Quick answer
Why do LEDs burn out even at the “correct” voltage? An LED is not a resistor — it does not limit its own current, so one series resistor has to do that job. Connecting a resistor in series helps prevent this damage. This article explains how to connect resistors for common anode and common cathode 7-segment displays.
Table of Contents
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.
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.
.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.
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.
One resistor for each current path
Why can one shared resistor damage LEDs that are connected in parallel? Because the I-V curves of two LED chips are never exactly the same. The chip with the slightly lower forward voltage takes most of the current, so the brightness becomes uneven, and in the worst case that chip fails from overcurrent. Therefore each LED current path should have its own current limiting resistor — connecting several LEDs behind one shared resistor is not a valid setup. This rule is stated in our application notes, and it applies to the segments inside a display in the same way as to single LEDs.
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.
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 = 3.5V, we can write down an equation for the current path L. Solving equation L will give us R4 = 120 ohms.
Solve equation L can get R4=120 ohm.
Quick forward voltage reference for resistor calculation
| Color class | Typical VF at 20 mA | Example: 5 V supply, 10 mA target |
|---|---|---|
| Red / Amber / Yellow | 2.0–2.4 V | R = (5 − 2.2) / 0.01 ≈ 280 Ω → use 270 or 330 Ω standard value |
| Blue / Pure Green / White (InGaN) | 2.8–3.4 V | R = (5 − 3.1) / 0.01 ≈ 190 Ω → use 200 Ω standard value |
This table is for a first estimation only. Please confirm the VF value from the datasheet of the part number you selected.
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.
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!
Multiplexing and PWM: check the peak current rating
The datasheet lists two current ratings. The continuous rating is typically 20 mA. The peak rating is typically 40 mA at 1/10 duty cycle and 1 kHz. A multiplexed display drives each segment at a higher instantaneous current for a shorter time, so the resistor should be designed for the peak current you actually use, and the duty cycle and frequency should stay inside the datasheet conditions.
The relation between current and brightness is not linear. For repeatable dimming, we suggest PWM control instead of analog current control.
What we correct most often on incoming schematics
One resistor per current path. Tying several segments to a single shared resistor looks economical on paper, but the segments then divide the current unevenly: the one with the lowest forward voltage takes more than its share, runs brighter, and fails first. The arithmetic itself takes two minutes. R = (VS − VF) / IF, so a part with a 2.3 V forward voltage on a 3.5 V supply at 10 mA needs 120 Ω; on a 5 V Arduino rail, 220 Ω is the value most designers settle on.
The second thing worth writing on the schematic is that forward voltage is not one number. Red, amber and yellow parts sit around 2.0–2.4 V, while blue, green and white sit around 2.8–3.4 V. A resistor sized for a red display is the wrong resistor for a blue one, and mixed-color displays need the calculation done per color, not once for the whole part.
Related 7-Segment and 16-Segment Display Resources
Continue with these closely related resources to verify selection, circuit, and application requirements.
- What is SMD / SMA / SMT / SMC / SMP / SME? What’s the difference?
- Choose lowest-cost colored LED for your product!
- Introducing 7-segment & 16-segment displays & internal structures.
- How to Represent Numbers and Letters in 7-segment and 16-segment LED display?
- What is RGB SMD LED? | SMD LED Packaging | Full Series RGB LED Comparison Table
- Introduce dot matrix display’s name, type, internal structure, and application scene
- Case-study – Custom LED display and module