When it comes to electrical resistors, then there are a lot of doubts like, for example, what is a resistor? What is the color chart and types of resistors? How to calculate the resistor tolerance? Anyway, we will answer all these questions and give you lots of tips and information about electrical resistors. Come on, guys!
What is a resistor?
The resistor is a passive electrical component that has the primary function of limiting the flow of electrical current in a circuit. To facilitate understanding, observe the example of water passing through a pipe represented in the image below. In this case, the flow of water is an analogy to the electric current flowing in an electrical circuit. Therefore, when we create resistance to the flow of water, the current will decrease.
The resistor has a higher resistance than the cables and tracks of an electrical circuit, forcing the reduction of the electric current that passes through it! As such, it causes a voltage drop.
The relationship between voltage, electrical current, and electrical resistance is described by Ohm’s Law. George Ohm was a German scientist who in 1827 discovered that electrical resistance is equal to the voltage divided by the current and this formula is one of the most important when it comes to electricity!
Electric resistors: Polarity!
A very common question when it comes to resistors is whether it has polarity! The answer is very simple, the resistor does not have a positive side and a negative side, there is no right side or a determined polarity to connect a resistor to a circuit. Therefore, you can invert the resistor as the electrical resistance will remain the same.
Electric resistors: Application!
To illustrate the importance of using a resistor, look at the example below. To light a red LED using a 5V source, 20mA of current is enough to light it, but if we connect the LED directly to the source, without a resistor, the LED will burn out quickly because there is nothing to limit the circuit voltage.
To avoid this problem, you must place a resistor between the voltage source and the LED, but it cannot be any resistor! It is necessary that it has sufficient resistance to limit the current to 20mA and that it creates a voltage drop suitable for the operation of the LED.
Electric resistors: How to calculate?
Using Ohm’s Law, we know that resistance is the voltage divided by the current. Therefore, the voltage on the resistor is 5V from the source minus the 2V voltage drop on the LED, resulting in 3V. Playing the values in the formula and dividing the resistor voltage which is 3V by 20mA which is the current that will pass through it, we will have the value of 150 Ohms for the resistance, as shown in the image below:
Therefore, if we put a 150 resistor in series with the source and the LED, the current is limited and now the LED lights up without burning, as the resistor limits the total current that passes through the circuit! Of course, this was just an example of a simple application, but almost any electronic circuit you find out there has resistors, either apparent or inside integrated circuits.
Electric resistors: What types?
There are several types of resistors and the symbols for the resistors are also varied. In the image below you can see this, the one on the left is defined by IEEE (I3E) and the one on the right by IEC.
Fixed resistors have a constant resistance value and they are the most common type. They can be axial or SMD, with SMD resistors being soldered on the boards and are the most used because they are small and easy to assemble on a large scale.
Variable resistors are those where it is possible to change the resistance value! Most variable resistors are adjusted by mechanical movement, that is, you need to move the adjustment manually.
When it is a voltage divider with 3 terminals, they are called potentiometers. When they are just a variable resistance, they are called rheostats. There are also digital potentiometers that are controlled by software, usually through the commands of a micro-controlled system with an Arduino, for example.
Another category is the resistances that vary from some physical quantity, such as temperature, light or voltage. This group includes LDRs and varistors, for example!
Electric resistors: Types of materials!
The type of material the resistor is made of is also very important. Wire resistors are the oldest and are nothing more than a resistive wire wrapped in a non-conductive material such as ceramic. They have low electrical resistance, can be very precise, and can also be adjustable. The disadvantage of wire resistors is that as they are wound like a small coil, they generate an inductance that can be significant in high-frequency circuits.
There are also carbon resistors or carbon compounds that are constructed with a mixture of a non-conductive ceramic and fine carbon particles. Although they are less accurate, they are still widely used since not every circuit requires great precision.
Carbon film resistors have greater accuracy than carbon composite resistors but have lower properties compared to metal film resistors or metal oxide film resistors. Metal foil resistors look like carbon foil resistors but have a metal layer instead of a carbon foil. These resistors have better accuracy and very good thermal stability, generating less noise in the circuits and therefore, they are the most used in audio circuits.
When it comes to durability, metal oxide resistors are the ones that have higher temperature resistance and greater reliability than metal film resistors. They have the highest available accuracy and stability, therefore they are more expensive than other resistors!
Electric resistors: Color code!
Most current resistors have a marking with colored bands that serves to indicate the value of the resistance and its tolerance. There is a color table to find out the value of a resistor, we have already talked about it in another article here on our website, we invite you for this extra reading. But anyway, let’s take a quick example by calculating the value of a resistor from the colors!
In our example in the image below, we have a carbon composite resistor with four color bands. The first range gives the first digit of the resistance value. The second track gives the second digit. The third range indicates a multiplication factor and the fourth range gives the resistor tolerance.
Looking at this table that you also find in the article suggested above, we see that the first color is brown, corresponds to the number 1. The second color, which is black, corresponds to 0, forming until now the number 10. As the resistor has only 4 colors, the 3rd color is the multiplier and red means to multiply by 100. Therefore, the value of this resistor is 10 multiplied by 100, resulting in 1000 ohms or 1k ohm or simply a 1k resistor.
The last color represents the tolerance or precision of the resistor. As in this case, the last color is golden, which means that the resistance value can vary by up to 5%. Therefore, the image resistor is a 1k ohm resistor with a 5% tolerance.
In the video below, you will find all this information passed in a very dynamic way, with practical examples, details of the types of resistors and also the measurement of the resistor tolerance using a multimeter.
We are finalizing this article further and we hope you have understood all the importance and functionality of resistors. If you have any questions, just leave them in the comments and we will answer them!