What is a resistor? How to calculate its value? How to choose it?

Introduction

In this course we will see what is a resistor, how to calculate its value and to know which resistor is adapted to your project.

Then we will see a software from our application “Arduino factory” to calculate the value of a resistor.

What is a resistor?

A resistor is a small component that opposes at the current to limit it.

It’s composed of a ceramic rod to limit the current. The both pins connect the resistor to the circuit thanks to nickel capsules. Finally, there is a spray to protect the resistor and display the rings of color.

The resistors are often used to limit the current in a circuit or as a pull down/up resistor.
Unlike a led, the resistor is an unpolarized component, which mean that’s mean you can plug it as you want it.

The unit for the resistor is the ohms and this is the symbol: Ω. The most known law which use a resistor is the ohms law (U=RI). We will use this formula in the next part.
The first resistors that have been created was formed by carbon layer. They are widely used because they are cheap. It has a tolerance from 5% to 10%.

Nevertheless this kind of resistor have a limited accuracy compared to metallic technology.
The new generation of resistor with a metal foil are constructed the same way as the carbon layer but they juste replace the carbon layer by metal conductor.

Therefore there is a bette stability of the temperature and a tolerance from 0.5% to 2%.

Now you know enough about how a resistor works. It’s time to calculate the value of the resistor you need.

You want to create a circuit and you don’t know what value of the resistor you need?
In this part we will answer to that problem. You will need some informations about your circuit, such as the tension of each component and the intensity of these. The purpose of a resistor is to limit the circuit into your circuit to not burn a component.

We can now take the example of a circuit of a led and an arduino board. This is the characteristic of the red LED:

V_{led} = 1,6 V and I_{led} = 8 mA

This is the mesh equation:

With the ohm’s law, we have this:

And we isolate R:

If you have plug the LED on the pin 3.3V of the Arduino board:

If you have plug the LED on the pin 5V of the Arduino board:

Now you can choose a resistor close to its value.

How to calculate the value of a resistor?

The value of the
resistor is calculated according of the color of its rings: 2 rings for the
value, one ring for the multiplier and one ring for the tolerance.

We will see now how to
calculate the value of a resistor with its rings.

First of all, you have
to know that each resistor has a different number of color’s ring:

Furthermore each ring as a different meaning :

Here is some examples:

For the 4 rings resistor:

Example

1st ring: blue -> 6

2nd ring : yellow -> 4

3rd ring: orange -> 10^{3 }Ω

4th ring : red -> ± 2 %

Conclusion

The value of the resistor is: 64 x 10^{3} = 64 000 Ohm = 64 kiloOhm

The tolerance is ± 2 %

For the 5 rings resistor:

Example

1st ring: blue -> 6

2nd ring : yellow -> 4

3rd ring: red -> 2

4th ring: orange ->10^{3 }Ω

5th ring: green -> ± 0,5 %

Conclusion

The value of the resistor is: 642 x 10^{3} = 642 000 Ohm = 642 kiloOhm

The tolerance is ± 0,5 %

For the 6 rings resistor:

Example

1st ring: blue -> 6

2nd ring: yellow -> 4

3rd ring: red -> 2

4th ring: orange ->10^{3 }Ω

5th ring: green -> ± 0,5 %

6th ring: violet -> 5 ppm/C°

Conclusion

The value of the resistor is: 642 x 10^{3} = 642 000 Ohm = 642 kOhm

The tolerance is ± 0,5 %

The temperature coefficient is 5 ppm/C°

Is there a 3 rings resistor?

A resistor has at minimum 4 rings but sometimes the last ring is missing. It doesn’t affect the accuracy of the value of the resistor because it miss only the tolerance. Therefore we will take the largest tolerance, which is 20%.

What is the multiplier and the tolerance of a resistor?

The multiplier:The multiplier ring is used to know at which power of 10 you need to multiply the significant digits. In fact, there are 220 ohms resistors or even 10 000 ohms. Thanks to the multiplier ring you know which power of 10 is situated the resistor.

The tolerance: When the resistor is made, the manufacturer give it a value. Nevertheless the value can change a little bit even if there is the same rings. That’s why there is a tolerance to notify the user that the value of the resistor may vary a little bit.

Tip !!

Do you think that calculate the value of a resistor is difficult and fastidious? We have created an android application which will do it by itself!

This application can be useful in all your project. You can find in it a resistor calculator, some course about the components and a remote control to control all your projects with bluetooth!