How to design an LED series current-limiting resistor for 7 segments LED display?

Why we need a current limiting resistor for LED?

  An LED is a fragile device. Applying excessive voltage and current to an LED would cause it to burn out. A simple and effective way to prevent burnout is to connect a resistor with the correct value in series. In this article, we will explain how to connect resistors for the most commonly used LED display configuration.

Two of the Most commonly used 7 segment LED display configurations

To minimize the external connections of a 7 segment display, two types of 7 segment display configurations, common anode and common cathode, are shown below as examples.

LED Basic

LED definition by Wikimedia were :

… A light-emitting diode (LED) is a semiconductorlight source that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the bandgap of the semiconductor.[5] ….

Wikimedia -LED

An LED is a two-terminal device where the current flows from the positive terminal to the negative terminal. The picture below shows a typical LED lamp with a clear explanation of its components.

Picture description:▲ LED Lamp explanation diagram came from Wikimedia

Explanation of Common Anode and Common Cathode Type 7-Segment Display Configurations

  In LED applications, it is common to use more than one LED. For example, a 7-segment LED display consists of a minimum of 7 LEDs. The mounting technology and the number of output pins for the display module affect its physical dimensions. By using the Common Anode and Common Cathode configurations, we can minimize the number of output pins and the outline dimension of the module.

We will compare the circuit diagrams for these configurations:

  • [A] 7-segment LED display without applying the Common Anode and Common Cathode configuration
  • [B] 7-segment LED display with Common Anode configuration (OPD-S3010LA-BW)
  • [C] 7-segment LED display with Common Cathode configuration (OPD-S3011A-BW)

We will discuss these circuits to clarify the main differences between these configurations.

Picture description:▲ common anode and common cathode configuration compare. OPD-S3010LA-BW  / OPD-S3010LA-BW 

  .From these circuit diagrams, Common anode and common cathode configurations can reduce the module output from 16 pins [A] to 10 pins [B]/[C].

What’s the difference between a common anode and a common cathode configuration?

  If you only need to turn on the LEDs, the common anode and common cathode configurations have no difference. For product designers, these two configuration options give them the freedom to design the LED module as needed.

Recommended configuration for a current limiting resistor in the 7-segment display

  The current limiting resistor should be placed in a location where there is only one resistor and LED for each current path. The recommended configuration for a current limiting resistor in the 7-segment display is shown below. Each LED’s current path should have one current limiting resistor. This configuration ensures that current flows into each LED independently of the others.

Picture description: ▲ Recommended configuration for a current limiting resistor in the Common cathode 7-segment display.

To turn on the LEDs, VP should be larger than VF, and the current through the LEDs is defined by the current limiting resistor and VP.

How to design the current limiting resistor to control the LED current?

Use the OPD-S3010LA-BW datasheet’s electrical parameters as an example to design LED current.

Picture  description:▲ OPD-S3011LA-BW datasheet

  First of all, you need to check all electrical characteristics, like Dominant wavelength, Luminous intensity, Power dissipation per dice suitable for your application. There are many reasons for how to design the current of a seven-segment display, which cannot all be discussed. We design our current based on the forward voltage and luminous intensity. For instance, if you do not need a luminous intensity of 40 (mcd), maybe you can design a 10 (mA) current for each LED. To turn on the LED, the VP must be larger than VF. Choosing VF = 2.3V and VP = 3.2V, we can write down an equation for the current path L. Solving equation L will give us R4 = 120 ohms.

Picture description:▲common anode 7 segment display Current limiting resistor calculation_part1
Picture description:▲common anode 7 segment display Current limiting resistor calculation_part2

 Solve equation L can get R4=120 ohm.

Use Arduino to light up a 7-segment display

  There are many devices that can be used to drive a 7-segment display and show numbers or digits. Examples include Arduino, Raspberry Pi, Micro-controller like the 8051, FPGA(Field Programmable Gate Array), or general-purpose IC with proper coding, such as the 74hc14 (driver) or 74HC47 (BCD to 7-segment decoder). Among these options, Arduino is one of the most popular choices due to its user-friendly interface, extensive open-source community, and abundant learning resources. The principles for turning on a 7-segment display are the same as for turning on a single-color dip-type LED lamp.

Picture  description:▲OPTO PLUS LED DIP TYPE LAMP Product page         

  The choice of the LED current limiting resistor in an Arduino system is a relatively simple task. A common choice for Arduino systems powered by 5V is a 220-ohm resistor. If you want to understand why 220 ohms is a good choice, you can watch a video made by Lazytomato LAB , which provides a simple and clear explanation (with English captions). OPTO PLUS will also provide additional example articles about other devices. Stay tuned for more information!

Video description:▲Arduino #6 – LED 與電阻的必學之術!LED & resistors 101! by LazyTomato Lab 懶番茄工作室

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