerance.
Even though they have a good performance in general, metal film resistors are not recommended for application where they can experience surge transient because of their size and construction, in this case a carbon composite resistor might be better suited. Another problem might be that they can be slightly inductive, affecting high frequency operations, where a surface mount resistor might be a better choice.
Wire Wound Resistor
One of the first types of resistors ever made, which basic structure hasn't changed much in a very long time. It is made by a resistance wire wound around a core usually made of ceramic. Its resistance is determined by the length, diameter and resistivity of the wire.
Although they are not the most used resistors by any means, they are the best option in very specific applications, mostly in high power operation where a huge amount of current is going to pass through them and where they could get hot, since they can maintain their stability even in high temperatures. They are known as well because of their really good tolerance that can go down to ±0.005%, and maintain it for a really long time of use.
Their usual characteristics can have a really wide range since the way they are constructed allow the manufacturer to be really precise in its design, but we can usually find them with really low resistance values (between 0.01 and 100k Ohm) and really high power levels, some rated up to 2.5kW.
Surface Mount Resistors (SMD Resistors)
SMD resistors use surface mount technology to take another step in the miniaturization of electronic design, their principal characteristic is its really small size, which brings a huge amount of advantages for most modern electronic design. One of the biggest advantages is the low cost of production, not only of the resistors, but from the manufacturing costs for the final design, since the way they are constructed allows a new level of production automation.
This is why SMD resistors are all over the professional and industrial electronics field, it allows the companies to have a faster and cheaper production. It is important to note, as well, that these resistors have a really small inductance, that makes them perfect for high frequency applications. They also have good tolerance and temperature coefficient.
On the downside, we have the problem that they are not that easy to replace, in fact, using SMD resistors for prototyping could increase by a lot the time of development of a design. Another disadvantage is the power rating (from 0.05 to 0.50 W) of the resistors, because of their small size they are not able to tolerate a huge amount of power, this is a characteristic that needs to be carefully taken in account at the moment of designing a circuit.
Many considerations must be taken at the moment of choosing the right resistor for every application, the most important ones might be electrical characteristics and accuracy, but sometimes physical and environmental characteristics should be in mind.