Learn how to Calculate LED Resistor?

Resistors are components that have the purpose of offering opposition to the passage of electric current, through its material. This opposition to the passage of electrical current is called electrical resistance or impedance, which has ohm as its unit of measurement.

The resistance causes a voltage drop in a certain part of the electrical circuit, but it never causes electrical current drops, despite limiting the current. This means that the electrical current that enters one resistor terminal will be exactly the same as the one that exits through the other terminal, but there is a voltage drop, that is, a potential difference between the resistor terminals. Using this, it is possible to use resistors to control the electric current over the desired components.

Principles of a led resistor

A common cause of happening is to burn a led by accident, probably for forgetting to put the resistors in series with the led. In the presence of many doubts about which ideal values ​​for these resistors, we will teach you how to calculate a suitable resistance for our projects and understand how and why to use them.

There are some generic “recipes” used by those passionate about electrical circuits, to size the resistors according to certain types of led, for example using a 1k ohm resistor on a diffuse or high brightness led with a power supply of up to 6V.

With the circuit in operation, the led will be protected from possible damage, and because it is a wrongly sized resistor, with a resistance much higher than what is needed, the led has a lower intensity in its brightness, due to the excess resistance determined by the resistor.

In order to obtain the maximum use of the brightness of the led, not to have any damage, or to offer danger to the led, keeping the electronic circuit balanced, we will show the necessary calculations to determine the adequate resistance.

Resistor need

Resistors are components of electrical circuits with the function of limiting electrical current or generating heat according to specific needs. The led is a light-emitting diode, used for several applications, such as lighting a place or as a sign, indicating a mode of operation of something, among other applications.

The diode, as well as any other component, has its limitations, so some protection is needed because if this limit is exceeded, the diode can be damaged, so it is essential to limit the current and voltage over the led in some way. The component used to limit the voltage and current in the diode is a resistor in series with the diode.

RETMA table

The RETMA table is adopted by the manufacturers in order to standardize the commercial values ​​of electronic components. It is useful to be aware of the values ​​available on the market when designing a new circuit. For example, if I want to obtain a resistance of 2 Ohms, I must know that there is no commercial resistor with that value. Therefore, a series of two 1 Ohm resistors should be chosen. The commercial values ​​of resistors (and capacitors) are powers of 10 multiplied by the values ​​below.

Commercial values ​​of resistors and capacitors.

In very special cases, it is possible to order a batch of components with an unusual value from a specific manufacturer. However, for most applications this requirement is easily circumvented by redesigning the values ​​or finding an equivalent resistor association .

The power of commercial resistors can be 1 / 8W, 1/4 / W, 1 / 2W, 1W, 2W, 10W, etc. In general, the power of a resistor is identified by its size, or by some inscription on its surface (for powers from 1W).

To identify the value of a resistor just look at its color code. Each code ring has a meaning:

  • First and second ring – digits;
  • Third ring – multiplication factor;
  • Fourth – precision.

For example, a resistor with the colors Brown Black Red Silver corresponds to 1k Ohm with an accuracy of 10%.

Identify the value of a resistor by looking at its color code.

How to calculate resistor

Colors to calculate the appropriate resistor for each led.

To calculate the appropriate resistor for the led you will need:

  • The voltage of the power supply, that is, how many volts will you use to power your led,
  • From the voltage supported by your led in volts,
  • And the current supported by your led in amps

When starting a calculation you need to know all the LED parameters that will be needed, because then you will know the voltage and limited current.

The formula for calculating a suitable led resistor is:

Equation for calculating resistors.

  • R is the ohm resistance of the resistor suitable for the LED, this is what you have to find out.
  • V is the voltage source you will use for the LED.
  • V led is the voltage in volts of the LED.
  • I is the current of the LED in amps

In the assembly of electronic circuits, the use of LEDs is common, as an example to indicate that the circuit is on, since a led flashlight or headlight is no longer a secondary electronic component and becomes the main component of the circuit .

Calculating the suitable resistor for LEDs connected in series

Now there are 3 red LEDs connected in series, each with a voltage of 2v, current of 20 mA and the power will be with a battery of 9v.

Converting the 20mA (milli ampere) of the led to ampere would give 0.02 A (ampere). Adding the voltages of the LEDs we would have a total of 6v.

Example of resistors connected in series.

So for these three LEDs in series a 150 ohm resistor would be adequate.

Calculating the suitable resistor for leds connected in parallel

For led connection in parallel it is recommended that you use a resistor for each led. It is not recommended to use a resistor for all LEDs as the leds can be damaged. In this case we have a red, a green and a blue led. So the correct thing is to individually calculate a resistor for each led.

For the red led:

Converting the 20mA (milli amps) of the led to amps, we obtain 0.02 A (amps).

Example calculating resistors connected in parallel to red led.

As it will be very difficult to find a 350 ohm resistor you can use a 390 ohm resistor, which is the closest to the calculated value.

For green led:

Converting the 20mA (milli ampere) of the led to ampere would give 0.02 A (ampere).

Example calculating resistors connected in parallel for green led.

As it will be very difficult to find a 325 ohm resistor you can use a 330 ohm resistor

For blue led:

Converting the 20mA (milli ampere) of the led to ampere would give 0.02 A (ampere).

Example calculating resistors connected in parallel to blue led.

As it will be very difficult to find a 300 ohm resistor you can use a resistor of the closest value, which is the 330 ohm resistor.

We will see below a video of how to calculate resistors in series and resistors in parallel. There are advantages and disadvantages in each association with regard to the division of the current. The theory is simple, but these resistive dividers are part of many electrical and electronic equipment that we use on a daily basis.