Calculation of resistors. Tips on how to do it!

Resistors are essential components within the electrical system as a whole, for sure all electronic circuits have resistors, from the simplest possible electronic equipment to the most complex ones. Before executing any electronic project, it is necessary to do the calculation of resistors beforehand, which can generate many doubts. Thinking about it we decided to show how to perform the calculation of resistors for a simple circuit with LEDs.

To do the calculation of resistors we will use as base simple circuits, which have LEDs, because besides being easier to understand how it is done, it is common in the assembly of electronic circuits to use LEDs, either to indicate the state of operation or operation of the circuit or even in the construction of a flashlight for example, where the LED becomes the main component of the circuit and ceases to be the secondary component.

In order for us to better understand how to calculate resistors, it is necessary to understand a little about them, such as what a resistor is, what a resistor is for and how it is a resistor association.

What are resistors and their applications?

Resistors are electronic components that have the ability to regulate the passage of electrical current, in addition to transforming electrical energy into thermal energy, that is, part of that electrical energy is dissipated by the resistor in the form of heat, this phenomenon is called joule effect.

There are several types of resistors, whether fixed or variable, they can be classified according to the type of material, and also by the constructive type, the most common being: coal resistors, wire resistors, carbon film resistors and metallic film resistors.

It is important to note that there are two ways to identify the value of the resistors, measuring with a multimeter or identifying them using a 

Resistor values ​​are tabulated and are therefore called commercial values, that is, after having done the calculation and having found the resistor value, this value may not be commercial, in this case always choose a resistor with a commercial value above the calculated as close as possible, or perform the association of resistors.

The commercial value resistors are shown in the table below so that the other values ​​can be found, just multiply by 10, 100, 1,000, 10,000, 100,000 or 1,000,000.

Commercial resistors:

Table as the commercial values ​​of resistors.

Sizing of resistors:

There are some methods that are already customary for hobbyists, to find the correct value of the resistor for the LEDs, methods that are generic, which seem to work, such as using a 1K resistor for any given LED tension.

These tips allow the LED to light up and protect it from damage. But the LED will certainly have its light intensity affected, due to the excess resistance imposed by the resistor. We are talking about an exact area, where every detail can make a total difference, so the need to do the due calculations of the resistors!

Thinking this way, to take full advantage of the brightness intensity of the LED, without danger of causing certain damage to the LED, other components and at the same time keeping the circuit well balanced and dimensioned, without excess or lack of resistance, voltage or current , we will do some examples below.

In order to perform the calculation of the resistors, it is necessary to have some important information of the components, such as the supply voltage, voltage that the components support, maximum current that they can support and the necessary voltage that each of the components need to function, that is, the voltage drop over each of them, such as the diodes. In this case, a type of diode will be used, which are the light emitting diodes, better known as LEDs.

Tips on how to dimension the resistors.

How to perform the calculation of resistors?

The electronic components have datasheets, and the datasheets are technical documents about each device, so just check the body of each of the components to search for their respective datasheet.

As we already know, the LED`s do not have something written on their body, or any information about it, so there is some difficulty in identifying proper information about the LED`s.

Knowing about the difficulty in identifying the correct LED and its respective technical characteristics, we provide a table, which contains the voltage and current rating for the LED. This table below is for 5mm resistors, so just identify the LED according to its color.

Voltage and current table for the LEDs.


The formula needed to calculate the appropriate resistor for the LED is R = (Vcc – VL) / IL, where:

  • R: resistance of the resistor suitable for the LED.
  • Vcc: Voltage of the power supply.
  • VL: LED voltage.
  • IL: LED current.

Example 1:

Dimension a resistor to build a circuit that lights a 5mm green LED, with a 9V battery.

The resolution is simple, just identify the maximum voltage and current for the green LED in the table, then replace the values ​​found in the formula, as shown below. Remembering that 20mA is the same as 0.02A.

R = (9 -2.5) / 0.02

R = 6.5 / 0.02

R = 325 ohms

In this case there is no 325 ohm resistor, so we will use the one with the closest value, but that is above the calculated value, so we will use the 330 ohm.

Wiring diagram for resistor and LED in series.

Example 2:

Turn on a red LED on a 4.5v power supply, using three 1.5V batteries each. Applying the values ​​in the formula looks like this:

R = (4.5 – 2) / 0.02

R = 2.5 / 0.02

R = 200 ohms

Now just check the table of resistors with commercial values ​​which is the closest to the calculated resistor, in this case both will be of the same value.

Calculating the resistor for LEDs connected in series:

Now there are two red LEDs and one blue connected in series, being fed a 9v battery.

In this example, as the current in the series circuit is the table, we will only use 20mA once, but we add the voltages of the LEDs, totaling 7V. Applying the values ​​in the formula:

R = (9 – 7) / 0.02

R = 2 / 0.02

R = 100 ohms

So for these three LEDs in series, a 100 ohm resistor would be the most suitable.

Wiring diagram for three LEDs connected in series.

Calculating the suitable resistor for LEDs connected in parallel

When connecting LEDs in parallel, it is recommended that you use a resistor for each one. It is not recommended to use a resistor for all LEDs, as there is a risk of damage to the components.

In this case, we have two red LEDs and a green one being powered by a 9V source. Therefore, it is correct to individually calculate a resistor for each of the LEDs. As there are two red LEDs, only one calculation is necessary for both.

For the red LED

R = (9 – 2) / 0.02

R = 7 / 0.02

R = 350 ohms

In this case there is no 350 ohm resistor, so we will use the one with the closest value, but that is above the calculated value, so we will use the 360 ​​ohm.

For the green LED

R = (9 – 2.5) / 0.02

R = 6.5 / 0.02

R = 325 ohms

As we did not find a 325 ohm resistor we have to use a 330 ohm one.

Wiring diagram for three LEDs connected in parallel.

Calculating the resistor for led`s connected in series and in parallel.

We have three red LEDs connected in series and one blue LED connected in parallel to the other LEDs and the power supply, which in this case is a 9v battery.
As in the previous examples, just add the voltages of the LEDs and dimension a resistor for them and dimension the resistor for the blue LED.

For red LEDs connected in series:

R = (9 – 6) / 0.02

R = 3 / 0.02

R = 150 ohms

So, for these three LEDs in series, a 150 ohm resistor is enough.

For the blue LED

R = (9 – 3) / 0.02

R = 6 / 0.02

R = 300 ohms

Therefore, in this case the most suitable resistor for the LED is 300 ohms.

Parallel series connection diagram.

The video below is a complete lesson on resistors, as it perfectly addresses what a resistor is, its effects on the circuit, the main types of resistors and their respective characteristics, as well as showing how to calculate and the ideal resistor for an LED.

We finish this article here and we hope to have helped you to clear all doubts about how to calculate resistors! If you still have any doubts or curiosity about this matter, leave it in the comments and we will respond.