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

Why we need a current limiting resistor for LED?

LED is a fragile device. Apply Excessive voltage and current to LED would cause burn out. A simple and effective way to prevent burnout is to series a resistor with the correct value. In this article, we will explain how to series resistor for the most used LED display configuration.

Two of the Most used 7 segment LED display configuration

  To minimize the outer connection of 7 segment display, two types of 7 segment display configuration, common anode and common cathode, shown below as an example.

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

LED is a two-terminal device that the current flows from the positive terminal to the negative terminal. The picture below shows a common LED lamp with a clear explanation of its component.

Picture description:▲ LED Lamp explanation diagram came from Wikimedia

Common anode and common cathode type 7 segment display configuration explain

For the LED application, It common to use more than one LED. For example, the minimum amount of a 7-segment LED display is 7 LED. For a module like a 7-segment LED display, the mounting technology for the module, and the number of output pins affects the physical dimension of the outline dimension. Use Common anode and Common cathode configuration to minimize the module output pin also minimize outline dimension.

We compare the circuit diagram of this condition:

  • [A] 7-segment LED display that don’t apply 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 circuit to clarify the common difference between these configuration.

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

From these circuit diagrams, Common anode and common cathode configuration can reduce the module output from 16 pin[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 LED s, the common anode and common cathode configuration have no difference. For the product designer, two configuration options give them the freedom to design the LED module wherever it needs.

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

The current limiting resistor should place in a place that only one resistor and LED for each current path.Recommended configuration for a current limiting resistor in the 7 segment display shown below.Each LED’s current path should have one current limiting resistor. This configuration makes sure current flows into LED independent from other LED.

Better Common cathode configuration for current limiting resistor  by Opto Plus LED
Picture description:▲ Recommend configuration for a current limiting resistor in the Common cathode 7 segment display. 

To turn on the LED, VP should larger than VF and the current through LEDs would define by current limiting resistor and VP. 


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

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

OPTO PLUS LED OPD-S3010LA-BW LED DISPLAY DATA SHEET
Picture  description:▲ OPD-S3011LA-BW datasheet

First of all, you need to check all electric 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 be discussed them all. We design our current from forwarding voltage and luminous intensity. For instance, if you don’t need 40 (mcd) luminous intensity, maybe you can design a 10 (mA) current for each LED.To turn on the LED, the VP must larger than VF. Choose VF =2.3V, VP=3.2V, we can write down an equation from the current path L.

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 so many devices that can be designed to drive the 7-segment display to show numbers or digits. Such as Arduino, Raspberry Pi, Micro-controller like 8051 , FPGA(Field Programmable Gate Array), or general-purpose IC with proper coding (e.g: 74hc14 (driver) / 74HC47 (BCD to 7 segment decoder), Arduino might be the most popular one due to friendly UI (User Interface), numerous open-source community and learning resource. The rule to turn on a 7 segment display the same to turn 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 Arduino is quite a simple task. 220-ohm resistor is a common choice for the 5V power Arduino system. if you want to understand why 220 ohm is a good choice. please see the video made by Lazytomato LAB with a simple and clear explanation (with English caption).
OPTO PLUS will provide another engineer example article about other devices. please stay tuned!!

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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