Why is there no current limiting resistor in LED?

Introduction: When using LEDs, it is necessary to design a current limiting circuit (which can be a current limiting resistor or a DC circuit) in circuit to ensure that current flowing through LED does not exceed maximum operating current of LED. The use of a current limiting resistor is most common and convenient circuit in conventional LED devices. Now there are more sophisticated and integrated LED chips with more features to use. This article discusses reason why current limiting resistor was not built directly into early single LED.

01

LED Current Limiting Resistor

1.1 Current Limiting Resistor

We know that when using LED, do not connect it directly to both ends of power supply[1] as there is a danger of burning LED due to excessive current. Typically, a current-limiting resistor must be connected to LED pin to control current flowing through LED within appropriate operating range.

The value of this current resistance should be based on:

Power supply voltage:

Pressure drop across LED tube, determined by color of LED;

Current disk range:

Three factors determine size of current limiting resistor.

▲ Figure 1.1 LEDs of different colors

LED turn-on voltage 1.2

Various LED curves of several different colors[2] give actual measured Volt-amp curves of several different colored LEDs. It can be seen that current-voltage characteristics of LEDs are very similar to ordinary diodes, except that conduction voltage differs depending on color of LEDs.

▲ Figure 1.2.1 CVC of LEDs of different colors

In some special cases, if conduction voltage of LED is basically same as applied voltage, and internal resistance of battery is relatively large, battery can also be used to drive LED directly.

For example, following diagram is from a Nintendo Wii interactive digital whiteboard with an infrared LED pen. For schools, HOWTO[3] shows application of direct connection of an infrared LED to fifth battery.

▲ Figure 1.2.2. Connect LED directly to both ends of battery

Since most LEDs require a current-limiting resistor to be connected, why aren't LEDs with built-in current-limiting resistors on market?

02

Why?

Someone asked this question in Why don't LEDs have built-in resistors? [4], and very detailed responses were given after that.

All night as I was trying to fit 16 current-limiting resistors on a small breadboard for a general purpose project, I wondered why LEDs don't have any internal resistance? I can't think of any flaws. I've never wired an LED without a series resistor, so this makes a lot of sense to me.

The questioner complained after soldering 16 current limiting resistors. Why is current limiting resistor built right into middle of LED?

2.1 LED with integrated current limiting resistor

Firstly, there are still such LEDs on market with built-in internal current-limiting resistors. For example, in a 6.5V red LED with a 3mm through hole, Kingbright L-934SRD-5V offers an LED[5] that incorporates a current-limiting resistor at 5V. The LED is 3mm in diameter and operates at 5V with current about 14 mA.

▲ Figure 2.1.1 Operational +5V red LED integrated with resistor

LP377GWH1-40G[6] shows specifications of a 5mm diameter green LED manufactured by YunSun in Shenzhen, China, which operates at +5V and operating current at +5V is 20mA.

In fact, LEDs with more complex internal circuits already exist. They not only contain current limiting circuits, but can also have flicker control circuits, dimming circuits, etc. I/O port can control color and brightness of up to 1024 LEDs.

▲ Figure 2.1.2 WS2812 light bar effect after illumination

2.2 LED built-in current limiting resistor

We have seen before that LED products with built-in current-limiting resistors are indeed a long time coming, but in terms of market response, this is a poor product (in terms of usage percentage and market penetration rate), main problems include:

The problem of light output: in fact, most LEDs used for lighting work without current-limiting resistors, and LED light output is maximized by a special constant current circuit;

LED series connection problem: In many applications, it is necessary to connect several LEDs in series to form a larger area display lamp. If there is a current-limiting resistor, a larger control voltage is required. At present, a constant current circuit is often used to drive LEDs of this series;

Different operating voltages and currents: We know that size of an LED's current-limiting resistor depends on several factors. Once current limiting resistor is integrated according to a certain condition, it will limit cases where an LED can be used;

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Heat dissipation. How to effectively dissipate power dissipated by current limiting resistor is also a factor affecting built-in current limiting resistor inside LED;

Low operating voltage: operating voltage of current circuits becomes lower and lower, gradually moving from original 5V to 3.3V or even lower, which makes devices integrated with LEDs unable to adapt to different operating voltages;

It is very convenient to use a SMD current limiting resistor: in fact, it is relatively easy to add a SMD current limiting resistor to circuit so that LED can adapt to different operating conditions. For high voltage operation (eg +12V) several LEDs can be connected in series.

▲ Figure 2.2.1 LED Series Current Limiting Resistor

03

Overview

In process of using an LED, it is necessary to design a current limiting circuit (this can be a current-limiting resistor or a DC circuit) in circuit so that current flowing through LED does not exceed maximum operating current of LED. The use of a current limiting resistor is most common and convenient circuit in conventional LED devices. Now there are more sophisticated and integrated LED chips with more features to use. This article discusses reason why current limiting resistor was not built directly into early single LED.

▲ Figure 3.1. 9V battery lights LED via current limiting resistor