O que é um resistor?

Hello guys and welcome to ""Mundo da Elétrica! "" In today's video I will explain in detail how resistors work! Come on guys!

To begin with. . .

what is a resistor? A resistor is a passive electrical component with the primary function of limiting the flow of electric current in a circuit. There are some symbols for a resistor.

The one in the left is defined by the IEEE and the right one by the IEC. Tha make things clearer I have here the example of water passing through a pipe! The water flow in this case is an analogy the electric current flowing in a electric circuit.

When there is a resistance to flow of water the flow will reduce. We can make the pipe narrower in a specific part A resistor is the same with the electric current! The resistor has a higher resistance than cables and tracks of an electrical circuit, forcing the reduction of electric current running through it!

It causes a voltage drop. The relationship between the voltage, electric current and electric resistance is described by Ohm's Law. George Ohm was a German scientist who in 1827 discovered that the 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 until today!

A common question we receive here in Mundo da Elétrica is if a resistor has polarity! People, it does not! A fixed resistor does NOT have a positive side or a negative side when connected in a circuit.

You can reverse a resistor that its resistance will remain the same. There is no right way or a specific polarity to connect a resistor in a circuit. An application for a resistor é to use it to connect a red LED to 5V source!

With 20mA of current we can already power the LED. If we connect the LED directly at the source, without a resistor to limit the current . .

. . .

. the LED will burn quickly because there is NOTHING to limit the current in the circuit! To avoid this problem, we put a resistor between the voltage source and the LED.

The resistor must be have enough resistance to limit the current to 20 mA, enough to turn the LED on without burning it. Using Ohm's law, we know that resistance is the voltage divided by the current. The voltage across the resistor is 5V from source minus the voltage drop of 2V in LED, which results 3V.

Dividing the resistor voltage, 3V for 20mA that is the current that passing through it, we got the value of 150 ohms for the resistor. Placing the 150 omhs resistor in series with the source and the LED, the current is limited. Now the LED can finally lights up without burning as the resistor limits the total current passing though to circuit!

Fixed value resistors might be axials or SMD. SMD stands for ""surface mounted device"". They are widely used because they are small and easy to assemble by machines in high scale.

The variable resistors are those where you can change the resistance value! Most variable resistors are adjusted by mechanical movement. You need to manually ajust it.

When it is a voltage divider and has three terminals they are called potentiometer. When they are just a variable resistor, they are called rheostats. There are also Digital Potentiometer that are controlled by software, usually by commands from a microcontroller system as an Arduino, for example.

Another category are the resistances that varies according to some physical quantity such as temperature, light or even voltage. This group includes the LDRs and varistors for example! The type of material the resistor is made is also very important!

Wire resistors are the older ones. And they are nothing more than a resistive wire wound in a non-conductive material such as ceramic, for instance. They have low electrical resistance, can be very precise and may also be adjustable.

The disadvantage of wire resistors is that as they are rolled up like a small coil, it generates an inductance that can be significant in high frequency circuits. Coal resistors or carbon compound resistors are constructed with a mixture of a ceramic and non-conductive carbon fine particles. Although less accurate, they are still widely used nowadays, since not all circuit requires high precision.

Carbon film resistor has a greater precision than carbon compound resistors, but inferior properties if compared to the metal film resistors or metal oxide film. The metal film resistors look like the carbon film ones, but has a metal layer instead of a carbon film. They have better accuracy and thermal stability generating a lot less noise in the circuits.

They are even widely used in audio circuits. The metal oxide resistors are still more durable and has a higher temperature stability and higher reliability than metal film resistors. They have the highest accuracy available and stability today and are obviously more expensive than other resistors!

Most current resistors have some colored bands to indicate the value of resistance and tolerance. This resistor is a resistor of compound carbon with four bands of color. The first band gives the first digit of the resistance value.

The second band gives the second digit. The third stripe indicates a multiplication factor and the fourth gives the tolerance of the resistor. You can try to memorize the meaning of each track of color, but can also use a color table to find the value of a resistor.

In the description of this video I put a link to an article in the Mundo da Elétrica website with a color table ready to be used. Anyway, let's see a quick example calculating the value of a resistor from its colors! Using the table you find on the Mundo da Eletrica website, we see that the first color, brown, is the number 1.

The second color, which is black, corresponds to 0, forming so far the number 10. As the resistor has only 4 colors, the 3rd color is the multiplier and red means multiplying by 100. so the value of this resistor is 10 * 100 = 1000 ohms or 1k ohm or just a 1k resistor The last color is the accuracy or tolerance of the resistor.

As the last color is the golden, it means the resistance value can vary by up to 5%! Thus, the resistor on image is a 1k ohms resistor with a tolerance of 5% Measuring it using a multimeter, we see that the value is 983 ohms, only 17 ohms less than 1000, or a variation of only 1. 7%, within the tolerance of 5%.

If the resistor you have there has 3 or 5 colors, see the article in the description to understand how to determine its value. Well . .

. that's what I had to say about resistors! If you have any questions, write it in the comments that we will answer!

If that is the first time you are in our channel sign up and check to receive our updates. Also visit the MundoDaEletrica. com.

br website and sign up for our VIP mailing list! Follow us on social networks and send your question! That's it folks and I see you on next video!